Solar battery backup: how solar keeps working in a power cut

Why solar switches off in a power cut

A grid-tied solar inverter only runs when it can sense the grid. The moment the grid fails, it shuts down within a fraction of a second. This is called anti-islanding, and it’s a safety rule: if your panels kept pushing power into the wires, a line worker repairing them could be electrocuted.

The inverter can’t tell the difference between the street and your house. Without extra equipment, it switches off entirely, so the panels on your roof sit idle while the lights are out.

What a battery adds

A backup battery system comes with a device, usually called a gateway or transfer switch, that disconnects your home from the grid when the power fails. With the house isolated, the battery forms a small local grid of its own, and the solar inverter can sense that and start producing again.

From then on, the system runs as a little island:

• in daylight, the panels power the house and top up the battery,
• in the evening and overnight, the battery takes over,
• the next morning, the panels start refilling it.

In good weather, with a battery sized for your essentials, that cycle can carry on for as long as the outage lasts. That’s the big advantage over a battery on its own, which runs down and then waits for the grid.

The exception: solar backup without a battery

Enphase’s IQ8 microinverters can form a local grid using only sunlight, which Enphase calls Sunlight Backup. They need a separate system controller to disconnect the house from the grid. There are real limits: it only works while the sun is out, nothing runs at night, and if your loads exceed what the panels are producing at that moment, the system trips off until it can restart.

It’s a useful feature, but it’s daytime-only backup. For evenings and overnight, you still need a battery.

AC-coupled or DC-coupled?

There are two ways to connect a battery to solar.

AC-coupled. The battery has its own inverter and connects to your home’s AC wiring alongside the solar inverter. It’s the usual choice for adding a battery to an existing system, because it works with most solar inverters. The trade-off is efficiency: power from the panels is converted to AC, then back to DC to charge the battery, then to AC again. AC-coupled batteries such as the Enphase IQ Battery 5P and FranklinWH aPower 2 are rated at about 89% round-trip efficiency.

DC-coupled. Panels and battery share one hybrid inverter, and solar power can charge the battery directly as DC. Fewer conversions mean slightly less loss. It’s the natural choice when installing solar and storage together. Tesla’s Powerwall 3, for example, has a solar inverter built in.

For backup, both work. The choice usually comes down to whether you already have solar and what your installer supports.

Sizing solar and battery for backup

Think of the two parts separately.

The battery carries you through the night. Size it for your essential overnight use plus a margin for cloudy days. For a fridge, lights, internet and a furnace blower, that’s typically 10 to 15 kWh.

The panels refill the battery and run the house in daylight. A typical home solar system makes far more on a sunny day than essentials need, so in summer the battery is the limit, not the panels. In winter, short days and snow can turn that around.

The power rating decides what can run at once. A battery’s continuous output (11.5 kW for a Powerwall 3, for example) caps how many big appliances can run together, however much sun there is. Air conditioning and electric heat are the usual problems. Our guide to whole-home or essentials-only backup goes through this.

Adding a battery to solar you already have

Most homes with solar can add an AC-coupled battery without replacing anything. Before you get quotes, check:

• Your export deal. A battery pays off fastest where exported solar earns much less than imported electricity costs. In the UK, for example, the price cap unit rate was 26.32p per kWh for October to December 2026, and many export tariffs pay a fraction of that.
• Your inverter. Older inverters may not be able to restart with a battery during an outage. Ask whether your panels will produce during an outage, not just whether the battery will run.
• Warranties. Make sure adding a battery doesn’t affect the warranty on your existing solar equipment.
• Space and placement. Batteries usually go on a garage wall or outside, near the main panel.

What it costs in 2026

US: EnergySage puts the average solar system at about $2.58 per watt before incentives in 2026, and a typical installed Powerwall 3 at about $13,500. The 30% federal tax credit for solar and batteries ended for systems placed in service after 2025, so 2026 buyers pay the full price unless a state or utility program helps.

UK: Solar panels and batteries are zero-rated for VAT until 31 March 2027. The government’s median price for installed batteries of 6 to 11 kWh was £890 per kWh in 2025/26.

For a full breakdown of battery prices and the costs that don’t appear on quotes, see what solar battery storage really costs. If you don’t have solar and aren’t planning it, read battery backup without solar instead.