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California Duck Curve. Too much renewables during day, not enough at night

California Duck Power curve

About 6% of San Diego CA power comes from rooftop solar. Even this relatively small amount of solar power creates serious problems on the grid. Once solar hits 15%, the problems get more pronounced. This is not just blather from utilities resisting change. When it gets dark, utilities have to ramp up other types of power quickly, something which not trivial. The Duck Curve for California for 2015 is moderate, as shown in the image. As more rooftop solar is installed, the curve becomes more pronounced.

More solar is produced during the day when people aren’t home. This production vanishes when they come home, as night falls, precisely when more power is needed. Also, the glut of production during the day can cause prices to be actually less than zero as well as creating grid problems, since the grid must always be in perfect balance between supply and demand.

As in most locales, solar output in California peaks in the naturally sunny middle of the day. But lots of people are at work and there isn’t much demand on the system. SDG&E’s system peaks closer in the evening, when people get home from work and the sun goes down. The drop in solar production almost perfectly coincides with the utility’s daily ramp up to peak demand.

The San Diego utility says solar rooftop users only pay about half of what their solar costs the company. This is probably at least partly true, made worse by a convoluted state-mandated rate structure. Time-of-use rates probably make more sense. Users would pay more for power on a baking hot summer afternoon than on a moderate Spring night. The ability to store power for a few hours would help enormously, however doing so is expensive. Other solutions though, are available now.

Avery imagines a world where your house pre-cools during the off-peak, where your electric water heater delivers demand response during the peak, and your electric vehicle charges based on price signals.

“All these things today are reality. They can happen right now,” he said.

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