5.11 With a battery, arbitrage time-of-use rates (SDG&E EV-TOU)

If your roof can't take solar or your budget is tight, there's still a way to cut the bill a lot: install a battery on its own and arbitrage a time-of-use rate. Take SDG&E's EV-TOU: the off-peak low is under $0.13/kWh, the evening peak as high as about $0.80 — a spread of more than $0.60. The play is simple: charge the battery full at the $0.13 off-peak, then draw from the battery the rest of the time. For one of our customers, modeling their past year of 15-minute usage data, a battery paid back in about 6 years at roughly a 17% annual return; shift usage off the peak deliberately and payback is even shorter — beating most passive investments.

If you already have solar + a battery, check one thing: is your rate plan EV-TOU-5? With a battery but still on the default plan, once the battery runs out overnight you're buying power at $0.30+, potentially throwing away hundreds or over a thousand dollars a year. On EV-TOU the battery discharges at the evening peak to offset the $0.80, and when it runs out overnight you buy the cheap $0.13 power (or on a cloudy day fill the battery at $0.13 to use at the $0.80 window — a 6× spread arbitrage). Switching plans is free; if you don't know how to do it in the utility portal, we'll switch it for you at no charge — customer or not.