Back of a fag packet calc/guess!
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!