Batteries are sold on a simple story: the panels make electricity in the day, you use it at night, the bills fall. The story is true. Whether it is worth the price of the battery is a separate question, and it comes down to four numbers that no installer’s quote works out for your house. Here they are, in the order to find them.
1. How much electricity you use after the sun goes down
A battery only saves money on the electricity it stops you buying. Without one, a solar household uses what it generates while it generates it, and buys the rest in the evening and overnight. The battery’s job is to shift the daytime surplus into that evening gap.
So the first number is the size of the gap: the kWh you use between roughly sunset and sunrise, on an average day. A smart meter’s half-hourly data gives it exactly; failing that, a plug-in energy monitor for a week, or an honest estimate from the appliances you run in the evening. For many households it is 4 to 8 kWh a day in winter and less in summer, when the evenings are light and the heating is off.
A battery larger than the gap is storage you will never fill from the evening side. A battery much smaller than it leaves savings on the table, which is fine, but then the case is weaker.
2. How much you would otherwise export
The battery can only shift what the panels make and the house does not use at the time. That surplus is the second number. On a sunny day a 4 kWp system makes 15 to 20 kWh, and a household that is out at work uses perhaps 3 of them as they are made. The rest is exported, for a few pence a unit, unless a battery catches it.
In midwinter the same system makes 2 or 3 kWh a day and the house uses it all. There is nothing to store. This is the point most quotes skate past: the battery earns its keep from about March to October and does little for the four months when your bills are highest, unless you also charge it from a cheap overnight tariff, which is a different calculation and a different reason to buy.
3. The price gap between buying and exporting
The saving on every stored kWh is the difference between what you pay to import and what you would have been paid to export. At typical prices that gap is substantial, and it is the number the whole case rests on. If you have a tariff with a cheap overnight rate, there is a second gap: charging the battery at the cheap rate and using it at the peak rate, every day of the year, weather or no weather. On some tariffs that is now the stronger reason to have a battery, and it changes the size you want.
Write both gaps down, in pence per kWh, from your actual tariff.
4. What each stored kWh costs you
Now the quote. Take the battery’s price, including installation and any inverter upgrade it needs. Divide it by the usable capacity in kWh (not the nameplate: the usable figure is on the data sheet), multiplied by the number of full cycles the warranty promises, and reduce the result by about ten per cent for the energy lost in charging and discharging. What comes out is the cost of every kWh the battery will ever deliver.
If that cost is below the price gap in number 3, the battery makes money across its life. If it is above, it does not, however it is described in the quote. Most honestly-priced batteries today land close to the line, which is why the first three numbers decide it: a household with a big evening gap, a big summer surplus and a wide price gap does well; a household with low evening use does not.
Things the arithmetic cannot see
Warranty terms. “Ten years” with a cycle or throughput limit is a different promise from ten years flat. Ask which you are getting.
The inverter. Adding a battery to an existing system may need a new hybrid inverter, or a separate battery inverter. Either way the price belongs in number 4.
Power cut backup. Most home batteries do not keep the lights on in a power cut unless a specific, extra piece of equipment is fitted. If that is why you want one, say so, and check the quote includes it.
VAT. Battery storage fitted to a home in Great Britain is zero-rated for VAT at the time of writing, as part of the energy-saving materials relief that runs to 2027. A quote should show no VAT on it. Check that rather than assume it.
What to do next
Find your evening usage, from your smart meter data or a week with a plug-in monitor, and your tariff’s import and export prices. With those two in hand, the battery quote can be judged in ten minutes using number 4. If you would like it done on your actual data, with the battery and the solar priced separately so you can see which part is earning, that is part of every Second Opinion.