What Size Solar Battery Do I Actually Need?

Almost everyone asks this question using the wrong number. They quote their monthly bill in pounds, because that is the figure they know. The bill is a poor guide to battery size, because it bundles together electricity you use at two in the afternoon with electricity you use at eight in the evening, and only one of those is the battery's job.

This guide shows you how to work it out properly before anyone quotes you.

Everything you need to know

• Battery size is driven by what you use after the panels stop generating, not by your total bill.
• The number you need is your annual consumption in kilowatt hours, and it is printed on your electricity bill.
• Whether anyone is home during the day changes the answer more than the size of the house does.
• A battery that never empties is money spent on capacity you are not using.

Start with the right number

Find your annual consumption in kilowatt hours. It is on your electricity bill, usually on the page that compares this year with last year, and it will be a figure in the low thousands. That one number tells an installer more than the size of your house, the number of bedrooms or what you pay each month.

If you cannot find it, your online account will have it, and your supplier must provide it on request. It is worth five minutes to get the real figure, because every quote you receive will otherwise be based on an estimate.

Why pounds will mislead you

Two households with identical bills can need very different batteries. One is on a cheap overnight tariff and already shifts half its usage into the small hours. The other is on a standard tariff and does everything at six in the evening. The first needs a battery to exploit the cheap window. The second needs one to cover a sharp evening peak. Same bill, different systems.

Unit rates also change. A bill from eighteen months ago describes a different world. Kilowatt hours do not move.

The question that matters most

Is anyone home during the day?

This single fact changes battery sizing more than anything else, because it determines what happens to your solar generation while the sun is up.

If the house is empty on weekdays, your panels generate into an almost empty house. Nearly all of it is surplus. That surplus either goes to the grid or into a battery, and the battery is what lets you have it back at six o'clock. These households get the most obvious benefit from storage.

If someone is home all day, a good proportion of your generation is used as it is made. The surplus is smaller, so the battery has less to capture, but it still covers the evening and overnight.

If you work shifts or the pattern varies, this is worth discussing at the survey rather than averaging out. An irregular pattern sometimes points towards a slightly larger battery and a tariff that rewards flexibility.

Working out your evening

Think about what actually runs after dark. Lighting is trivial. Television and computers are modest. The things that move the needle are heat and motors: an electric oven, a tumble dryer, a dishwasher on a hot cycle, an immersion heater, air conditioning in summer.

A household that cooks a full meal, runs a dryer and puts a dishwasher on uses a substantial amount of electricity between five and ten in the evening. A household that eats out twice a week and uses the washing machine on a timer uses very little.

You do not need to calculate this precisely. You need to know roughly which of those two households you are, because it is the difference between one size of battery and the next one up.

A rough guide

This table is a starting point for a conversation, not a specification. We do not quote a battery from a table and neither should anyone else. It is here so you can tell whether a quote you have been given is in a sensible region or wildly out.

What goes wrong when it is undersized

An undersized battery empties before the evening is over. You get the benefit until about nine o'clock and then you are back on grid electricity for the rest of the night.

That is not a disaster. It is still better than no battery. But it is frustrating in a specific way, because you can see it happening in the app and you know that a slightly larger unit would have carried you through. Adding capacity later is possible with a modular system, though it costs more in total than sizing it correctly at the outset.

What goes wrong when it is oversized

An oversized battery never empties. In summer it fills by lunchtime, sits full all afternoon, and the surplus that would have filled it goes to the grid anyway. In winter there is not enough generation to fill it at all.

You paid for capacity that spends most of the year unused. Nothing breaks and nothing underperforms, it is simply money that would have done more work elsewhere, on more panels or on better equipment.

This is the more common error, because a larger battery is an easier thing to sell. Be sceptical of anyone who recommends the biggest unit without asking what your evening looks like.

How a time-of-use tariff changes the answer

A time-of-use tariff charges less for electricity overnight. With one of those, the battery has a second job: it fills from the grid in the cheap window and runs the house through the expensive part of the day, whether or not the sun has appeared.

This matters most in winter, when solar generation is at its lowest and the battery would otherwise sit mostly idle from November to February. A household on a time-of-use tariff can often justify more capacity than the same household on a flat tariff, because the battery is working all year rather than seasonally.

We do not sell tariffs and have no arrangement with any supplier. But if you are considering one, tell us at the survey, because it genuinely changes what we recommend.

Why the roof and the battery must be sized together

A large battery behind a small array is a jug under a dripping tap. There is not enough generation to fill it, so it never does its job.

A large array with no storage is the opposite problem. You generate a great deal at midday, use a fraction of it, and export the rest.

The two have to be designed as one system. This is the most common fault we see in quotes people bring us for a second opinion: a battery sized from the bill and an array sized from the roof, with nobody checking that the two match.

What happens at the survey

We ask for your annual kilowatt hour figure, look at the roof, ask about your daily routine, and ask whether a tariff change is on the cards. Then we size the array and the battery together and quote a fixed price for the system that results.

You get the sizing in writing, with the reasoning behind it, so you can see exactly why we have recommended what we have.

FAQ

Can I just get the biggest battery available?

You can, but in most homes a proportion of that capacity will sit unused for most of the year. The money is usually better spent on additional panels, which increase what you generate rather than what you can store.

What if I get it wrong?

A modular system can be added to later, so an undersized battery is recoverable by stacking a second unit. An oversized battery is not recoverable, which is why we err towards sizing carefully rather than generously.

Does the number of bedrooms matter?

Far less than people expect. A four bedroom house with two occupants who are out all week uses less than a three bedroom house with a family of five. We size against consumption, not against the house.

Will an electric car change the sizing?

Yes, significantly, because a car is a large load and usually charges overnight. Tell us if one is planned, even if it is a year away, because it changes both the battery and the tariff conversation.

How do I know a quote is sensible?

Ask the installer what annual kilowatt hour figure they sized it against. If they cannot tell you, they did not size it, they picked it. That is the single most useful question you can ask anyone quoting you.

Can I start with solar and add a battery later?

Yes, and for some households that is the sensible order. Adding a battery to an existing array is routine work for us. It costs a little more in total than doing both at once, because some of the labour is repeated.