What a 10kWh battery actually covers

Ten kilowatt hours is around the point where a home battery stops being sized for a typical evening and starts being sized for a whole night, an electric car, or serious tariff arbitrage.

Whether it earns its price depends on whether you can actually cycle it, and that is measurable before you buy.

Bound the return first

A 10 kilowatt hour usable unit cycling fully once a day shifts 3,650 kilowatt hours a year. At the current cited rates, each cycled unit is worth roughly 10.7p when shifting solar surplus (the capped 26.11p import times 87% round trip minus the 12p export forgone), or about 14.7p when charging at Intelligent Octopus Go’s 8p off peak rate.

So the ceiling is £390 to £540 a year, and it is reached only by a household that genuinely empties the battery daily. Everything below is about whether that household is yours.

What it covers

Measure rather than guess.

1. Get your half-hourly consumption from your smart meter data.
2. Take a representative fortnight, avoiding holidays.
3. Total each day’s consumption from roughly 4pm to 8am.
4. Look at the range, not just the average.

If your overnight total is routinely near or above ten kilowatt hours, the unit will empty most nights and every kilowatt hour you bought is working.

If your overnight total is half that, you have bought capacity that will sit part-charged, ageing without earning, and the smaller unit returns more per pound.

Usable, not nominal

The advertised figure is often nominal. What you get is usable capacity, after the reserve the system holds back to protect cell life.

Ask for the usable figure in writing and compare quotes on price per usable kilowatt hour. A ten kilowatt hour nominal unit may deliver meaningfully less than a competitor quoting nine usable, and at these prices the difference between £500 and £550 per usable kilowatt hour is several hundred pounds of real money.

Where the larger size genuinely pays

An electric car charged at home. This is the clearest case, because it moves the overnight demand line by itself: 8,000 miles a year at 3.5 miles per kilowatt hour is roughly 2,500 kilowatt hours from the wall, about seven kilowatt hours a night. A household that was marginal for ten kilowatt hours becomes one that cycles it.

Electric heating. Storage heaters, or a heat pump running overnight.

A wide tariff spread. Every additional kilowatt hour shifted each day earns the spread, 14.7p net on the rates above, and capacity is the constraint on how much you can shift. A larger battery raises the cap directly, which is why the arbitrage case favours size in a way the solar-shifting case does not.

High evening consumption generally. A busy household that cooks, showers and runs appliances after dark.

A large array. More summer surplus to capture rather than export, though remember the seasonal shape: the surplus ceiling only exceeds ten kilowatt hours a day through the brighter half of the year.

Where it does not

A modest household with a small array, no car and a narrow tariff spread will find a ten kilowatt hour unit rarely cycling fully. The economics of storage are driven by throughput, and unused capacity produces none: a unit that averages half a cycle a day has halved its ceiling to under £300 a year while costing the full price.

In that case a smaller unit at a better price per usable kilowatt hour, cycling completely every day, returns more, which is the sizing method’s core rule.

Power rating matters as much as capacity

Capacity is how much it holds. Power rating is how fast it can deliver.

A ten kilowatt hour battery with a modest continuous output cannot cover a large simultaneous load. If your evening involves an oven, an electric shower and a car charging at once, that is several kilowatts of simultaneous draw; whatever exceeds the battery’s continuous rating imports at full price even with energy still stored. Check the continuous power figure against your real evening peak, not just the capacity.

This is where the better-engineered units justify their premium.

Cycles and lifespan

Larger batteries generally last longer in calendar terms for the same household, because each day’s demand represents a shallower cycle.

That cuts both ways. A battery that only ever discharges to half will reach its year limit with cycles to spare, which means you paid for capacity that never got used. Check the warranty’s structure: a guaranteed remaining capacity after a stated number of years, cycles or throughput, whichever comes first.

Then run the clock arithmetic: daily cycling is about 365 cycles a year, 3,650 a decade, and double-cycling on an arbitrage pattern doubles that. Divide the warranted cycles by your expected cycles per year and see which clock you reach first. If it is the year clock by a wide margin, the unit is larger than your household needs; if it is the cycle clock well inside the warranty years, check the warranty covers your operating mode at all.

Grid charging is essential at this size

At ten kilowatt hours, a large part of the return usually comes from arbitrage rather than from storing solar surplus, particularly in winter when there is little surplus to store.

That requires the system to charge from the grid, not only from the array. Some solar-oriented configurations do not. Ask explicitly, in writing, because without it you have bought a large battery that sits mostly empty for five months of the year and earns nothing while it does.

The sizing rule

Take the upper end of your measured overnight consumption, not the average, and look for a unit whose usable capacity sits near it. Then check the power rating against your largest simultaneous evening load, and confirm grid charging.

Model it in the ROI calculator with degradation applied over the period. The right size leaves the battery nearly empty most mornings and rarely empty by nine in the evening, and at ten kilowatt hours that description fits an EV household far more often than it fits anyone else.