How long will my battery system last?
Some batteries will also provide longer backup than others. The 13.5-kilowatt-hour capacity of Tesla’s Powerwall, for instance, outranks Sunrun’s Brightbox at 10 kilowatt-hours. But those systems have the same power rating, at 5 kilowatts, which means they offer the same “maximum load coverage,” according to WoodMac’s director of solar, Ravi Manghani.
“Typically, during a power outage, one wouldn't aim to draw at the maximum 5 kilowatts," a load roughly equivalent to running a clothes dryer, microwave and hair dryer all at once, Manghani said.
“An average homeowner typically will draw 2 kilowatts maximum during an outage, and an average of 750 to 1,000 watts during the course of the outage," he said. "This means a Brightbox will last for 10 to 12 hours, while a Powerwall will last for 12 to 15 hours.”
Certain applications and programs already on the market, such as Sense and Powerley, can also give homeowners an idea of their usage. But in a Catch-22, the apps might require power to function, though data on past power usage could help homeowners identify which appliances to prioritize.
Recent data suggests that many homeowners installing energy storage systems are opting for two batteries instead of one for greater backup capacity.
John Berger, CEO at residential solar and storage company Sunnova, told Greentech Media that the company has seen an influx in demand for storage from existing customers looking to update their systems, as well as new customers asking for batteries from the start. In terms of how long the system can last, however, Berger offers what he called “a rather unsatisfying answer.”
“It depends on how much power your home uses, how big it is, what the weather is in your particular area,” he said. “Some of our customers may be able to have a whole home backup with one or two batteries, and then in other cases that may still not be enough.”