Using Batteries for Backup

Recently, a high-profile supplier of battery packs to the solar industry announced that it was discontinuing its plans for a 10kWh battery backup. It’s a good time to visit the question of what backup is and whether batteries are the way to best meet a homeowner’s backup needs.

Are Backup Batteries a Significant Solution for Power Outages? 

Many consumers are concerned about power outages. But are batteries the right way to deal with that concern? Consider that Eaton’s 2015 Blackout Tracker report found that of the more than 3,500 outages in the U.S., the average duration was only 49 minutes. The Electric Power Research Institute has also estimated that 57% of U.S. power outages last less than five minutes. So even if blackouts aren’t particularly long, they are concerning, and homeowners are interested in being more prepared. They are also showing interest in using batteries to control how they consume and manage energy in their homes. The withdrawal of the 10kWh backup-only battery pack before it was introduced to the market shows that homeowners may be getting wise to the limitations of batteries as a strategy for dealing with long-term outages. With the increasing interest in storage systems connected to solar, the question comes down to whether a storage system can do both well.

For a battery to serve well as a backup device for an outage of a day or more, the battery must be either large enough to have at least a day’s worth of power, or (more likely) the homeowner should have their home rewired with a sub-panel for the critical loads. The U.S. Energy Information Administration says that the average U.S. household consumes nearly 30kWh a day, and a battery system large enough to provide that is both very expensive and bulky. A critical loads panel cuts that size somewhat, but even with a smaller storage system, the outage would need to take place at a time that the battery happened to be fully charged. And if the outage is due to a weather condition, you wouldn’t be able to count on solar the next day to recharge the battery if the grid is still down.

A battery that is designed to help a homeowner consume the power their solar produces every day, manage their power costs in an area with time of use rates, abide by local limitations on feeding power back into the grid, or react to demand response requests should be sized based on the customer’s daily cycle of solar production and energy usage. Chances are, a system providing those benefits to an average-sized home would be around 4-6kWh, based on some homes that Enphase has modeled. If that storage system had to do double duty and offer backup as well, it would need to be larger by at least a day’s worth of critical loads, increasing the size significantly.

Only 5% of Americans experience an outage of any length in a year, but for those who want to play it safe, the most cost-effective approach today may be to use a gas-powered generator for extended outages and keep their storage system focused on what it does best: enabling self-consumption and managing the cost of energy.

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