Tesla Powerwall: Not much cheaper and also a bit wimpier than existing batteries It's pretty and batteries are ugly – but Tesla's Powerwall is more like an incremental change than a radical disruption. Given that the hype over Elon Musk's Tesla Powerwall announcement has reached all the way from The Verge calling the colours “lickable” (no, not likeable) and tossing in a hypegasm over Musk's “best keynote …
Powerpack probably consists only of made-to-order systems. When I went to the product page, I found a link allowing businesses a chance to inquire about their commercial version. Alternatively, it might be vaporware.
When the rumours first started, I wondered if Tesla would be selling 're-manufactured' batteries from their cars. Range anxiety, but more importantly size are not as big an issue when the batteries are stuck on the wall in the house, especially if the price is good and the warranty reasonable.
An ordinary solar inverter just converts all the power it gets from the panels to AC, in the case of the battery you only want to convert what you need, so you need smart inverter.
Anybody work out what kind of DC to AC inverter / controller you'd need to run a few of these things?
I assume if you want to charge the batteries you'd also need a AC to DC controller.
But yeah, without the price of the DC to AC (and potentially AC to DC) inverter/controller (3 phase?) the numbers still aren't complete.
I guess at the end of the day battery tech will flatten out the power demand curve and this should result in the flattening of the power price curve over a 24 hour period too.
Pro-logic -
The inverter you'd need is a function of how much power you want to draw. If you want (say) 6 kW continuous, that would be (for eg) two 3 kW inverters.
I didn't price these up, because you'd need the inverter capacity whatever battery you're using, it remains the same.
Richard Chirgwin
The Register
I'm not up on the technology, but doesn't a 'standard' solar inverter just dump direct to the mains? It's not stored locally at all? So the business model is based on the feed-on tariff rather than off-grid use?
I'd hazard a guess that a run of the mill UK customer has a fairly low base load - lights, fridge, TV, computers - with occasional medium level draws from dishwashers, washing machines, and possibly driers, and an occasional but extreme load from a kettle or oven. Without looking at the spec, I note merely that the oven is fed from a 30A breaker... which means you'd need a 40A/250V = 10kW peak supply.
With regard to the oven. A standard electric oven alone will have a 2kW heating element, or possibly 3kW. Maybe a second such element for the grill function. Because they are thermostatically controlled, and unlikely to be on at the same time many ovens can be run from a standard 13A socket, or fused spur unit.
An electric cooker on the other hand may well consist of:
1x 3kW ring
2x 2kW rings
1x 1 or 1.5kW ring
1x 2kW oven element
1x 2kW grill element
AND a 13A socket (common in the UK) run from the same circuit
With out applying diversity this little lot adds up to 15kW or so, which implies a 65A circuit at 230V. This would be very expensive in cable alone. Diversity recognises the thermostats and the fact that it will never all be on at the same time and hence you can get away with a 30A (fuse) or 32A (MCB) circuit.
Because people have different preferences it is common in the UK to have a 32A "cooker outlet" in the kitchen, even if you have a gas cooker, and even if all you have is a single electric fan oven at 2kW you will often find it is powered from this outlet.
M.
The announcement spurred me to look at off-grid for a house I am building in a region with good sun and poor electrical supply. I calculated that 10kWh would probably be enough and with 6.5h of sun per day on average I calculated I'll need about 1.6kW of solar panels (obviously you should get more to deal with winter but other provisions can be made).
I also looked at the output from my spreadsheet that I use to track my home energy usage in the UK. It seems I use 12-14kWh per day on average. Now according to my energy company that isn't bad for a three bed house but I know it could be better because I'm not that careful. But 10kWh is within reach and if we made more concessions it would be even better.
Now I just need to figure out if Tesla will sell one for DIY install!
You might look at forklift batteries. They are heavy but only need to be placed once. Will work with standard off-grid inverters if spec'ed to match.. Very few intra-connections as opposed to the usual lead-acid battery grid. Usual need for electrolyte monitoring. I'd prefer less energy density like in a lead-acid for safety reasons.
Having actually taken care of off-grid property, there is a different mentality over time, one becomes very conscious of power usage and lives within means. Heating/cooking/refrigeration is often done with gas. In dry climates an evaporative cooler is the answer.
I am in Australia and have a 4kw system during summer months I get around 25kw of power from my system each day; however during some periods throughout the year I only get 2-3 kW in a day. To go off grid you need enough storage to cover the very low periods of solar production. Having been to the uk many times your low periods are much longer than ours. I calculated I would need an absolute minimum of 50kw of storage to cover my usage in low solar periods and that is with a 4kw system
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Inverters typically convert DC or direct current to AC or alternating current. The AC can be used in an "Off Grid" application to make the Solar electricity usable by everyday standard equipment - Motors, Refigerators, Electronics, etc.
The AC produced by the inverter can be backfed into the main power system if they allow it. They call these systems "grid-tie". Lots of extra equipment like automatic transfer switches and utility power quality inspections needed to get the utility to pay any money or balance out your bill. Let's just say that utilities hate paying their customers.
Resistance heating elements technically don't really care if the power is DC or AC. Only the electronics that control the elements do. You could easily run lighting, electric baseboard or a kettle directly from DC power. However, batteries won't like high loads like that unless the battery banks are pretty big. You have to match the battery bank to the load over the time the load will be present.
If you want a cheap system and are willing to compromise then the offgrid style system will be best for you if your municipality will allow it. Total up the wattage of all the equipment you use, add a safety factor, calculate the size of the DC to AC inverter and batteries you need to satisfy the load and then size the solar panels so they can supply enough power to the batteries to run the equipment through the system. Every time you convert from DC to AC or back you will lose at least 10-15% from inefficiencies.
You don't make any money off such an "Off Grid" system you just don't have much of a bill.
With some management of load types you could come up with a reasonably priced system. You will likely have to do with wood heating, air drying and no air conditioning but you could run almost everything else you need from solar or wind power just not at the same time like when connected to the main power.
Just remember, almost everything about solar is a compromise in one way or another.
"How much more expensive than a 'standard' solar inverter is one that also has batteries in a hybrid power set-up."
Price of the batteries plus charge management kit with batteries being the lion's share of the figure.
These kinds of battery setups tend towards 600V or so, so the 12V out of solar panels needs stepping up anyway - and you need to take a heckuva lot more precautions with DC than AC as arcs aren't self-extinguishing.
You also need to factor in a loss of ~35% over the charge/discharge cycle if batteries aren't just being used for peak/brownout mitigation, which in turn means higher overall energy consumption whether that's from power lines or local generation plant (solar/windmills/generator/hydro/etc)
I'm in the middle of moving to a country where the the power is notoriously unreliable (230V mains often droops to less than 130V) so I can see a need for this kind of thing, but on the other hand the cost is around 10 times the annual income of the average person there.
Tesla claims a 92% round trip efficiency. This is possible with efficient converters with FETs to reduce the forward diode drop and the 2 KW limit - I would imagine that is at the 2 KW rate. Running a 15 watt shaver might consume 50 watts or more as it runs things far from optimal.
That said. A 4 KW option, based on one 2 KW and 2 more 1 KW inverters sounds more rational, with the ability to run a few separate circuits.
My house has 24 breakers, 12 on each 120 volt phase of my 240 volt system, so for a few extra $$ it would be easy to split off some of the large loads to a dedicated line. The home solar people set their houses and batteries up like this all the time.
The DC to AC inverter technology would be basically the same. The only difference would be the kind of protection the inverter provides for the batteries, aka, how hard it will suck the life from them!
In the case of these Tesla systems, it sounds like the answer is "very gently", which mean only a low capacity inverter is requried, which is actually cheaper (less amps is easier to work with).
Other things they might have done is play about with the supply voltage to the inverter. If the batteries are run in parallel and feed the inverter a higher voltage, it means the current is reduced for a given load. (Watts = V x A), and current is the bit that makes all the inverter bits expensive. Most off-grid solar inverters run from 12v or 24v DC battery stores, which means the current for a 1.5KW kettle on a 24v system (assuming 100% efficiency) is over 60 amps (double that for a 12v system), which is more than the current an entire house uses on 230v mains, so you can imagine the size of the conductor required... and then you have to switch it into an oscillating form and feed it into a step up transformer, so the oscillator needs to be beefy and so does the primary winding of the transformer!
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that is one thing I keep seeing skipped or completely left out of the conversation when comparing this to lead acid batteries. It's like they are comparing a bunch of raw parts to the complete system.
And this wall box is designed to charge off the grid at night when rates are cheap and then augment the power during the day when electric rates are high. Remember, Musk is the CEO of Solar City and in that capacity, he knows what it means to sell people on cheaper electric rates. With Solar City, they will pay for the solar panels and installation and give you a guaranteed fixed rate on electricity for X years and the rate is cheaper than from the grid. So this battery thing is mostly about filling it up with cheap electricity at night and using it at peak rate times. So it has to have a charger and inverter in there an the electronics to sense and control based on time or maybe even internet control besides load control. A pile of batteries does none of that.
The Nedap Powerrouter has this functionality - indeed, many of the inverters designed for the German market now do this, as the market there is moving away from FIT. The householder wants to use as much self-generated power as possible.
http://www.powerrouter.com/en/producten/frontpage.html
They also do a retrofit model...
http://www.powerrouter.com/en/powerrouter-familie/powerrouter-unifit.html
The Tesla batteries also aren't hugely revolutionary - alternatives already exist:
http://www.akasol.com/en/storage-for-renewable-energies/neeoqube/
A Nedap setup will allow 5KW or 3.7KW peak loads (according to model).
A lot of the cost is tied in to the intelligence of how to handle the charge/discharge, particularly when electricity is charged at different rates at different times (which includes self-generated power). What you really want is something which can determine when you are generating more than you are using, and only store that electricity. The problem with the Nedap solution (in this country) is that the power is stored directly, and doesn't pass through the inverter to go through the generation meter. Thus you don't get your FIT payments until it comes out of your battery. Thus you pay for any inefficiencies.
Mind you, it does open up the possibilities of charging batteries elsewhere and putting them into the system fully charged to then be used and pass through the generation meter at full FIT earning potential. Seems like a lot of effort for a small return...
Richard,
I live where it is hot and sunny. Most of us don't start our day with "flipping the switch on 1,200 watts of air-conditioner and fire up the 2,400 watts' worth of iron to get the wrinkles out of shirts". The house is usually cooler in the morning, and we would be unlikely to have the single 5kW "cool only" split system air conditioner in your link. What we would have is solar panels which would be generating power when the sun came up and the house started to get hot. If we had bought our aircon in the last year or two we would be more likely to have two modern 2.5KW systems which would draw a maximum of 0.42KW each, but we probably would not have both of them running flat out at the same time.
If you live somewhere really hot, it is also unlikely that you would be wearing a long sleeved, crisply ironed, shirt with a tie :-)
Would a single 7kW or 10kW system give me all the power I need with a maximum draw of 2kW? Probably - Except for a few days in the middle of winter where it might be a bit tight, but I could always look at running a cheap 4kW petrol generator for a few hours to charge the batteries. I would also look at putting a "soft start" power supply on my fridge, or paying the extra for a DC powered one.
Would I buy one now? No, but I might in 5 years where the cost is likely to have come down by half.
Actually, Tim, I live in an area where (in summer at least) if you want your house to be cool in the afternoon you had better start the air-con in the morning or else it won't cope with the existing heat. And I do work in an office where ironed, collar-and-long-sleeves shirts are a must. Luckily, we do not need to wear a tie unless meeting with clients.
Hi Neoc, I'm (semi-)retired now so I am at home for much of the day near Perth and get full advantage of solar power. Short sleeve shirts are OK here, and when I was working I kept a tie in the office/car for the 3 meetings a year I might need one. Before I retired, we set the aircon to come on at about 10:00am so we had free sunshine cooling the fabric of the house for when we came home.
Before aircon some people used to sleep on the beach here because their houses were too hot at night. It could be worthwhile to look at how well your house is insulated and if it is worthwhile getting ventilation or additional insulation/heat reflection for your roof-space - We found that putting up some solar screening on our exposed window helped (deciduous trees are good if you can spare the water).
Sounds like your house is not designed for passive cooling...but then looking around at houses and apartments in hot countries, the majority don't pay even lip service to passive cooling. Having done some research on this, Australia seems to have the most practical experience and designs.
The toaster, kettle, and iron are not continuous demands. It's not totally beyond the wit of man to schedule demand-side things differently if the supply capacity is limited.
Even the aircon may not be a continuous demand - the blowers may be on continuously, but the heat pump may operate under thermostatic control.
Details, I know.
Are readers aware that there are a handful of local-grid-scale (11kV, hundreds of kW?) battery+inverter trials going on in the UK? This isn't just to store energy between peak and off peak, it's also to locally peak lop such that grid infrastructure upgrades can be deferred.
E.g.
UK Power Networks, Leighton Buzzard, 10MWh (but how many MW?):
http://eandt.theiet.org/news/2014/dec/uk-power-networks-battery-storage.cfm
Western Power Distribution/Sheffield University, Wolverhampton (says 2MW, may mean 2MWh?):
http://eandt.theiet.org/news/2014/jun/battery-storage-project.cfm
Richard Chirgwin also, probably to make his point, only quoted the continuous power output of the powerwall and then gave examples of non-continuous use, i.e. kettle. Tesla actually quote a 3.3kWH peak output which should be more than enough for most homes and certainly removes the need to double-up.
Would I buy one now? No, but I might in 5 years where the cost is likely to have come down by half.
Or Tesla has gone bust.
Or the price has gone up.
Or no one sells them because they all caught fire or died early.
Or teh world reserves of lithium have been dpelted to the point where lithium itslef is worth the same as gold.
Nope. I'll not be an early adopter on this one.
I would also look at putting a "soft start" power supply on my fridge,
I had one of those things back in the early 90's, it worked really well on the basic fridge/freezers, but as soon as they introduced electronic control panels... Basically, most modern fridge/freezers with electronic control panels already include "soft start" as it helps them with their energy efficiency rating...
"I live where it is hot and sunny. Most of us don't start our day with "flipping the switch on 1,200 watts of air-conditioner "
With appropriate design it's possible to virtually eliminate external heating effects (dualskin roof, solar chimney, double glazing with heat rejecting outer panes, etc) and it's far more efficient to solar-cool using a hot water collector and SolarFrost system than to use solarPV and electrical pumps.
The problem with complex electrical/electronic solutions is that they're not always appropriate, especially in hot climates to drive cooling or water pumps (hot water collection and sterling engines are probably a better fit) or desalinisation setups (a greenhouse-style distillation plant is surprisingly effective, if somewhat large, but space isn't generally a problem in most parts of the world)
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The second or third generation of this product could be very interesting if they go for alternative battery technologies like the AL based one that was in the news recently or doped Li. Can't help but think when it doesn't have to go anywhere, power density is not a hugely critical factor. Longevity is.
Longevity and ability to handle high current flows was what made the Al batteries so interesting. Less need to have excess capacity if you can safely pull huge currents and no need worry about badly behaved loads like kettle and fridges. As well as being inherently cheaper we could get away with using less of them.
I get that small is beautiful, but from what they advertize it is not small and the price is very high.
I just purchased a deep cycle battery flooded/lead/acid to run my computer during power outages. It's just a little bigger than a normal car battery and it would take 5 of these at a cost of $700 to equal the 7KW Powerwall.
My battery is 7" x 13" x 9" (times 5 is 4095 cubic inches) and the Powerwall is 51.2" x 33.9" x 7.1" which is 12323 cubic inches. Where's the small in that?
I'd check your math there. Five lead acid batteries giving 7kWh would work out at 116Ah @12V. A regular car battery is something like 40-50Ah
I have no doubt that I could have gotten something wrong, but the lead acid battery which I purchased is rated at 115Ah @12V. This is typical of deep cycle batteries in that price range. A car battery is just a thin plate affair designed to give high cranking amps - not the same thing at all (and definitely not recommended).
Checking online, yes 115Ah is achievable for that kind of price. There are still three issues I can see. Firstly cooling. You can't just strap them into a closed box the exact size of the pack, plus you need management electronics. Secondly lifespan. The cells I looked at all seem to be rated at about 500 cycles, to get 10 years or more use out of them you need to be quite conservative over your use and underrate the capacity (use at least 6 or 7 of them). Thirdly weight. They were over 25kg each, by the time you get 6 strapped together that's going to need some fairly hefty brackets to hold them on the wall.
Secondly lifespan. The cells I looked at all seem to be rated at about 500 cycles, to get 10 years or more use out of them you need to be quite conservative over your use and underrate the capacity (use at least 6 or 7 of them). Thirdly weight. They were over 25kg each, by the time you get 6 strapped together that's going to need some fairly hefty brackets to hold them on the wall.
I think you're right about lifespan. The high quality lead/acid batteries for this kind of thing are much more expensive - presumably for a reason. That said, I've heard people say they get many years out of these when used for trolling motors or running utilities in their motor home. In my case it's going to be a floater so I'm expecting a very long lifetime. This is of course a different use than daily discharge.
Regarding your other comments. I don't seen any need to cover batteries up since I'd be proud to show them off. A closed box is not needed. And putting them on the wall is an odd place to my way of thinking. Regular metal shelving will take that kind of weight, it's no more than storing engine blocks or transmissions.
Anyway, I'm just curious about what the real advantages of these Powerwalls are. Personal preference is certainly legit, but I'm not getting the rest of it. I note too that it is apparently not currently feasible to recycle lithium-ion batteries. Where these batteries may shine is in longevity though. We'll see.
"They were over 25kg each"
Such things would never be "wall mounted"
See http://www.geekzone.co.nz/imagessubs/3192f7702bd84653858252ab395df9a7.jpg
This is the most common way of setting up battery banks. The photo's from a New Zealand telephone exchange and the bracing is heavy to ensure the array can survive a Mag 7.2 earthquake.
"...rated at about 500 cycles..."
If the pack is only storing 7 kWh, at roughly $0.15 each (YMMV), that's about a dollar pay back per cycle (generously assuming input power is free). So if the system costs any more than $500, there's no point if commercial power is available.
At home load leveling is an expensive hobby.
There are two factors where lead acid differ from lithium
1st off even deep cycle batteries cannot withstand being discharged as much as lithium
second is that the number of cycles for lithium is higher.
I think you get between 200 and 400 cycles from lead acid typically at 50% discharge and lithium does 1000s of cycles at 80% discharge before the capacity drops below 70% of its original capacity.
If you redo your sums with that factored in you will find lithium is cheaper in the long run.
Even "deep cycle" lead acid batteries do not really like deep discharge at all. If you want ten years of life you are probably looking at no more than 25% of nominal capacity discharge. To get 7kWH at 12V you will need more like 2400AH - that's about 20 120AH domestic batteries. The life will depend partly on the peak discharge current - if this is C/10, that will be a little under 3kW.
You can of course get better results from traction batteries but these will cost more.
Overall I agree with the article - what Musk is proposing is an incremental improvement. But I have to say that if I were to fit out a narrowboat or small Dutch barge, I'd be looking at his battery very carefully (the acceleration and vibration on those things means domestic batteries should be fine.) It looks as if it could work out as a significant incremental improvement. For off grid use in the UK, not really.
"A regular car battery is something like 40-50Ah"
lead acid batteries come in a dozen different design strategies.
A regular car battery is designed to deliver 2000A to turn the starter motor, at cost of capacity.
Deep cycle ("traction") batteries are optimised for energy storage but are current limited and intended ot be floated most of their lives.
Caravan and boating ("hotel") batteries are designed for long-life off-charge and low leakage, etc. They can provide 12V for a long time but only at relatively low currents.
Gel mat batteries have longlife, low maintenance but very low currents.
That's why it's critically important to have the right battery for the job. Car batteries don't last long when used to power mains inverters. Deep cycling kills them in very short order - yet these are the kind of batteries the average supplier will try and foist off on consumers (who generally know no better - the same people who think that WD40 is a lubricant)
"A regular car battery is designed to deliver 2000A to turn the starter motor, at cost of capacity."
I'd like to see one of those.
Starter motors are typically a few kW, and the current required is in the hundreds of amps. A car battery might briefly produce a 2kA short circuit current, but it's going to be brief. In fact, in many cars a short will blow the safety fuse built into the positive feed.
Traction batteries as used in fork lift trucks are designed to be cycled and their anode design facilitates this (it's wound, more like an electrolytic capacitor than a typical Pb cell.) They're expensive but they provide about C/5. To get a repeatable 7kWH out of them you're looking at around 600kg, and at C/5 you'll get a bit over 2kW. It's telecoms batteries that are designed to be floated, at quite a low voltage (around 13.7V), to provide emergency standby power. As such they may only discharge very rarely.
To summarise, for domestic use in an offgrid application you're going to need a lot of lead and a lot of sulphuric acid.
Only El Reg is on my Adblock exclude list. So I guess I must have picked it up from El Reg!
I wonder if its because I'm sending Do Not Track, so it just sends completely random ads.
That or it saw the two powerwalls side by side and found an image that had similar features and decided to use that as the ad.
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Tesla just need to add some capacitor into the mix to cope with short term peak loads of kettle / toaster etc
I realise that in the US they are saddled with a domestic electrical system that can't easily supply high power to point loads, but 1.5kW for a kettle as mentioned in the article is very, very rare in the UK and, I suspect, in Europe and most of the rest of the 220V+ world.
A cheap electric kettle here would be 2kW with many available at 2.5kW and a not inconsiderable number at 3kW. Half the time to boil the same amount of water and probably fractionally more efficient because of that.
Example: a £5 kettle rated at 2.2kW.
I'd like to see the capacitor bank that can supply 3kW for 3 minutes :-)
Yes 115V is not much good for boiling water, it does however heat the cable very efficiently!!!
However, it is possible to have your house wired 220V in the US, as I believe that is what some large appliances (washer/dryer/cooker) use, so in principle you could add a "kettle bay" and use a proper European kettle.
Beer, no such problem...
P.
"However, it is possible to have your house wired 220V in the US"
All USA houses are 220V wired. The problem is that the earth is centretapped and most appliances are running from one side of the transformer to earth.
Imbalance currents can be quite an issue.
(As an aside a lot of SE Asian countries use 220/240V with NEMA outlets - which has predictable results when american tourists plug in their 110V equipment. Thankfully universal PSUs are almost "universal" these days.)
@Alan Brown
"You'd need a fair few farads to handle that"
True, but they do exist, and are getting better all the time. For example, Maxwell Technologies makes several modules that could conceivably be used in this application, as high as 600 farad capacity. Inverter electronics would of course be needed. Given super capacitors' high discharge-recharge cycle count, maybe a hybrid supercapacitor-battery system would be best.
Amazing how capacitor technology has advanced -- when I was in high school, we were told that a 1F capacitor would be the size of a railroad car, and now I see this 350F 2.7V capacitor the size of a D-size battery, for $12! Of course, you'd need something like 106 of them to run that 1.5kW kettle for 3 minutes, ignoring inverter losses.
"when I was in high school, we were told that a 1F capacitor would be the size of a railroad car,"
Yep.
"I see this 350F 2.7V capacitor the size of a D-size battery, for $12! "
It is truly amazing, but there is still at least one little problem if you want to use these for real world applications. That voltage (what, other than microprocessor core voltages, runs off 2V these days?).
To get a useful working voltage, it gets more expensive than you might at first expect, but don't panic.
Still doable though. Maxwell Technologies do a range of supercap modules at useful voltages such as 48V (and higher).
165F at 48V will cost you around £1000 (quantity one) (price from Mouser UK).
Then again, no need to panic, the stored energy goes up as the square of the voltage, so at 48V rather than 2.4V you should get four hundred times as much stored energy, which means in energy per dollar terms the higher voltage one is somewhat better value.
Mmmmm. Interesting.
Now, that kettle equation, with working.
1.5kW * 200 seconds (3minutes, rounded) = 300 kJ.
Stored energy = 0.5 * C * V * V
At 2.7V in 350F we see 0.5 * 350 * 2.7 * 2.7 = 1275J or very roughly 1kJ (for $10).
I make that three hundred little supercapacitors providing the energy to run the kettle for three minutes (ignoring theoretical and practical losses). It's not bad is it.
At 48V in 165F we see 0.5 * 165 * 48 * 48 = 190080J or very roughly 200kJ (for £1000).
So a couple of the 48V modules would do the job, and cost less than the 2.7V ones.
It almost starts to look interesting :)
InLog helpfully suggested: "Tesla just need to add some capacitor(s) into the mix to cope with short term peak loads of kettle / toaster etc."
You might want to look up the definition of Farad.
Hint: 1.0 Amp-seconds. Pay attention to voltage rating.
How big is your house?
100A is common for new installations these days, but 80A is common and 60A is very common for houses built before perhaps the mid 1980s. I even saw a couple of 40A "main cutouts" in my electrician-ing days.
There's quite a nice section in "the regs" about maximum demand and diversity. Essentially, while you can quite safely say that it's unlikely that anyone would have two 2kW fan heaters, the Sunday roast, the washing machine, dishwasher, lawnmower and 10kW shower all running at the same time, it's actually possible that some of that will happen.
There are three things that all the coverage seems to have missed. I'm not sure if the first or second apply in the US, but I'm certain the last will.
Firstly note that practically all existing inverter systems (solar PV) will switch off if the grid supply fails, even on the sunniest days when the panels might be generating a couple of kW. This is simply because the regulations governing "off grid" systems are an order of magnitude more difficult to comply with than "feed-in" systems. While Musk's battery pack potentially could act as a UPS for your house, I suspect that the cost of an installation that meets regulations will make the $3,500 purchase price look cheap.
Secondly, this is not a money-saving exercise. Plain fact is that even with today's reduced "feed in" tariffs, I suspect it's much more economically viable to use the grid as your "storage" mechanism rather than keeping it locally.
Lastly, at 2kW continuous output, if the thing does not disconnect in the event of a failure of grid supply you will have to engineer-in some kind of load shedding. The simplest way to do this would be to connect the output of this battery pack to a select number of low-power circuits, probably the lighting and the one that feeds the boiler and heating pumps. Even if you want to take a chance and connect it to one or more sockets circuits, you will definitely have to isolate the system from the circuits providing power to your cooker, your electric shower and any permanent electric heating appliances. Again, re-arranging the distribution in your house in this way will cost.
M.
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"Lastly, at 2kW continuous output, if the thing does not disconnect in the event of a failure of grid supply you will have to engineer-in some kind of load shedding. "
"essential" and "non-essential" circuits have been in use in commercial installations for decades. Applying that sensibility to domestic circuits isn't difficult unless you have stupid-ass ringmain circuits feeding all your wall outlets.
US is typically 200 Amp service rating for new build homes. 100 amp is normal service rating for most older US homes and is minimum code in many locales. 40 amp service went out with the Dodo bird. Code says new service and breakers are required to sell most homes. No more fuses, period.
3kW isn't enough to power most US homes even intermittently. You would struggle with 10kw in most large modern homes.
Regulations are stacked in favor of the utilities and it may even be illegal to go off grid if you have existing grid power available.
You say a 4.4kW limit wasn't a big deal in your flat.
Maybe so - but I wonder how you managed. In the home I grew up in, 4.4kW wasn't enough to power the lights (roughly 2kW if they were mostly on, and they often were with five residents) and kettle (3kW) simultaneously. Then there was 7kW needed to power the immersion heater to provide hot water, and gawd knows how much for the electric cooker.
The washing machine used 3kW when heating water, and we had a 3kW electric radiator in one room.
A limit of 4.4kW total available electrical power really would have been a big deal in that home, wouldn't it?
Having 100 A at 240 V available - i.e., 24,000W - wasn't a case of over-engineering at all, just a case of providing what was needed.
These days, I live with a mains gas supply and improved efficiency lighting everywhere - and a smaller house. My energy bills are a much lower drain on the household budget than my parents had to deal with despite the proliferation of gadgets. But even now, I'm sure a 4.4kW limit would be a problem - my kettle consumes electricity at a nominal 3kW, which doesn't leave much over.
Thought it might be helpful to do some of the maths on the Powerwall. We live in Northern England (not too hot, coldish in winter) and run a couple of electric cars (a PHEV and a pure EV). So we have relatively high electricity consumption 14,000 kWh per year.
Of the daily average (38.4kWh) we reckon roughly 55% (21.1 kWh) is at peak rate, and 45% (17.3 kWh) is at off peak rate. Our utility charges 12.17p/kWh for peak supply and 6.83p/kWh for off peak juice. Hence our total annual electricity bill is made up of £937.09 peak rate power + £430.29 off peak power, for a total of £1,367.38.
In all the following calc, we're looking at just the battery costs, acknowledging that inverter costs would be additional. We also assume the deal is funded from cash on hand, i.e. not taking into account finance costs (which would be pretty minimal anyway given current interest rates).
The first calc we did was to say "how would Powerwall work out if we wanted simply to use it to arbitrage the (substantial) price differential between peak and off peak utility rates?"
Answer: we'd need 21kWh capacity to fully cover the daily peak requirement, so we'd need to buy 3 of the 7kWh units at $7K each, which makes a total of £5,850 in GBP.
We would then use 21 kWh of OFF PEAK (6.8p/kWh) energy to charge up the batteries overnight, plus 17.26 kWh of OFF PEAK energy to run the normal daily off peak load. Thus we would be purchasing a total of 38.26 kWh OFF PEAK energy per day. The 3x7kWh Powerwall units would have completely eliminated the need to purchase PEAK rate electricity from the utility. The peak rate power bill would be ZERO.
Net effect: the annual electricity bill falls by £409.31 to a total of £958.07, versus the £1,367.38 we started with. Payback for a 3x7 Powerwall system would thus be around 14.3 years, if the sole purpose was to exploit the price differential between Peak and Off Peak utility rates.
If it were possible to use two of the 10kWh units instead of the 3 x 7 kWh config discussed above, the capex for the batteries falls to £4,550 and the payback reduces to 11.67 years. However, it is not yet clear that the 10 kWh unit can be used in this way. The Tesla website describes the 7 kWh Powerwall as suited for "Daily Cycling" (which is what we need in this example) and the 10kWh as suited for "Weekly Cycling". If that's just marketing guff, then there'd be nothing to stop us using two of the bigger units instead of three of the smaller ones. However, if the control system and liquid cooling are optimised for the weekly application, then the 10 kWh unit may not be applicable for the daily charge-discharge cycles we need here.
However, and it's a big however, the economics under the UK Government's renewables incentives regime are MUCH more compelling than simply using the batteries to exploit the peak/off peak price gap.
Let us assume we assume we install a 12kW solar array on a dual axis solar tracker. At our latitude, experience (from the exactly similar unit installed on our neighbour's property) indicates that we'd get a MINIMUM of 15,000 kWH energy production per annum.
Given the batteries' ability to store renewably generated power when the sun is not shining (or the wind is not blowing) a system on this scale would effectively eliminate the need for grid energy purchases: so the utilities bill falls to £0.
And then then the Government renewables generation incentive kicks in on top. This subsidy pays end users 14.45 p/kWh for every unit of electricity generated, regardless of whether it is consumed in the home or exported to the grid. There is an additional smaller subsidy paid per kWh actual exported, but we've sized this system to match the on-site power needs, so very little will actually get exported and we've thus ignored this second subsidy for the purposes of this calculation.
On the calculations above, the Government subsidy – which is guaranteed for 20 years to encourage uptake – totals £2,167 per annum. Add that to the £1,367 saving on the utilities bill, then the total benefit of Solar + Powerwall system is £3,534 per year.
Once again assuming the 3 x 7 kWh Powerwall set-up at £5,850 and a (generous) £15,000 for the solar unit, including installation, inverter and integration of the two subsystems, total capex would be £20,850. Payback on these numbers would be 5.9 years. Payback shortens to 5.53 years if the 2 x 10 kWh Powerwall units are used.
Typo, sorry.
Sentence currently reading "Answer: we'd need 21kWh capacity to fully cover the daily peak requirement, so we'd need to buy 3 of the 7kWh units at $7K each, which makes a total of £5,850 in GBP."
SHOULD read "Answer: we'd need 21kWh capacity to fully cover the daily peak requirement, so we'd need to buy 3 of the 7kWh units at $3K each, which makes a total of £5,850 in GBP."
Don't forget installation costs, and the VAT, which adds 20% to all the above.
So what you're saying is that it is uneconomic to use these because they will not pay back within their warranty period, unless you steal from the poor.
The 14.45p/kWh comes directly from everyone else's electricity bills.
Everyone who does not have the system is paying you for all the electricity you use.
Who can install these systems? Those who own their property and have either large enough savings to buy outright, or a good enough credit rating that a bank will loan them the upfront cost. In other words, the well-off.
Who pays for the systems? The poor and lower-middle class.
Isn't that simply evil?
I have just attempted to priced up an equivalent spec'd 7.KWh system using deep cycle Lead/Gel batteries applying :
1. Batteries do not go below 50% of capacity to ensure pre-longed life.
2.Inverter efficiency 90%.
3. Gel filled to enable rapid charging.
4. There is always enough surplus charge off the solar panels to charge.
5. Life expectancy greater than 10 years.
So required capacity (7000 W x 2) / (0.9 x 12V) = 1296Ah which is also 56.Mega joules.
This can be achieved using 11 off 135Ah Gel deep cycle which Victron sell at around £340 each,
so 11 batteries come in at £3700 ( as Victron claim of 12 years design life. )
If one includes an inverter/charger then add another £3500, (Does the Tesla solution include an inverter/charger or just a power management?)
If Tesla sell a 7kWh at £1875 ($3000), then this is quite competitive when compared with an equivalent specified Lead/gel battery set up at £3500, especially as Tesla have a 10 year guarantee vs the average 2-5 years for Lead/gel batteries)?
I do admit these are back of fag packet calcs and am unsure to how makers arrive at Ah ratings, some quote Ah capacity for a 12v battery to the point in time where it only outputs 10.5V. (30% charge remaining) So I am ready to be corrected & learn!
" some quote Ah capacity for a 12v battery to the point in time where it only outputs 10.5V."
Just above that is the point at which my charge controller turns off the battery output - though that threshold can be altered but it's best not to chance it and have the batery fail.
"Batteries do not go below 50% of capacity to ensure pre-longed life."
When I was doing research into lead acid battery backup, I found that at 50% discharge you are typically looking at a life of a few hundred cycles. You might get a year rather than ten years. At 25% discharge you might be getting into multi-year life. You really have to dig into manufacturer data sheets to get this information.
There are two reasons why car batteries are so big: One is to get enough plate area for the starting current, the other is to ensure only a low discharge percentage to give acceptable life. I believe a typical 65AH car battery can be replaced by a lithium battery of no more than 7-8AH.
When I first looked into this I found like you that most deep cycle batteries, that their service life is around 300 to 500 charge cycles. It does appear with batteries, as with most things, you get what you pay for, The 230Ah Victron Gel lead calcium grid batteries are £340 each & almost 1 and a half times the price of the cheapest, it was also significantly larger being 410x176x227mm.
However as you implied, without the data sheet to back it, 12 years Design life could mean any thing, such as only as a designed for a 25% discharge limit from full to achieve 12years life, as you stated.
There are batteries by Trojan (T105 RE) which claim 1000 discharge cycles to 80% from full or 10 year life, at a reasonable £110 (to good to be true?) and industrial quality Solar-One flooded batteries which employ HuP technology which there SO-6-85-21 can deliver 1000Ah, +2100 80% discharge cycles from full for 20 years plus at £2000. So you pays your money & takes your choice!
In all cases, it still makes the Tesla Powerwall look good value if it can live up to the spec/hype!
"1. Batteries do not go below 50% of capacity to ensure pre-longed life."
You have similar restrictions with li-ion batteries. The anodes swell and contract with charge cycles, so the usual rule of thumb is to keep them in the 20-80% range for automotive use (8 year lifespan). For standby power they'd need to be treated even more conservatively in order to get longer life.
Lead Acid traction batteries may not be particularly energy-dense but they have the least demanding characteristics for maintaining endurance over any other electrical storage system.
Unless Elon's managed to pull a rabbit out of a hat (nano engineered anode materials) I can't see how the claimed capacity and endurance can be achieved with more than 10% cycling. The specs shown should be good for riding out brown/blackouts but I can't see these devices being suitable for transforming offpeak power into all-day availability - and it's worth noting that if night-time consumption rises "too much" then the whole notion of "offpeak" goes away anyway.
It would be interesting to know if Tesla have built in extra capacity into its battery array to enable the 80/20 charge cycle to be applied by its charge management system .This would help along with careful cooling towards reaching their claimed 10 years service life.
My first thought about the Powerwall, was it'd be great if it could be fed from solar or wind devices and to then do a simple re-wire of the house lighting circuits, and replace existing bulbs with LED's.
The re-wire wouldn't be expensive, as it just needs disconnecting from the output of the distribution unit and re-wired into the Powerwall output. Likewise, no DC->AC->DC conversion required if the bulbs are all DC to begin with - so no invertors needs.
Likewise, extra DC outlets could be provided to run/recharge DC devices (routers, iPads, mobiles, etc)
OK, so lots of "low energy" bulbs consume little power, but LED's can consume much less, so although you might still be dependant on the grid for high powered AC-mains items, you could possibly save quite a bit of energy cost.
I guess in the end it comes down to the power provided by the Powerwall, vs upfront cost (and some re-wiring, plus new LED bulbs) vs lifetime. Oh, and the cost of the solar panels/wind turbines too. Ah....I can see there might be an issue now :( Oh well.
By 7Kw/h and 10Kw/h I naively assumed this meant it would be able to output up to those levels of power continuously for as long as the batteries lasted. If it can only offer 2Kw then its not going to be of so much use, here in Italy most people have a 3KW maximum before there power is cut so being able to exceed this would have been a good seller, and as a lot of us have PV it could store our power rather than selling it back to the providers for less then we pay them for it.
Dear El Reg - read the specs again - it's 2kW continuous with a "Peak Power: 3.3 kW" read the presskit http://www.teslamotors.com/presskit/teslaenergy
Further it's all very well talking about existing technology potentially being cheaper - but the whole point of getting away from lead acid batteries is because a) you need a dedicated room to put them in and b) they give off hydrogen when charging - leading to a potentially explosive situation.
As for your off-the-grid comments - did you watch the presentation? It's pretty clear he means developing countries could use it to live off the grid - the sorts of places that don't know what an AC unit is or a dishwasher.
Yup, I live in one of those countries, we have no solar, but shore power is normally 8 - 12 hours per day, the battery bank keeps us going during outage, if we need to power washing machines/iron we wait for shore power or start the generator.
The Tesla units would have been an option for us, but we have just replaced the battery bank 16*225Ah, 5 years time though it will be on the list, people in the Europe and other parts of the world, where, if the power goes out it makes the news, need to remember this is not the norm for everyone.
These batteries have cute bezels and presumably a nice Tesla-flavored marketing campaign, but they won't and can't change the world.
In order to take advantage of new forms of power, we must re-evaluate our uses of the old forms. Working from a much lower energy budget, it would be insane to come up with ideas like air-conditioners, hair driers, clothes driers, etc. Working from primarily DC, our home would look more like an RV - small fridge, LED lighting, a couple of laptops.
In addition, there must be a feedback loop- the user and his devices must intelligently communicate with the power source and work from a single energy/economic budget in order to cooperate intelligently in fulfilling the user's goals. For example - knowing the batteries are low, the power system would decline to power the microwave, but would continue to supply the laptop. The PowerWall looks like an attempt at 'greenwashing' a standard home, but magically requires no changes from the user's lifestyle.
This power wall is like all other Tesla projects - some attractive bezels, marketing hype, and fairly standard tech underneath, all with fairly poor systems thinking, such as - how does this product fit into reality?
The problem here is that most people don't know how much power they use, or what the alternative product really costs. Also, look at our target markets.
Lead-acid batteries are terrible - just think of how often you have to replace them in cars. Have you figured more frequent battery replacements in your numbers? Also think about the extra controllers and inverters and stuff. Does the way the powerwall is presented, it looks like it comes with its own electronics and inverters? Better figure that in with your math. Lithium batteries, even at a much higher cost per amp hour are a much better bargain in terms of lifecycle costs. An integrated unit with built-in electronics will be even better.
Someone who wants a single one of these for net-metering and the occasional 2-hour power-out is not expecting this to be a complete solution. He isn't going to run around his house and turn everything on the moment the power goes out. He is going to notice how his neighbors are out and be happy that he has just lights and air conditioning. He will be ok with turning off the TV and waiting till the power comes back on to start baking cookies and running the iron.
Someone who is using this to cut the cord will not look like the average homeowner. They will already have more energy-efficient appliances, and have fewer of them. They will likely have insulated their home really well and put solar panels on the roof and a ground-source heat pump. Their electricity requirements will be much less than the average. These people are willing to spend the premium to go off grid, and in the grand scheme of the super efficient home and solar generation 20 or even 30 grand is not as big and scary as it sounds, especially if it is simply rolled into the house loan. For them it is not a question of batteries or generators - it is a question of which battery and how much of it.
"The problem here is that most people don't know how much power they use, or what the alternative product really costs. Also, look at *our* target markets."
http://forums.theregister.co.uk/user/74820/
Anyway, as per the previous, how the fuck do I get if off the wall and put it in my car?
Oh.. silly me. I'm not 'rich enough to ask'.
I'll get me coat.... again.
I have been in the solar field for 14 years and lived off-grid
very comfortably for the last four years. With a properly designed system
there are few compromises to make. Learn your loads and your limits.
Batteries have been the weak part of the system and L-ion batteries are
head and shoulders better than FLA (flooded lead acid) much easier to
maintain, much deeper depth of discharge before damaging the batteries,
and most importantly, up to 3-5,000 cycle-lives (10+ years).
Use common sense with them. Don't short circuit the terminals, charge and
discharge residential L-ion batteries at a capacity/2 (C over 2 hours) rate or less. Cars
and planes have a cooling system that permits faster rates. Slow rates
are easy with solar charging.
Inverters make AC power from DC and determine the rate at which you can power devices.
Batteries determine the overall capacity of your system in kWh. FLA batteries can be discharged to 50% depth of discharge and remain viable for 3-5 years. Lifetime depends on caring for them, keeping plates covered with electrolyte, using distilled water to refill them, and don't allow ANY batteries to
remain at a low state of charge for long periods of time.
Use LPG, propane, or natural gas for heat applications, furnace, water heater, clothes drier, or cooking as much as possible. Fossil fuels are still more cost effective than electricity for heat. Solar is good for water heating and space heating but needs backup for cloudy days. Living off-grid requires a generator backup for cloudy periods.
Buy the most efficient appliances you can find, like LED lights. Seal windows and doors as tightly as possible. Have as much insulation as is practical. We use 13-17 kWh per day, even with a big-screen TV, two full sets of office equipment, and a booster pump for well water.
The Powerwall will make living without grid power much more practical, and early adopters will begin to erode the nearly 100% market share of traditional central power plants. US utilities complain about >5% penetration of solar as a "non-dispatchable" (intermittent) energy supply while Germany and Denmark already have 30% penetration.
There is plenty to learn.
"Use LPG, propane, or natural gas for heat applications, furnace, water heater, clothes drier, or cooking as much as possible. Fossil fuels are still more cost effective than electricity for heat. "
That will change - and quickly - if CO2 control regulations kick in as hard as I suspect they will in the next decade.
Where I live in the US you don't just plonk some solar panels on the roof, hook them up and go, you have to negotiate with the power utility a small systems generation package. Since solar is so common its a proforma, just a whole lot of words, but hidden in the fine print is things like 'no batteries'. The electricity you generate isn't free, either -- you have to pay standing charges and the like which are typically the utility dumping a share of the infrastructure cost on you. As a giveback we do get total energy tariffs, but what that means in real life is that we swap surplus power generated during the day (which the utility sells to my neighbors at Tier 4 rates) for power used at night (which we'd typically be paying Tier 1 rates for) -- in plain English, they're making 25-30c a Kw/hr on our surplus power. (...but that doesn't stop the utility from grumbling about them....they'd like to see these tariffs disappear....)
So you think "I'll just disconnect from the grid entirely". Enter H&S or its clone -- its actually illegal to do this in some parts of the US.
As far as power use goes, if you're at the solar panels stage then you've probably gone through the house systematically reducing power consumption. You should be using tiny amounts of power compared to a decade ago (and remember that if you have A/C that immediately bounces back on the power draw for cooling -- less power means less heat which means less energy needed to remove it). This is where our naff 110volt plugs help, BTW -- compared to a ring main they're a joke, a maximum of 2Kw a circuit, but now everything's so low power it doesn't matter any more. (So if you're using older appliances, lights and so on -- spend the money on upgrading them, not on a battery....)
"So you think 'I'll just disconnect from the grid entirely'."
Most of those people don't even see the irony of becoming "self sufficient" and "independent" of modern society, with their imported Chinese solar panels and high tech Japanese battery packs.
One hail storm and they'll be down at the local Red Cross begging for a generator.
The $3,500 one is 10 kWh.
At (for example) $0.10 per kWh, that's a dollar. YMMV, feel free to adjust.
If the power that goes is in completely free (e.g. solar surplus), and if the conceptual in-and-out cycle is once per day-and-night, then congratulations, you'll save $1/day or $365 per year.
Personally, I wouldn't even walk across the street for $1 per day. I'm certainly not going to pay $3,500 plus plus plus for the privilege.
Batteries are to energy storage what a yak is to transportation.
The good news is that they "only" need to get twice as good about one or two more times and they'll be simply wonderful.
Having read through, it looks as though you can't (without a lot of expensive safety kit to prevent you frying a power worker who is trying to fix a fault) use this as a whole house UPS. There are similar constraints on powering your normal house wiring from a backup generator. You have to be safely disconnected from the grid before you power it up.
So it seems to be a potential method of buffering the grid by storing off peak electricity and then feeding back into the grid during peak load. Presumably the circuitry would be much like that for solar panels, which disconnect during a power failure. Which can be frustrating if the power is out and the sun is shining.
There has been talk of using the batteries in electric cars to do much the same, although not much has come of it so far. So this could be an option if you don't have or want an electric car, especially if you live in an appartment in a high rise block without a dedicated (electric powered) parking slot. In this case solar power isn't an option either, of course.
So in the UK this is looking (as with solar power in the home) like yet another opportunity for the affluent investor who owns property and can afford to tie up capital over ten years or more to be subsidised by the less fortunate to meet Government green targets.
What is needed in the UK is massive buffering capacity for renewables such as offshore wind farms which currently seem to waste a lot of power (where the grid can even accept their power) and also screw up the economics of running conventional gas generation to cover for when the wind isn't blowing.
Any advance in storage technology is good.
However at the momemt the major benefit seems to be for the developing world where the is no grid or an unreliable one.
This may all change, of course, if supply in the UK exceeds demand due to loss of generating capacity, and we have to restructure our domestic power to cope with rolling brown outs. Again the affluent are likely to cope better than students in bed sits and tenants (especially those on housing benefit) who will suck up the pain and subsidise the rest.
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