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Home Battery Guide: How Batteries Work With Solar
A plain-English guide to how home batteries work with solar, and what to expect if you add one.
In this guide
A home battery stores electricity so you can use it later. Paired with solar panels, it holds the extra power your roof makes during the day and gives it back in the evening, on cloudy days or when the grid goes down.
How a home battery works with solar
A home battery charges from your solar panels whenever they make more power than your home is using, then runs your home when they can't keep up. Without a battery, that surplus flows straight to the grid; with one, the battery fills first and only the overflow is exported.
A typical day:
Daytime: your panels power your home and charge the battery.
Evening: as production falls, the battery covers some or all of your home's needs, depending on how it's programmed.
Outage: if the grid fails, the battery disconnects your home from the grid within seconds and powers your backed-up circuits.
Next morning: the sun recharges the battery, which can help carry you through an outage that lasts more than a day.
Solar panels without a battery shut off during an outage, even on a sunny afternoon. That rule protects utility crews working on the lines, and a battery is what lets your solar keep running while the grid is down.
Why homeowners add a battery
Homeowners usually add a battery for backup power, to use more of their own solar, or both.
Backup power. Severe weather and aging equipment cause outages across the country. A battery keeps your essentials running on its own, even when the power fails at night or while you're away.
Using more of your own solar. Many utilities pay less for the power you send them than they charge for the power you buy, and many charge more during the late-afternoon and evening peak. A battery lets you run your home on stored solar at those times.
In select markets, Sunrun's distributed power plant (DPP) programs may also pay eligible customers for sharing stored energy when demand peaks.
Read more: Benefits of a home battery
What a battery can back up
A battery can back up your whole home or a set of essential circuits, and you choose which during design.
Whole-home backup connects the battery to your entire electric panel. Many Sunrun customers with a single battery are able to have whole-home backup, depending on the size of their panel and the largest appliances on it.
Partial-home backup covers only the circuits you pick, such as the refrigerator, lights, internet and outlets in a few rooms. Heavy loads like a pool pump, an EV charger or a second air conditioner usually stay off the backup.
More batteries add both power (how many things you can run at once) and stored energy (how long you can run them), so larger homes or homes with big appliances may need two or more. Sunrun installs lithium-ion batteries from brands including Tesla and Lunar Energy. Availability depends on where you live, and your advisor matches the equipment and number of batteries to your goals.
How long a battery lasts in an outage
A single battery is designed to provide about 8 to 12 hours of backup for essential loads, and your home could land above or below that. Runtime depends on how full the battery is when the outage starts, which circuits it backs up and what you run while the power is out.
Small, efficient loads barely dent a battery. Anything that heats water, cools air or runs a big motor drains it much faster.
Approximate energy use for common loads (illustrative):
Load | Running time | Approximate energy used |
Refrigerator | 24 hours | 2 kWh |
Five 40-watt light bulbs | 2.5 hours | 0.5 kWh |
50-inch LED TV | 5 hours | 0.75 kWh |
Laptop | 6 hours | 0.5 kWh |
Electric water heater | 1 to 2 hours | 4 to 8 kWh |
Pool pump | 4 hours | 7 to 8 kWh |
The two habits that help most are starting the outage with a full battery and saving heavy appliances for when the sun is back.Read more: Battery duration: how to make your backup last
Battery modes: how your battery decides when to run
Your battery's mode tells it when to use stored energy, and the right one depends on your utility's rates and billing rules. Sunrun batteries typically run in one of three:
Time-based saves stored energy for the hours when grid power costs the most, keeping a 20% reserve for outages.
Self-powered uses your solar first, then the battery, then the grid, also with a 20% reserve.
Backup-only keeps the battery full for outages and sends extra solar to the grid.
In many cases, Sunrun manages your battery's mode for you.
Read more: Battery programming and modes
Batteries and your electric bill
Whether a battery lowers your bill depends on how your utility charges. On time-of-use rates, a battery can cover your home during expensive evening hours with solar you stored earlier. Where the utility pays little for exported solar, as under California's NEM 3.0, keeping that power for your own use generally gets more value from your panels. With full net metering and flat rates, a battery mostly adds backup rather than savings.
Cost depends on the equipment, how much of your home it backs up, the installation and local incentives. You can include a battery in a Sunrun Subscription with $0 to little down, or buy it outright.
Batteries vs. generators
Both can keep the lights on in an outage, but they work differently. A battery starts on its own the moment the grid fails, runs almost silently, produces no exhaust and recharges from your solar panels. A standby generator burns natural gas or propane, needs professional servicing and can run large loads for as long as its fuel lasts.
Already own a generator? You can keep it, but it must stay electrically separate from your solar and battery equipment to keep everyone safe and your warranty intact. Sunrun doesn't install generators, so talk with a licensed electrician about connecting one.
Read more: Home batteries vs. generators
What installation involves
Most battery installations take one to two days. Before install day, we design your system, apply for permits and, if needed, visit to check your electric panel.
Where the battery goes depends on your climate and your home. In mild West Coast climates, batteries usually mount on an outside wall; in colder places like the Northeast, they usually go indoors, often in a garage or utility room. Either way, the spot needs clearance for cooling, service and emergency access, and it has to meet local building and fire codes.
Some homes need electrical work first, such as a panel upgrade or moving a few circuits. We'll flag anything like that during design.
Monitoring, maintenance and warranty
Batteries need very little upkeep. They switch on by themselves in an outage and mostly need clear space around them and the occasional firmware update. You can check your battery's charge and your home's energy use in the Sunrun app, and Tesla Powerwall owners can also use the Tesla app.
Warranty. Every battery Sunrun installs is covered for at least 10 years, through Sunrun or the manufacturer, depending on your location and plan. Manufacturers also guarantee the battery will keep a share of its usable capacity at year 10, typically 60% to 70%. With a Sunrun Subscription, Sunrun covers repair or replacement of your battery for the full length of your agreement.
Lifespan. Most home batteries last about 10 to 15 years, so you may replace one during the life of your solar panels. Extreme heat or cold and frequent deep discharges shorten battery life, which is one reason placement matters.
Are home batteries safe?
Yes, when they're certified and installed correctly. Sunrun only installs lithium-ion batteries that nationally recognized safety laboratories have tested and certified. Each one has a built-in battery management system that watches temperature and charge and is designed to shut the battery down if it detects a problem. Our crews install to local and national electrical and fire codes, and Sunrun monitors your system 24/7.
One limit matters: home batteries aren't designed to power life-support or other lifesaving medical equipment. If someone in your home relies on that kind of equipment, plan a separate backup for it.
Adding a battery to solar you already have
If you already have Sunrun solar, you may be able to add a battery without replacing your panels. Add-on batteries are available in select states and utility areas, and eligibility depends on your equipment and agreement. Check if your home qualifies.
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Home battery questions
No. Home batteries aren't designed to back up life-support or other lifesaving medical equipment. Keep a dedicated backup for that equipment, and ask your utility whether it offers programs for customers who depend on medical devices.
No. The battery detects the outage and takes over your backed-up circuits automatically within seconds. You can watch its charge in the app and turn off heavy appliances to make it last longer.
Often, yes, though it depends on your battery and inverter. Some systems make adding a battery simple; others need more work. If you expect to add an EV or other big loads, tell your advisor so the design leaves room to grow.
Most last about 10 to 15 years. Every battery Sunrun installs is covered for at least 10 years through Sunrun or the manufacturer.
Old batteries need proper recycling, and many manufacturers run recycling programs. Sunrun builds end-of-life handling into its environmental management system, with the goal of decommissioning, redeploying, reselling or recycling used equipment.
Explore more about home batteries
Notes
*Actual savings vary by customer. Recommended battery operation mode varies by home. Some homes are not incentivized to use stored energy except during outages. Backup time depends on battery charge, the circuits backed up and household use. Appliance energy figures are illustrative; your appliances may use more or less. Sunrun's customer agreements contain the terms of all Sunrun warranties, guarantees and maintenance.