How Solar Batteries Work
A home solar battery does two jobs: it keeps your essential circuits running when the grid goes down, and it stores the solar your panels make at midday so your home can use it in the evening. Which job matters more depends on where you live and what you need to keep on.
How a solar battery works, step by step
- Your panels make DC power. Sunlight on the panels produces direct current.
- An inverter turns it into AC. Your home runs on alternating current, so the inverter converts the panels’ output before it reaches your electrical panel.
- Your home uses solar first. Whatever your home is drawing at that moment comes from the panels.
- Extra power charges the battery. When the panels make more than you use, the surplus fills the battery before anything goes to the grid.
- The battery covers the evening. After sunset, your home draws from the battery instead of buying power from the utility.
- When the grid fails, a gateway disconnects your home. A transfer switch or backup gateway separates your house from the grid in a fraction of a second, and the battery and panels keep your backed-up circuits running.
That last step is the one people miss. Solar panels on their own shut off during an outage. Grid-tied inverters are required to stop sending power when the grid is down so line crews can work safely. Without a battery and a backup gateway, a home with solar is dark in a blackout like every other home on the street.
AC-coupled vs. DC-coupled batteries
There are two ways to wire a battery into a solar system, and both can provide backup when they are installed with the right equipment.
- AC-coupled batteries have their own inverter and connect on the AC side of your home. They are the usual choice for adding a battery to an existing solar system.
- DC-coupled batteries share one hybrid inverter with the panels and store the power before it is converted. They lose a little less energy along the way and are common on new installs.
For most homeowners the difference is invisible. What matters more is how much the battery stores, how much it can power at once, and which circuits are backed up.
Essential circuits or the whole home
A typical home battery stores somewhere around 10 to 15 kilowatt-hours and can supply roughly 5 to 11 kilowatts at once. That is plenty for a refrigerator, a well or sump pump, a furnace blower, lights, Wi-Fi and medical equipment. It is not enough to run central air, an electric range and an electric water heater all at the same time.
So most homes back up an essential-loads panel: a short list of circuits that keep you safe, fed and dry. Homes that want everything on usually need two or more batteries. The right answer comes from your own list, not a rule of thumb.
How long will a solar battery last in an outage?
Add up what you would keep on. These are typical daily figures for common essentials:
| What you keep on | Typical energy per day | Notes |
|---|---|---|
| Refrigerator | 1–2 kWh | Runs on and off all day |
| Well pump | 1–3 kWh | Needs a big burst of power to start |
| Sump pump | 0.5–2 kWh | More in a storm, when outages happen |
| Gas furnace blower | 3–6 kWh in winter | The furnace needs power to run |
| Lights, Wi-Fi, phones, TV | 1–3 kWh | LED lighting uses little |
| Medical equipment (CPAP, oxygen) | 0.5–3 kWh | Check your device’s rating |
| Central air or electric heat | 15–40+ kWh | Usually needs more than one battery |
A home keeping the refrigerator, sump pump, furnace blower, lights and Wi-Fi on usually needs about 8 to 12 kilowatt-hours a day. One battery of that size covers about a day, and on a sunny day the panels recharge it, so a multi-day outage becomes manageable instead of a countdown. Try the battery backup calculator to pick your own loads and see the hours.
Battery vs. generator
A standby generator runs whatever its fuel allows, but it needs fuel deliveries, yearly service and a weekly exercise run, and it makes noise. A battery is silent, needs no fuel and recharges from your own roof, but it holds a fixed amount of energy. Many homes that already have a generator add a battery so the generator rarely has to start. Solar + battery vs. a generator →
What a battery is worth in your state
- Pennsylvania: net metering credits exports at the full retail rate, so a battery is mainly about backup power. With winter storms and rural well pumps, that is the reason most Pennsylvania homeowners add one. Battery backup in Pennsylvania →
- New York: NYSERDA offers an upfront incentive for home batteries, and some utilities pay battery owners for discharging during summer peak events. New York battery programs →
- Ohio: exports are credited at the generation rate, not the full retail rate, so storing midday solar and using it yourself in the evening is worth more than sending it to the grid. A battery earns more here than in a full-retail state. Battery backup in Ohio →
- Massachusetts: utilities run a program that pays battery owners to discharge during peak demand events, on top of backup value. Battery backup in Massachusetts →
How long do home batteries last?
Lithium-ion home batteries are typically warrantied for about 10 years, with a guarantee that they will still hold around 70% of their original capacity at the end of the warranty. A battery used daily to shift solar into the evening ages faster than one that sits full and waits for outages. Batteries lose a little capacity in extreme cold, which is why they are usually installed in a garage or basement rather than outdoors in northern states.
Can you add a battery with $0 down?
Yes. A battery can be part of a $0-down lease or PPA or a purchase, and it can be added to an existing solar system later. Battery cost depends on capacity and how your home is wired, so it is quoted from your own list of essential loads rather than a sticker price. Get a quote with a battery →
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Solar Battery Questions
Do solar panels work during a power outage without a battery?
No. Grid-tied solar inverters shut off when the grid goes down so line crews can work safely. To keep power on in an outage you need a battery and a backup gateway or transfer switch.
How long will a solar battery power my house?
It depends on what you keep on. A home running the refrigerator, sump pump, furnace blower, lights and Wi-Fi typically uses 8 to 12 kWh a day, so one battery of that size covers about a day, and the panels recharge it when the sun is out.
Can a solar battery run my well pump?
Usually, yes. Well pumps need a large burst of power to start, so the battery and inverter have to be sized for that surge. It is one of the most common reasons rural homeowners add a battery.
Can a battery run central air?
Sometimes, but central air uses a lot of energy and often needs more than one battery. Most homes back up essentials and leave central air off during an outage.
Do batteries work in cold weather?
Yes. Lithium-ion batteries lose some capacity in extreme cold, which is why they are usually installed in a garage or basement in northern states.
Can I add a battery to the solar I already have?
Yes. An AC-coupled battery can be added to most existing solar systems without replacing the panels or inverter.