Is a 5kWh battery big enough?

A five kilowatt hour battery is the smallest size commonly offered, and whether it is enough depends entirely on one measurement: how much electricity your household uses between sunset and sunrise.

For a small, low-consumption home it may cover most of an evening. For a house with an electric car or electric heating, it will be empty well before morning. Both outcomes are calculable in advance, from data you already have.

Measure your overnight use

Do this before comparing any quotes.

1. Get your half-hourly consumption data. If you have a smart meter, your supplier can provide it, usually through an app or a data portal.
2. Choose a fortnight that reflects normal life, avoiding holidays.
3. For each day, total the consumption from roughly 4pm to 8am.
4. Look at the range across the fortnight, not just the average.

If that daily total sits comfortably below five kilowatt hours, the small unit will cycle fully and earn its keep. If it is routinely double that, you will be buying a battery that empties early every night, and the sizing method points you at a larger unit before any brochure gets a say.

Usable, not nominal

The advertised figure is often nominal capacity, which is not what you get to use. Every system holds back a reserve to protect cell life, and the share it will actually deliver is the depth of discharge.

So a battery sold as five kilowatt hours may deliver meaningfully less. Ask for the usable figure in writing, and compare quotes on price per usable kilowatt hour rather than on the headline number.

This matters more at small sizes, because the reserve is a larger proportion of a small battery’s value to you: half a kilowatt hour held back is ten percent of this unit.

Put a ceiling on what it can earn

Before either case, bound the return. A 5 kilowatt hour usable battery cycling fully once a day moves 1,825 kilowatt hours a year. At current cited rates each cycled unit is worth roughly 10.7p when shifting your own solar (the capped 26.11p import times 87% round trip, minus the 12p export you gave up), or about 14.7p when charging at Intelligent Octopus Go’s 8p off peak rate for evening discharge.

So the small unit’s absolute ceiling is £195 to £270 a year, reached only with a full cycle every single day. Whatever it costs installed, divide by a realistic fraction of that ceiling before assuming it pays.

The case for starting small

There are real arguments for the smaller unit:

It cycles fully. A battery earns by moving energy. Capacity that sits unused earns nothing while still ageing and still having been paid for. A small battery that empties every night is working harder per pound than a large one that never drops below half.

Worked: a household using 6 kilowatt hours each evening. The 5 kWh unit shifts 5 a day, 1,825 a year. A 10 kWh unit shifts 6 a day, 2,190 a year: 20% more benefit, for perhaps 50% more money. Per pound invested, the small unit wins, because the constraint is the household’s evening, not the box.

Lower capital at risk. Storage economics depend on your tariff and your pattern. A smaller first purchase is a cheaper way to find out whether the theory matches your household.

Some systems expand. Several are modular, so capacity can be added later. Ask whether yours is, and what the added modules cost against buying the larger unit outright now.

The case against

Fixed costs do not scale. The inverter, the installation labour, the electrical work and the certification are much the same whatever capacity you choose. Illustratively, a 5 kWh installation at £3,500 is £700 per usable kilowatt hour while a 10 kWh one at £5,000 is £500: the small unit carries the fixed block on half the capacity. This is the mirror image of the cycling argument, and which one wins depends on your evening figure.

Power rating, not just capacity. Small units often have a lower continuous power output. If your evening involves an oven, a shower and a car charging at once, the battery may be unable to supply enough at that moment even with energy still stored, and the excess imports at full rate.

It caps your arbitrage. If you are also charging cheaply overnight to discharge during expensive hours, the amount you can shift each day is limited by capacity, and in winter, when the roof contributes little, arbitrage is most of what the battery does. A small battery caps that earning directly.

Where the small unit usually wins

A five kilowatt hour battery tends to suit a household that:

• Uses modest amounts of electricity overnight, five kilowatt hours or less on the measurement above.
• Has no electric car and no electric heating.
• Has a wide gap between import and export rates, so each shifted unit is worth a lot.
• Wants to test the idea before committing more capital, ideally with a modular system that can grow.

Where it usually does not

It tends to disappoint a household with an electric car charged at home, electric heating, or high evening consumption, because the constraint binds every single night and the return is capped by a capacity that is too small rather than by anything else. That household is also the one with the most units to shift, so the larger battery’s worse price per kilowatt hour is repaid by far higher throughput.

Decide it on your own figures

Take your measured overnight consumption, the usable capacity on the quote, and the gap between your rates. Then model the two candidate sizes against each other in the ROI calculator, which applies degradation and replacement over the projection. The right size is the one where the battery is nearly empty most mornings and rarely empty by nine in the evening: empty at 7pm means undersized, half full at dawn means you bought someone else’s evening.