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.”

What are RV Lithium Batteries?

Simply put, RV Lithium Batteries are rechargeable 12-volt batteries that have become a popular replacement for lead-acid batteries. This is particularly true of folks who have solar power on their rigs.

RV lithium batteries are based on a newer, more efficient lithium-ion technology known as LiFePO4 or lithium iron phosphate. For the purposes of this post, whenever we talk about “lithium” we’re referring to this specific technology.

Aside from the technology on the inside, the difference between lithium batteries and lead-acid batteries essentially boils down to the efficiency of use and lifespan.

Lithium batteries can be fully discharged without damage. But once a lead-acid battery is discharged below 50 percent, it suffers permanent damage and will no longer recharge to its full capacity. Because lithium batteries can safely be fully discharged without damage (so their rated capacity is fully usable) a lithium battery provides much more usable amp hours than a lead-acid battery rated at the same capacity… about double!