What do solar panels and batteries do in a power outage?

In a power outage, solar panels alone shut off. Panels tied to the grid are designed to switch off when grid power cuts out, for safety. That keeps power off the lines while crews fix them. A home battery can keep some of your power on. How much, and for how long, depends on the battery, how it is set up, and what you run.

Why do solar panels shut off when the grid goes down?

The U.S. Department of Energy (DOE) says home solar systems generally depend on the grid to make power. It also says they are designed to switch off when grid power cuts out.

The panels make direct current, or DC. A box called an inverter turns the DC power panels make into the AC power the grid uses. AC is alternating current. DOE says the traditional kind, a grid-following inverter, needs an outside signal from the grid to work. When the grid goes away, that signal is gone.

The safety reason is the people on the lines. When the grid is down, crews work on the lines to bring it back. They expect those wires to have no power. Power flowing back from a home could reach them. DOE warns that power from an islanded system could injure workers who think the lines are disconnected from power. For that reason, it says, many solar systems are set up to sense this and disconnect from the grid.

DOE calls this islanding: a small part of the grid keeps running on local power while the rest is down. A system that islands by accident is the danger. A system built to island on purpose, cut off from the lines outside, is how a battery keeps some power on in a home.

Not every flicker shuts the panels off. DOE says a small home inverter rides through small disruptions, and shuts down if one lasts a long time or is larger than normal. So a quick blink may pass. A real outage will not.

A sunny day does not change any of this. With panels alone, your home loses power along with the rest of your street.

What does a home battery change?

DOE says that for solar panels to make power on their own, they need two things: a properly configured inverter and a storage system. A storage system here means a battery.

With both in place, the system can run apart from the grid. DOE describes home solar with battery storage that detects the loss of grid power and switches to islanded mode to keep the power on. Islanded means your home runs on its own power, cut off from the lines outside.

Not every battery setup does this. DOE says solar with a battery depends on advanced inverters to run without the grid in an outage, and only if the system is designed to do so. A system that isn't designed for it won't keep your power on, even with a battery. Ask before you assume.

When the system is built for it, solar with storage can keep delivering power in a long outage, even at night. The panels make power by day. The battery holds some of it for later.

DOE's homeowner guide puts the use plainly. Storage lets you use it later, like at night, or even when the power goes out.

What can a battery keep on, and for how long?

This page gives no number of hours, because the answer depends on your battery and your home. Two sizes on the battery matter most.

• Energy capacity is the total amount of energy it can store, usually in kilowatt-hours. A kilowatt-hour (kWh) is an amount of energy. Think of this as the size of the tank.
• Power capacity is the amount of energy it can release at a given time, usually in kilowatts. A kilowatt (kW) is a unit of power. Think of this as how far the tap opens.

The two work together. Energy capacity is how much the battery holds. Power capacity is the most it can hand out at one moment. A battery with plenty stored still can't run more at once than its power capacity allows.

What you run is the other half. Every light, fridge, pump or fan you leave on draws from the battery. The more you run at once, and the longer you run it, the faster the stored energy goes. Two homes with the same battery can get very different results in the same outage.

Some energy is lost along the way, too. DOE says storing energy is never 100% efficient. Some is always lost when power goes into a battery and comes back out.

How a battery is wired matters as much as its size. Ask which circuits or rooms it is wired to run in an outage.

If someone gives you a number of hours, ask what it assumes. How full is the battery at the start? What is running? Are the panels making power, or is it night? A number with no answers behind it tells you very little.

Does a battery take your home off the grid?

No. A battery helps while the grid is down. It does not end your tie to the utility. DOE says that when service comes back, backup systems reconnect to the grid.

Illinois Shines rules say any marketing about your utility must explain that you stay a utility customer, responsible for a utility bill. That stays true with panels, and it stays true with a battery. Read why you still get a utility bill with solar.

On a normal day, your panels still work with the grid. In Illinois Shines words, net metering is a credit on your bill for the power your panels send to the grid. See how net metering works for new solar in Ameren Illinois territory.

Where do battery costs have to be listed?

Illinois Shines is a state program that provides an incentive for solar projects. If your offer comes through it, you will see a Disclosure Form. You sign the Disclosure Form before the contract.

See also: how Illinois Shines works.

If a battery is part of the offer, the Illinois Shines consumer protection rules put its costs on that form. You should see battery costs and fees on the Disclosure Form.

Look for the battery on the form before you sign anything. Check that what it says about the battery matches what you were told. Then check it against the contract. If the battery is missing from the form, ask why before you go further.

If you sign in person, the agent must go through the whole form with you and take your questions. That is a good time to ask about the battery line by line.

See also: the Disclosure Form and your right to cancel.

This site makes no claim about what a battery will do to your power bill. Read the numbers on the form, and ask about anything you don't follow.

What should you ask an installer about backup?

DOE's homeowner guide says to ask your solar installer if they offer battery storage options. Go further than a yes or a no. These questions get at what a battery will do in your home.

• Will this system keep any power on in an outage? Or only if I add a battery?
• Is the inverter set up to run my home without the grid?
• Which circuits or rooms does the battery cover? Which ones go dark?
• How much energy does the battery store, in kWh? How much power can it give at once, in kW?
• Can it run the things I care about most at the same time, like the fridge, a sump pump or the furnace fan?
• Do the panels charge the battery during an outage?
• If you quote me a number of hours, what is running in that number, and how full is the battery at the start?
• Who owns the battery? Who services it, and what happens to it if I sell the home?
• Where are the battery's costs and fees on my Disclosure Form?
• What happens when grid power comes back?

See also: PPA, lease or buy, selling a home with solar, how to check a solar company.

Write down the answers. Compare them with the Disclosure Form and the contract before you sign.

Quick answers

Do solar panels work in a power outage?

Not on their own. Panels tied to the grid are designed to switch off when grid power cuts out. A battery and the right inverter let a system keep some power on.

Why do solar panels shut off when the grid goes down?

For safety. Power sent onto the lines during an outage could hurt workers who think the lines are dead.

How long will a home battery keep my power on?

It depends on how much the battery stores, how much power it can give at once, and what you run. Ask how any number of hours was worked out.

Does a battery take my home off the grid?

No. You stay a utility customer, responsible for a utility bill. A backup system reconnects to the grid when service comes back.

Where do battery costs have to be listed?

On the Illinois Shines Disclosure Form. If the offer includes a battery, its costs and fees must be there.