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For reference, the price for fixed-cost plans is around 10c/kWh.
As someone who’s been constantly running an electric heater in the garage while painting my car, I was quite lucky with the timing.
It’s not literally free, though. Transfer prices are fixed, and there are taxes and some other minor costs associated with it, so where I live, it still adds up to around 6c/kWh even when the price drops to zero. The cheap prices are due to an excess of wind power, but once the wind dies down, prices usually spike hard.
This is actually an area that's developing quite quickly. In 2023, California managed to put almost 14mw worth of storage on the grid; if they keep building out at that rate, peaky/transient power sources like wind and solar will have someplace to park until someone needs that energy. Almost 12mw of that was utility storage; it's like the utilities have the chance to get out of the business of producing power themselves and into the role of renting storage (or buying surplus energy then selling it later when it's needed)
Granted, 14mw isn't a lot in the scale of California, but the rate of growth in grid-storage over time is humongous
10gwh is last report I have of CA utility battery storage.