When you’re buying a solar battery, you want to get the right size for your home. For the average UK household using 3,400 kilowatt-hours (kWh) of electricity per year, this means a 9-10kWh battery.

If you get it wrong in either direction, it could end up costing you more. Get an undersized system and it’ll become overworked and need to be replaced sooner, while an oversized battery will be unnecessarily expensive.

But if your installer chooses an appropriate model for your home, you can make the most of the electricity your panels generate without overpaying.

In this guide, we’ll explain why it’s important to get the right size of battery, how your installer will work out which size suits your home, and the range of sizes they’ll have at their disposal.

If you’re wondering how much a solar & battery system could save you, just answer a few quick questions below and we'll provide an estimate for you.

What size solar battery should you get?

A battery that’s roughly 9-10kWh is suitable for the average UK household.

A good rule of thumb is to match your battery size to your daily electricity usage, and with the average UK household using 3,400kWh of electricity per year – or about 9.3kWh per day – a 9-10kWh battery is ideal.

It follows that you’ll need a smaller battery if you consume less electricity.

The table below shows how the right battery size changes based on your electricity usage.

What does ‘solar battery size’ actually mean?

A battery’s size is measured in kWh. It refers to a quantity of electricity, just like you’d talk about water in litres, for example.

So if your solar panel system produces 7kWh on a given day and you use half of this electricity as it's being generated, a 5kWh battery can store the remaining 3.5kWh.

However, a solar battery will only discharge to its Depth of Discharge (DoD) – that is, the maximum amount of its capacity you can use without putting it at risk.

For instance, a 5kWh battery with a 90% DoD will discharge 4.5kWh before needing to recharge.

You can increasingly find 100% DoD batteries which don’t come with any discharge limits, meaning the entirety of their capacity is usable – except for a small, hidden reserve used to operate the Battery Management System.

Why is battery sizing important?

It’s crucial to get an appropriately sized battery to ensure your investment pays off.

If your battery is too small or too big, it may well result in you saving less or spending more than you should.

If your battery is too small

An undersized battery won’t allow you to make the most of the electricity your panels produce.

When the battery’s limited storage space runs out, you’ll have to sell the rest to the grid. You can get paid for this via an export tariff, but it tends to be less profitable than using it in your home.

With a correctly sized battery, you'll charge up with off-peak grid electricity overnight, then use this as cheap backup energy throughout the day, if and when your panels aren't producing enough.

But with an undersized battery, you'll often need to import expensive grid electricity during the early evening peak.

If your battery is too big

Getting an oversized battery is a waste of money.

Batteries cost around £3,000-£7,000, depending on their size, so going big can mean spending thousands of pounds more than you need to.

With the right import and export tariffs, you could make the most of it, but you’d primarily be using it for energy arbitrage – that is, buying electricity during cheap off-peak periods, then selling it back to the grid at peak times.

This would provide extra savings, but almost certainly not enough to justify spending thousands of pounds more on a larger battery.

For example, let’s say you buy 5kWh of extra storage to exclusively use for energy arbitrage, and fill it with 24p per kWh electricity every day before selling it back for 32p per kWh.

These rates are based on Intelligent Octopus Flux, which is the best tariff to use for energy arbitrage, as of April 2026. You can find cheaper import rates elsewhere, but this export rate makes it the best option.

Over a year, you’d make £146 with this approach, and across a battery’s lifespan, you’d earn roughly £1,400-£1,800. It’s not usually worth the investment.

How to work out what size solar battery you need

A qualified solar panel installer should work out what size battery you need, so this shouldn’t be left up to you – but it’s good to know what they’ll base their recommendation on.

Here are the most important factors your installer will consider:

1. What your budget is
2. How much electricity you use
3. Whether your electricity usage will increase
4. How much space you have
5. Whether you want emergency backup power

Here's a bit more detail on each factor.

1. What your budget is

Batteries are expensive, to the extent that going up a size or two can cost you thousands of pounds extra.

So even if a 10kWh battery would suit your home better than a 5kWh one, it only makes financial sense if you can afford it.

That’s why your budget will ultimately have the largest influence on which battery size you choose.

Sunsave Plus is the UK’s first solar subscription, and it completely removes the upfront cost of switching to solar. If you’re wondering how much you could save with a solar & battery system, enter a few details below and we’ll provide an estimate.

2. How much electricity you use

After budgetary concerns, your household’s electricity consumption is the most important factor in deciding which size of battery you should get.

As we explained further up the page, a good rule of thumb for battery sizing is to match it to your daily electricity usage.

If you check your last year’s consumption – which you should be able to find on your supplier’s app or an energy bill – and divide it by 365, you’ll get a decent idea of your household’s ideal battery size.

So if you use 2,000kWh per year, a 5-6kWh battery should suffice – whereas a home that consumes 6,000kWh per year should probably get 16-17kWh of battery storage.

3. Whether your electricity usage is going to increase

If you think your electricity consumption will increase in the long term, you should get a larger battery from day one.

You can make some savings with energy arbitrage until your needs increase, at which point you’ll be perfectly placed to maximise your returns.

The other option is to wait and add more batteries later on, though this is almost always the worse choice financially (more on this below).

So if you’re likely to get a heat pump in the next few years – or if you’re planning to add a member or two to your household – you should take that into account when you’re buying your system.

However, purchasing an electric car and charger shouldn’t affect the size of battery your installer recommends, since you won’t charge your EV from your battery.

Instead, you should charge it with cheap, off-peak grid electricity that you access with an EV tariff.

Bear in mind though that batteries typically last 10-12 years, so there’s no need to plan too far into the future.

Fortunately, every Sunsave Plus subscription is backed up by the 20-year Sunsave Guarantee, which includes a free battery upgrade.

4. How much space you have

Most households will be able to find a suitable place to fit an appropriately sized battery, but it’s still worth checking that you have enough space before you buy.

The average 10kWh battery takes up about 0.9m³, so make sure you have enough room. They’re generally taller than they are wide, which tends to help.

The best place to put a battery is outside your home or in your garage, according to the most recent guidelines. This is because higher temperatures and small spaces usually cause batteries to degrade faster.

5. Whether you want emergency backup power

If you want to keep the lights on when power cuts happen, you’ll need a bigger battery.

On top of your standard daily usage, you’ll have to keep a reserve of electricity in your battery that stays untouched, just in case you need it.

Your installer should help you work out how much electricity you’d like on hand when the grid goes down, and size up your battery accordingly.

Depending on whether you’d like to supply power to a single socket, one or more circuits, or your entire home during blackouts, you’ll typically need 20% to 40% more battery storage.

To learn more, read our guide to home battery backup for power cuts.

Should you get an oversized battery?

You should only get a battery that’s too big for your current needs if you anticipate your home’s consumption levels rising in the near future.

If you’re looking to get a heat pump, or you might add people to your household soon, then it’s worth considering a larger battery.

Otherwise, it’s not worth it to pay thousands of pounds more for an oversized battery. Even if you take full advantage of a time-of-use tariff, you probably won’t make your money back on your initial investment.

Can you add more battery storage later on?

Most batteries come as stackable modules, meaning an installer can easily add more storage at a later date – but it rarely makes sense to actually do it.

If you wait 10 years to expand your capacity, you'll be adding modules to a system that’ll need replacing soon. In this case, you'd be better off just buying a whole new, larger battery.

If you decide to buy more modules after 10 years, it’ll be too close to the point when your original battery will need replacing. Instead of adding storage, it’d be more cost effective to just buy a new battery instead.

If you make that choice sooner (say, in a few years) it won’t be worth replacing your original battery, as it should still be in good condition – but you’ll kick yourself for not getting more storage initially.

After all, you’ll have to pay for labour again, whereas adding a few modules to your system on your original installation day shouldn’t affect your costs in any meaningful way.

Next steps

The average UK household requires a 9-10kWh solar battery.

However, you should generally leave it up to an installer, who’ll size your battery according to your budget, how much electricity your home consumes, and the space available in your home.

And if their recommendation sounds wrong, you can always ask other installers to perform the same calculations, just in case.

If you’re wondering how much a properly installed solar & battery system could save you, just answer a few quick questions below and we’ll provide an estimate.

Solar battery sizes: FAQs