drees wrote:I still question the low capacity of each unit. 2-3 kWh per unit would be much more usable and should reduce overall costs. I can't imagine anyone wanting less than 5-6 kWh. Given the advances in battery tech the last two years they should be doing better.
While I agree that almost no one needs just 1.2 kWh, I think there are two issues which are more important than the absolute size of battery: durability of the battery and energy-to-power ratio. Let's look at both:
It seems clear that Enphase has chosen a more durable battery than Tesla and this fact is reflected in their respective warranties. The end result is that the lifetime costs of the AC Battery are on the order of 1/2 that of the Powerwall.
This ratio is important in the following ways:
1) It determines the maximum rate at which the battery can be charged or discharged. That rate is equal to C/Energy-to-Power Ratio.
2) It determines the minimum amount of storage you must purchase in order to get a particular amount of power capbility.
3) Alternatively, it determines the minimum power level that you must purchase to get a given quantity of storage.
So let's compare the two products:
Tesla Powerwall: 7 kWh/2 kW = 3.5 hours
Enphase AC Battery: 1.2 kWh/0.27 kW = 4.4 hours
Basically, both companies have chosen about the same ratio. While I haven't done the analysis myself, my guess is that this ratio has been determined to be the best compromise for the initial target market.
In Enphase' case, they have limited flexibility when it comes to the power of the inverter, as it appears their goal is to maximize their use of their standard inverter technology. Perhaps they have decided that 1.2 kWh is the largest battery that they can reasonably sell with a 270-W inverter.
I also think Enphase designed their product in order to ensure the weight was not beyond a particular value. The ease of single-person installation is a very attractive aspect of the Enphase product.
As time marches on, I imaging that both products will become available with both higher or lower Energy-to-Power Ratios.