How to Use a Solar Backup Battery
Turns out, there’s more than one way to use a Tesla Powerwall or Enphase Ensemble battery.
Battery storage for solar panels offers some obvious, and not so obvious benefits, depending on how you use it. Eric Kilens, senior solar advisor at Granite State Solar, explains the different battery modes.
Battery Backup Mode
“Most people are interested in batteries for battery backup mode,” says Eric. “A customer can use his or her battery for a backup power for primarily their critical loads like their well pump, their heating system, refrigerator, freezer, essential lights, and maybe some outlets to power light electronic equipment.”
Think of it like a generator, with some major upsides. It starts automatically when the power shuts off (it’s so seamless, you might not even notice that there’s been an outage). There’s zero maintenance. And it runs on the power of the sun collected by your panels, so you never have to worry about filling up the tank.
This last one is particularly important because, during an outage, solar panels without a battery won’t work. “With a normal grid-tied system without a battery, when there’s a power outage, the solar array will automatically shut down,” explains Eric. “This is a safety requirement.” But not so if you have a battery. The battery acts as a brain between your home and the grid so your panels can continue to safely produce power.
That alone is a big upside, but now consider what happens if you’re facing a multi-day outage: “So long as the sun’s out, your solar can produce power to recharge your battery,” says Eric. “With a battery system and PV array, you have an infinite source of power. There’s no need to refuel a gasoline or propane tank.”
Another way to use a battery is self-consumption mode. For a grid-tied system without a battery, you buy electricity from the utility when your system isn’t producing power, like at night. Then, during the day, if your panels are making more energy than you can use, that power gets sent to the grid and the utility pays you for it. Ultimately, the two cancel out so you’re left with no electric bill. This system is called net metering.
But, if you have a battery, the extra power during the day is saved so you can use it at night instead of tapping into the grid.
“Here in New Hampshire, the net metering tariff is less than the cost buying power from the grid,” explains Eric, “so it’s always in the customer's best interest to self-consume as much of the power their PV system generates.
Utility Battery Programs
Finally, batteries can help make the grid more resilient through utility battery programs. Eversource, the main utility in New Hampshire, has such programs in Massachusetts and Connecticut and is testing it here as well.
The way it works elsewhere is that stored solar energy can get discharged to the grid during a peak event, or when a lot of people are all pulling a lot of electricity from the grid. (Think: everyone in the state turns on an air conditioner because of a summer heatwave.) Ramping up energy production during these peak times is very expensive for utility companies, so using electricity that already exists is a cost-effective solution. And the battery customer sees a benefit too—to the tune of $1,100 a year on average for discharging during a peak event. (The actual amount depends on how much your battery actually contributes to the grid.)
Plus, the utility isn’t stealing your safety net. “If there’s a potential storm coming, then the utility will cancel that discharge event for that day so they aren’t discharging a customer’s battery and then the power goes out,” says Eric.
Again, such a program doesn’t exist in New Hampshire yet, but it’s something our solar advisors keep a close eye on. Even without this in place, there’s still plenty of value in having a battery.
By Julia Westbrook