I think this will work out...
A 10KWh unit + inverters & charge controller and a budget of $12K should do the trick for us. We can add another later if need be.
When we built our home we were aiming at being off grid, unfortunately, like many things, there was too much project left at the end of the money. What we did manage is to get an 11KW generator (LPG fueled) installed, as well as the all-important generator/cutover controller (a GE unit). The entire home is easily run from the backup generator, including all motor start loads, of which the Air Con is the worst.
When power goes off the generator auto-starts within 30 seconds and power comes on a couple of seconds later when it and the AC stabilize. The cutoff unit 'sees' power from the generator, disconnects the grid side and connects the generator with a loud CLACK! When the grid power comes back on the controller unit slowly synchronizes the two AC signals, reconnects the mains, and shuts down the generator. The electronics in the house are never aware of the switch back to grid power.
The generator has been used for 80+ hours in the last 10 years, saving much in what would have been spoiled food during one particularly long outage, and running the central heat, stove/oven, and hot water heater. The LPG for all of these is stored in a 2000 Litre tank.
To take this system and turn it into what we want we need to mimic the utility company, putting our "mimic" in-between them and us, and electrically moving the generator so it can feed either the charging system OR replace the mimic should the need arise. The utility company OR the backup generator could be used to "top-up" the batteries, which one gets chosen for the task would depend on the time of day or if the grid is even alive at the time (it has been known to go off-line). If we then add a few solar panels to the battery maintenance mix the usage of the grid would begin to decrease.
As of this writing our electrical bills are $220/month, ten years ago they were $100/month. In one year we spend $2600+ on electricity. The house has CFC lights, LPG Hot water heater, LPG oven & stove-top, and an LPG clothes dryer. There are four desktop PCs running 24/7, 2 of which are servers, all have their own UPS so they don't go dark during the 35 seconds or so that it takes the generator to start powering the house. Electrical outlets to the entertainment stack are switched, allowing us to power off those parasitic loads when not in use.
A budget of $12K should do it. If we borrow the money @ 5% over 5 years it results in a bi-weekly payment of $104.41 (annual payment of $2714.66), replacing an existing $220 monthly payment (annual payment $2640) . We pay 74.60 more per year for the next 5 years, after that we earn $2640 a year. During the term of the loan and beyond the cost of electricity will continue to increase, so these are conservative numbers.