Is a solar battery worth it?

Solar generates most of its power in the middle of the day. If you are out or not using much at the time, that surplus goes to the grid for a few pence per unit. In the evening, when you switch the lights, oven and TV on, you then buy power back at the full rate.

A battery breaks that pattern. It stores the daytime surplus and runs the house from it later. The measure that matters here is self-consumption, the share of your own solar you actually use. Without a battery that is around a third. With one it commonly rises to two-thirds or more, depending on how much you use and when.

It helps to put rough numbers on it. Take a home that generates 4,000 units of solar a year, where a unit is one kWh. Without a battery it uses about a third of that, so roughly 1,300 units, and exports the other 2,700. At an import price of around 28p a unit, the self-used solar is worth about £364 a year off the bill. The exported units earn a Smart Export Guarantee payment, the SEG, of perhaps 4p to 15p each.

Add a battery and self-consumption climbs to about two-thirds, so roughly 2,600 of those 4,000 units now run the house instead of being sold. That is an extra 1,300 units a year kept off the bill. At 28p that is about £364 of further saving. Against it you lose the SEG you would have earned on those 1,300 exported units, perhaps £50 to £130 depending on your export rate. So the battery nets you somewhere around £230 to £310 a year on top of the panels.

A battery is a separate cost on top of the panels. As a rough guide, a 5 kWh battery is around £2,000 to £4,000 fitted, and a 10 kWh battery around £4,000 to £7,000. Prices have fallen over recent years as hardware got cheaper and more installers entered the market.

Now run the payback. Take the worked example above, where the battery adds around £230 to £310 a year. A £3,000 battery saving £270 a year pays back in a little over 11 years. A cheaper battery, a wider gap between import and export prices, or heavier evening use shorten that. A pricier battery or a high export tariff lengthen it. That is why a battery typically pays for itself in around 9 to 15 years, usually longer than the panels take.

The panels are the part that does the heavy lifting on your bill. The battery adds to the saving, but it is the slower of the two investments, so it deserves its own look rather than being bundled in as a given.

A battery suits a home with plenty of evening and overnight use and enough daytime surplus to fill it. Families who are out in the day and busy in the evening fit this well, as do homes charging an electric car at night. The more expensive grid power you can replace with stored solar, the faster the battery earns its keep.

It makes least sense if you use little electricity overall, if you already use most of your power during the day, or if your solar array is small and produces little surplus to store. Someone retired or working from home may already self-consume a high share, which leaves less for a battery to capture. In those cases a battery can sit part-charged for much of the year and the numbers do not stack up.

Sorting it by household helps. A working family that is out by day and home by evening is the strongest case, because the daytime surplus the battery stores is exactly what they need at night. A home with an electric car charged overnight is stronger still, since that demand alone can empty a battery most nights. A retired or home-working household sits at the other end. They are in during daylight, so they already use a large share of their solar as it is made, and there is less left over for a battery to hold. A small two-bed flat with a modest array falls in the same camp, simply because the panels make little surplus to begin with.

There is a trade-off with the Smart Export Guarantee, the SEG, which is the payment you get for power you send to the grid. Storing power instead of exporting it means a little less SEG income. For most homes that is a good swap, because the power you avoid buying is worth more than the power you would have sold. On a high export tariff, where the export rate is close to or above the price you pay for power, maximising export can beat storing it.

A battery also opens up time-of-use tariffs. With a smart meter and the right tariff, the battery can charge from the grid when overnight electricity is cheap, then run the house at peak times. That works on top of storing your solar. It only pays if the gap between cheap and peak rates is wide enough to beat the small losses from charging and discharging.

This arbitrage matters most in winter, when there is little solar to store and the battery would otherwise sit idle. A wide gap between an overnight rate and the peak daytime rate lets the battery keep earning year round, charging cheap and discharging dear, rather than only working through the sunny months. A flat-rate tariff, or one with only a few pence between cheap and peak, removes that second job and leaves the battery to earn from solar alone.