Energy Storage System (ESS)
An energy storage system, or ESS, is a stationary home battery that stores electricity for later, usually charged by rooftop solar or cheap off-peak power. Building code treats it as fire equipment rather than an appliance, so it has to be listed to UL 9540 and it can only go in certain places.
A home energy storage system is a battery cabinet rated in kilowatt-hours, wired between your solar array, your panel, and the utility. A Tesla Powerwall or an Enphase IQ Battery is the version most Arizona owners know. It charges while the sun is up or rates are low, then carries the house through an outage or the pricey hours of a time-of-use plan.
Section R328 of the International Residential Code governs the install. It is new in the 2021 edition. The battery has to be listed and labeled to UL 9540. UL caps a single unit at 20 kilowatt-hours. Indoors, in a utility closet, basement, or storage space, the total is 40 kilowatt-hours. In a garage, on an exterior wall, or outdoors, the total is 80. Units sit at least 3 feet apart, unless a UL 9540A fire test documents a shorter distance.
Where it can go is what shapes the design. R328.4 allows four spots. Detached garages and accessory structures. Attached garages separated from the living space. An exterior wall or outdoor spot, at least 3 feet from any door or window into the house. And enclosed utility, storage, or basement rooms with finished or noncombustible walls, where unfinished wood framing gets 5/8 inch Type X gypsum board. Then a hard line: an ESS shall not go in sleeping rooms, or in closets or spaces opening directly into sleeping rooms.
For example, a Fountain Hills home pairs two 13.5 kilowatt-hour batteries with rooftop solar, almost always on a garage wall. R328.4 keeps them out of the living space, and a garage clears both caps. Pick that wall carefully. Every battery publishes an operating temperature range, and an unshaded west wall in Phoenix is the hottest surface on the lot in July. One local wrinkle: R328 did not exist in the 2018 IRC. If your AHJ still enforces that edition, the electrical code, NFPA 855, and the maker's instructions govern instead.
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