Guide: batteries
Is a home battery worth it?
A battery does not make electricity. It moves it from a cheap hour to an expensive one, or from your roof at midday to your kettle at seven. Whether that is worth several thousand pounds depends on a price gap, a quantity and a lifespan, and all three are checkable.
Last reviewed: September 2026
The whole decision in one line
A battery earns money equal to the number of units it shifts each year multiplied by the price gap between the units it displaces and the units it stores, less round-trip losses. Divide the installed cost by that annual figure and you have a payback. Everything else in a battery sales conversation is detail on top of that sentence.
The two ways it earns are shifting cheap off-peak grid electricity into peak hours, and storing your own solar surplus rather than exporting it at a low rate. Many households do both, which is why a battery often makes more sense alongside solar than on its own.
Usable capacity is not nominal capacity
A battery advertised at a given kWh figure will have a usable capacity below that, because manufacturers reserve headroom to protect cell life. Ask for usable capacity in writing and do the arithmetic with that number.
Two more specifications matter as much as capacity. Continuous power output, in kW, decides whether the battery can actually cover your evening peak or whether you keep importing anyway when the oven and the kettle are on. Round-trip efficiency decides how much you lose on every cycle: you pay to store more units than you get back.
The specifications to ask for
- Usable capacity
- The kWh you can actually cycle, not the nameplate figure
- Continuous power
- The kW it can deliver, which decides whether it covers your peak
- Round-trip efficiency
- The share of stored energy you get back after losses
- Warranty terms
- Years, throughput or cycles, and the retained capacity guaranteed at the end
Cycles and warranties decide the lifetime
Battery warranties are usually expressed as a number of years and either a cycle count or a total throughput in MWh, with a guaranteed retained capacity at the end. A battery cycled hard every day reaches a throughput limit sooner than one cycled lightly, so the warranty that looks generous in years may be the tighter one in practice.
Work out roughly how many cycles a year your intended use implies, compare that against the warranted throughput, and you have a realistic service life to divide the cost over. A payback longer than the warranty is a warning, not a technicality.
Tariffs do most of the heavy lifting
On a flat-rate tariff, a standalone battery has very little to earn from: there is no price gap to exploit and no surplus generation to store. On a time-of-use tariff with a meaningful difference between cheap and peak rates, the same battery can earn every day.
We do not publish specific tariff rates here, because they vary by supplier and go stale quickly. Take the rates from your own tariff or from the one you intend to switch to, check the terms on the supplier's own page, and use those figures. Also check the exit terms: a battery business case built on one tariff is exposed if that tariff changes.
Why payback varies so much between households
- Evening consumption. A household that uses little in peak hours has little to shift.
- Tariff structure. A wide cheap-to-peak gap transforms the arithmetic; a flat tariff flattens it.
- Solar surplus. With panels, the battery can capture exports that would otherwise earn a low rate.
- Usable capacity against your actual daily shiftable load. Capacity you never cycle earns nothing.
- Continuous power against your real peak. An undersized output rating means you import anyway.
- Installed cost, including any electrical work, which is the denominator in every payback sum.
Common mistakes
- Sizing on nameplate capacity rather than usable capacity and actual daily shiftable load.
- Assuming a full cycle every day of the year when consumption and generation both vary.
- Ignoring round-trip losses, which quietly reduce every year of benefit.
- Buying storage for backup power without confirming the system actually supports it, and at what output.
- Bundling the battery into a solar quote so neither purchase can be judged on its own.
- Treating a tariff that exists today as a fixed feature of the next ten years.
Questions to ask an installer
- 01What is the usable capacity, and what continuous power output can it sustain?
- 02What round-trip efficiency should I assume for the whole system, not just the cells?
- 03What does the warranty guarantee, in years, cycles or throughput, and what retained capacity at the end?
- 04Does this need a hybrid inverter, and is that included in the price?
- 05Will it provide backup during a power cut, and what does that add in cost and equipment?
- 06Which tariffs does the system support today, and what happens if I switch supplier?
- 07What is the full installed cost including any consumer unit or earthing work?
Work through your own case
Our battery calculator is not published yet, so the practical route today is to do the arithmetic by hand: daily shiftable kWh multiplied by the price gap, multiplied by 365, reduced for round-trip losses, then divided into the installed cost. If you also have panels, use the solar calculator to see how much surplus you are currently exporting, since that surplus is what a battery would capture.
See how much you are exporting first
Storage mostly earns by capturing what you would otherwise export, or by shifting cheap hours. The solar calculator shows the export side of your own figures.