What size solar battery do I need? (UK sizing guide)

Most UK homes land on a battery somewhere in the 5–13.5 kWh usable range (kWh means kilowatt-hours, the same unit your electricity bill is measured in). The size that's right for you comes down to how much electricity your household uses between sunset and sunrise. There's no single correct number: a flat with modest evening use and a family home charging an EV overnight need very different batteries. Here's how to work out roughly where you sit before you talk to an installer, and if you want a figure for your own home rather than a rule of thumb, our savings calculator models it for a home like yours.

Start from your evening and overnight usage, not your panel size

The instinct is to size a battery to match your solar array. That's backwards. A battery's job is to store daytime surplus and release it when the sun isn't out, so the number that actually matters is how much electricity you use after your panels stop generating, typically from early evening through to breakfast.

A rough way to build that number:

• Evening electricity use (cooking, lighting, TV, washing machine, kettle): commonly 2–4 kWh for a typical household.
• Overnight baseline (fridge, freezer, router, standby devices): often 1–2 kWh across the night.
• Anything big and specific (an EV charging overnight, a heat pump running, a tumble dryer): this is where the number can jump a long way, covered below.

Add those together and most UK households without an EV or heat pump land on 4–7 kWh of overnight and evening demand, which is why 5 kWh and 10 kWh are the two most common battery sizes sold today. It's a planning range, not a target to hit exactly; your actual usage is on your energy bill, and a smart meter or in-home display will show you the real shape of your day if you have one.

The common sizes and what they're really for

• ~5 kWh: suits a smaller household or a home with modest evening habits. Covers a normal evening plus overnight baseline, with some cushion, on days when solar tops it up well.
• ~10 kWh: the most common size for a family home. Covers a fuller evening (cooking, appliances, a couple of people home) plus overnight baseline, and starts to give useful headroom on darker winter days.
• ~13.5 kWh (a size some well-known battery units land on): worth considering if you're charging an EV at home, running a heat pump, or you specifically want to ride through a dull winter day with minimal grid import.

These aren't hard tiers. Battery systems are increasingly modular, so "10 kWh" might really be two 5 kWh units, and some homes stack two or three units together. The size itself matters less than whether it matches your actual overnight and evening load.

Where diminishing returns kick in

Every extra kWh of battery costs money, and each additional kWh earns you less than the one before it, because it only pays back on the days you'd otherwise have imported that much and had the solar surplus to fill it.

To make that trade-off concrete, here's the rough money involved (2026 UK installed prices, which vary widely by brand, whether the battery is retrofitted or fitted alongside new panels, and by installer):

• a small ~5 kWh battery lands around £2,500–£4,000 installed
• a mid ~10 kWh battery around £4,000–£6,000
• a large ~13.5 kWh battery around £6,000–£9,000 for a modular stack (a single premium all-in-one unit like a Tesla Powerwall can run higher, closer to £8,500–£10,500 installed)

Those are ballpark ranges, not quotes. For a figure that reflects your home, see how much solar panels cost in the UK and our solar battery storage guide, or run the savings calculator. With the cost in view, the diminishing-returns pattern is easier to see:

• The first 5 kWh or so tends to get used most days, filled by the panels and drawn down most evenings. High utilisation, solid payback.
• The next 5 kWh gets used less often, perhaps filled on sunny days and drawn down on the odd evening you'd otherwise import. Useful, slower payback.
• Beyond roughly 13–15 kWh for a typical home, you're often sizing for the worst-case dull winter day rather than a typical one: capacity that sits mostly empty most of the year.

None of that means a bigger battery is wrong. If you specifically want to minimise winter grid dependence, or you're stacking EV and heat pump load onto the same battery, the maths changes. It means "biggest I can afford" isn't automatically the right buying decision on its own.

EV charging changes the answer significantly

Charging an electric car adds a large, lumpy overnight load that dwarfs normal household use. A typical home EV charge, topping up after a day's driving, can be 7–30+ kWh depending on how far you've driven and your car's battery size. That's often bigger than your whole-house evening and overnight load combined.

Two practical patterns follow from this:

• Many EV owners size their home battery to cover normal household evening/overnight use and let the car charge separately on a cheap overnight time-of-use tariff, rather than trying to store enough solar to fully charge the car from the battery too.
• Some owners deliberately go bigger, at 13.5 kWh or with stacked units, specifically to shift more EV charging onto stored daytime solar rather than grid import, particularly if they drive a lot and want to minimise both cost and carbon.

There's more detail on this trade-off in our guide to solar and EV charging in the UK.

Heat pumps and time-of-use tariffs also move the number

A heat pump running through a cold evening or overnight can add several kWh to your daily draw, particularly in winter when solar generation is at its lowest anyway, which is exactly when a battery's timing advantage matters least, since there's less surplus to store in the first place. If you have or are planning a heat pump, it's worth discussing sizing specifically with your installer rather than assuming standard household ranges apply.

Time-of-use tariffs (like Octopus Go and similar) change the calculation in the other direction: a battery doesn't only fill from solar surplus, it can also fill from cheap overnight grid electricity, then discharge through the expensive evening peak. That means a battery earns its keep even on dull days with little solar generation, which shifts some households toward a slightly larger size than "solar surplus alone" would justify. We cover the mechanics in solar and Octopus Go / time-of-use tariffs.

Why oversizing wastes money

A battery only pays back if it's regularly filled and regularly drawn down. Buy more capacity than your household (plus EV, plus heat pump, if relevant) realistically uses, and the extra kWh mostly sit idle: capital spent on capacity that isn't earning anything back. It's the same logic whether you're storing enough for cooling, an EV, or ordinary evening use: size to your actual load with sensible headroom, not to the largest unit on the price list.

If your circumstances are likely to change soon, such as an EV purchase or a heat pump install, it's worth flagging that at the sizing stage. Most modern battery systems are modular and can be expanded later, so a modest starting size with room to add a second unit is often a more sensible bet than overbuying now for a future that hasn't arrived yet.

Get a number for your own home

Rules of thumb get you in the right zone; your actual bill, tariff, and household habits get you the real figure. Our solar battery storage guide covers the broader value case for storage in the UK, and our savings calculator models generation, self-use, storage and export together for a home like yours. Whatever the estimate suggests, an MCS-certified installer survey is what turns a sensible planning range into a battery that's actually matched to your roof, your tariff and your household.

Sizing is only one of the questions worth settling before you buy. How long the battery will last is a separate one, covered in how long does a solar battery last?, and if you're hoping a battery will keep the lights on in an outage, note that it won't unless you specify backup hardware: see do solar panels work in a power cut?.

Before any of that, it's worth confirming your roof itself is a good fit: check your roof for solar takes a couple of minutes and gives you a direction and suitability read before you spend time sizing a system around it.

Frequently asked questions

What size solar battery do I need for an average UK home?

Most UK households without an EV or heat pump land on 4–7 kWh of typical evening and overnight electricity use, which is why 5 kWh and 10 kWh batteries are the most commonly installed sizes. Your actual figure depends on household size, habits, and whether you're charging an EV or running a heat pump. Your smart meter data or energy bill is the best starting point.

Is a 5 kWh or 10 kWh battery better?

Neither is universally better; it depends on your evening and overnight load. A 5 kWh battery suits smaller households with modest evening usage; a 10 kWh battery suits a fuller family evening plus more headroom on darker days. Sizing to your actual usage, rather than the biggest unit available, gets the best payback.

Does an EV change what size battery I need?

Yes, significantly. A typical home EV charge can be 7–30+ kWh, often larger than your whole-house evening and overnight load. Many owners keep the home battery sized to normal household use and charge the car separately on a cheap overnight tariff, while some choose a larger battery specifically to store more solar for EV charging.

Can I add more battery capacity later if I undersize it?

Usually, yes. Most modern home battery systems are modular, so you can add a second unit later if your needs grow, such as an EV purchase or a heat pump install. Starting with a sensible size for current usage and expanding later is often more cost-effective than overbuying capacity you don't yet need.

Related

• Solar battery storage UK: worth it in 2026?
• How long does a solar battery last?
• Do solar panels work in a power cut?
• Solar and EV charging in the UK
• Solar with Octopus Go and time-of-use tariffs in 2026
• Check if your roof is suitable for solar
• Find listed installers covering your postcode