I am not sure that the math is quite fair.I'm getting efficiency of only around 80%.
I have 2 Powerwalls. No solar. They discharge during peak and charge at night. Two batteries are more than enough to meet home energy needs during peak.
I'm calculating it by looking at the monthly data on the Tesla iOS app. I can see how much I drew from the grid and how much Powerwalls discharged in a given month. The difference is the loss, so for example, if loss is 20kWh and PW discharge is 100kWh, efficiency = 100 / (100+20) = 83.3%.
Am I calculating the efficiency correctly? If so, why is it so low? (you can ignore January b/c that's when I got them installed, so Powerwalls had to be charged first)
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efficiency= home use (B)/net grid use (E), I.e. what the fraction of the power used is relative to how much power pulled from the grid. That brings January up to 87%, and February up to 93%.
All the best,
BG