An enormous Tesla-installed power storage facility is up and running at Southern California Edison’s Mira Loma substation in Ontario, California. The facility, announced last fall, holds enough energy to power 15,000 homes for four hours — 80 megawatt hours of electricity with a peak output of 20 megawatts.

It’s designed to reduce the need for “peaker plants” — electricity generation facilities that run when electricity demands are particularly high, such as on a hot afternoon when air conditioners are running full tilt. They’re expensive to install and maintain, especially when in some areas they might only be used for a few hours a day — or even a year.

The 396 Powerpacks that Tesla installed (it acted as its own general contractor) for SCE were all assembled at the company’s Gigafactory in Nevada. Though 80 megawatt hours of batteries might seem like a lot, it’s the same amount of battery capacity that the company puts into its cars in just three or four days of production.

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