How Long Will a Solar Battery Power My House?

"How long does a solar battery last?" has two answers. One is hours: how long the battery keeps your house running when the power goes out. The other is years: how long the battery itself lasts before it needs replacing.

This article covers both. For the bigger picture, including whether you need a battery at all, see our guide to solar batteries and backup power.

How many hours will a solar battery run your house?

Two numbers on a battery's spec sheet answer this question:

• Capacity, in kilowatt-hours (kWh). How much energy the battery stores. Think of it as the size of the fuel tank.
• Power, in kilowatts (kW). How much the battery can deliver at one moment. Think of it as how wide the fuel line is.

Runtime is capacity divided by what your home draws. Here's simple arithmetic with round numbers, not a promise for any one battery or home:

• A battery holding 10 kWh, running essentials that draw about 1 kW, lasts about 10 hours.
• Trim the load to about 0.5 kW and the same battery lasts about 20 hours.
• Let the load climb to about 2 kW and it lasts about 5 hours.

Your home's draw also changes through the day. Overnight, most houses use far less than at dinnertime, which is why a battery that seems small on paper can often carry essentials until morning.

What drains a battery fastest in an outage?

Anything that makes heat with electricity, or runs a big motor, uses energy quickly. These are the loads that empty a battery in hours instead of a day:

• Electric heat. Baseboard heaters and electric furnaces draw a lot of power for as long as they run.
• Central air conditioning. A large motor that runs for long stretches on hot days.
• Electric dryers, ranges and water heaters. Each one can use more in an hour than your essentials use in several.
• Well pumps starting up. A pump may run only briefly, but every start needs a burst of power. Virginia Cooperative Extension notes that a motor can draw about three times its running current as it starts.

Some loads look big but aren't. A refrigerator cycles on and off to hold its temperature, so it doesn't run all day. The same extension guide suggests estimating its running time as about a third of the hours it's plugged in.

The practical fix is choosing what the battery covers. Most homes with a battery back up a set of essential circuits, not the whole house, so the big loads simply stay off during an outage.

How does solar keep a battery going for days?

A battery on its own runs until it's empty. Paired with solar panels, it gets refilled. During an outage, the system separates from the grid, and each day the sun is up, the panels power your home and send the extra into the battery.

That daily refill is what turns an overnight battery into multi-day backup. How much comes back depends on the weather and the season. A clear June day refills a lot. A short, cloudy December day refills less, and snow on the panels pauses charging until it slides off.

A few habits make the most of each day's sun:

• Run the washer, dishwasher or well pump around midday, while the panels are producing.
• Keep evening and overnight use to the refrigerator, heat, lights and Wi-Fi.
• Check your monitoring app to see what the panels are adding and what the battery has left.

The same app you use to track production every day becomes your fuel gauge in an outage. Our article on solar panel maintenance covers what to watch for in it.

Should you keep a reserve in your battery?

A battery that empties itself every evening isn't much help when a storm hits at night. Many batteries can be set to hold back part of their charge for outages instead of using all of it every day. Ask how much your system will keep in reserve, and whether you can change it.

A reserve matters if you join a battery program. In Massachusetts, ConnectedSolutions pays battery owners to send stored power to the grid during summer events, which run between 3 and 8 p.m. and last up to 3 hours each. Program events draw down the battery on peak afternoons, so ask your installer how the battery is set up during events and how much charge it keeps for backup.

How many years does a solar battery last?

Home batteries are built for years of daily charging, but they don't last forever. Most are lithium-ion, and lithium-ion batteries slowly lose capacity. Research from the National Renewable Energy Laboratory notes that the ways a battery wears out depend on time, use and temperature.

In plain terms, three things age a battery:

• Use. Each full charge and discharge is a cycle, and cycles add up.
• Time. A battery ages even when it sits mostly full and rarely gets used.
• Heat and cold. Extreme temperatures speed up wear, which is one reason installers pay attention to where a battery goes.

An older battery doesn't suddenly stop. It holds a bit less each year, so the same essentials run for a somewhat shorter stretch on one charge.

What does a solar battery warranty promise?

Manufacturer warranties show how long a battery is expected to last. They are usually stated in years, and many add a limit on use, counted in cycles or in total energy delivered. They also promise the battery will still hold a minimum share of its original capacity at the end of the term.

Two common examples show the pattern. One widely installed battery's U.S. warranty promises 70% of its rated capacity after 10 years, with unlimited cycles for solar and backup use but a cap on total energy for other uses. Another manufacturer's warranty runs 15 years or 6,000 cycles, whichever comes first.

When you compare quotes, ask three questions about the battery warranty:

1. How many years does it run, and is there a limit on cycles or total energy?
2. What share of the original capacity does it promise at the end?
3. Does joining a battery program change the coverage?

Batteries installed as part of an Evergreen Solar system are covered by the same 25-year warranty as the system, so the battery and the panels are protected for the same stretch. Every solar package also includes a 25-year power production guarantee. Our original post on how solar panel batteries work covers the basics.

How do you make sure a battery lasts long enough?

Runtime comes down to sizing: deciding which circuits the battery covers, and matching its capacity and power to them. Backing up essentials takes less battery than backing up the whole house, and a home with a well pump or plans for a heat pump or electric car needs more power. Your installer works through that choice with you at the consultation and site visit.

If outages are what worry you, bring a list of what you lost the last time the power went out. Questions? Call Evergreen Solar at (508) 507-2908.

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