Home batteries
A battery moves electricity through time. That is the whole trick.
Storage does not create energy. It lets you buy or generate electricity when it is cheap and use it when it is expensive. Whether that is worth several thousand pounds depends almost entirely on your tariff and your habits.
Research snapshot: September 2026
A battery without solar
You do not need panels to own a battery. On a time-of-use tariff you can charge overnight at a low rate and discharge through the expensive evening peak. The saving per unit is the gap between those two prices, minus round-trip losses, multiplied by how much you actually shift each day.
That makes the maths refreshingly simple to sanity check. Work out how many kilowatt hours you can realistically move per day, multiply by the price gap, multiply by 365, and compare with the installed cost. If the answer is uncomfortable, no amount of app screenshots will fix it.
A battery with solar
Paired with panels, a battery soaks up midday surplus that would otherwise be exported at a low rate, and returns it in the evening at the rate you would have paid to import. The value per stored unit is roughly the difference between your import and export prices.
That is usually a bigger gap than the export rate alone, which is why batteries improve annual savings. It is also why they often lengthen payback: the extra saving is real, but so is the extra capital.
Usable capacity versus nominal capacity
Nominal capacity
The headline figure in kilowatt hours, describing the total energy the cells nominally hold. It is the number on the datasheet and in the advert.
Usable capacity
What you can actually cycle, after the reserve the system keeps back to protect the cells. This is the number that does the saving, so compare quotes on usable capacity.
Also ask about continuous power output in kilowatts. Capacity decides how long the battery can run your house; power output decides how much of your house it can run at once. A large battery with modest output will not cover an oven, a kettle and a car charger together.
Round-trip efficiency matters too. Some energy is lost going in and coming out, so a portion of every stored unit never reaches your sockets.
Cycles, degradation and warranties
Battery warranties are usually written as a number of years, a total throughput in megawatt hours or a cycle count, and a guaranteed remaining capacity at the end of that period. Whichever limit is reached first typically ends the cover.
Read the retained capacity figure carefully. A warranty promising a certain percentage of original capacity after ten years is telling you plainly that the battery will hold less then than it does now, and your savings estimate should allow for that.
Check who backs the warranty, whether it survives the installer going out of business, and what happens if the manufacturer leaves the UK market. Ask whether labour and call-outs are included or only the hardware.
Time-of-use tariffs
A battery is only as clever as the tariff behind it. Flat-rate electricity gives a battery with solar a job, but gives a battery without solar almost nothing to do. Tariffs with distinct cheap and expensive periods are what make standalone storage viable.
Before committing, confirm that the battery and inverter can be controlled to charge and discharge on a schedule, and that they work with the tariff you intend to use. Check whether any supplier-specific control features require a particular piece of hardware.
Why payback varies so much
- The size of the gap between your cheap and expensive electricity prices.
- How many full cycles you genuinely achieve each year rather than in theory.
- Usable capacity and round-trip efficiency, not the nominal headline figure.
- Whether solar is present, and how much surplus it produces in summer versus winter.
- Installed cost including electrical work, and whether it was bundled with a solar quote.
- Degradation over time, which slowly reduces the amount you can shift each day.
- Tariff changes, since the deal that justifies the battery today may not exist in five years.
Questions to ask
- What is the usable capacity in kWh, and the continuous output in kW?
- What round-trip efficiency should I expect in normal use?
- What exactly does the warranty guarantee, and after how many cycles or years?
- Which tariffs and control systems is this battery compatible with today?
- Where will it be installed, what does it weigh, and what are the ventilation and fire safety requirements?
- Is this quote priced separately from the solar, so I can see each part on its own?
- What does the annual saving estimate assume about my daily shifted kWh?
Test a battery against your own tariff
The solar calculator lets you add an optional battery cost so you can see what it does to annual benefit and payback before anyone visits.