What Size Solar Battery Do I Need?

The right battery size depends on when you use electricity, not on how big your roof is or how big a battery you can afford. The aim is a battery that holds what you would otherwise export in the day and covers what you would otherwise buy in the evening, with enough charging power to fill it when it matters.

The Energy Saving Trust notes that a typical home battery might be around 10 kWh, and says there is no exact formula for working out what you need. That is true. But there is a method, and it is worth understanding before anyone quotes you.

Key points

• Capacity (kWh) is how much a battery holds. Power (kW) is how fast it can charge and discharge. You need both to be right.

• Start from your evening and overnight electricity use, not your annual total.

• On a cheap overnight tariff, the length of the cheap window and your inverter's charging power set a practical limit.

• Electric cars and heat pumps change the picture completely.

• Batteries lose capacity with age. Size for the battery you will have in year ten.

Capacity and power: two different numbers

Capacity, in kilowatt-hours (kWh), is how much energy a battery can store.

Power, in kilowatts (kW), is how quickly it can take energy in or give it out.

A large battery behind an inverter with modest charging power cannot fill up quickly, and cannot run a kettle, oven and shower at the same time on its own. That matters most if you plan to charge overnight on a cheap tariff: if the cheap window is a few hours long, the charging power decides how much you can actually get in before it ends.

Also look at usable capacity, not just nominal. Batteries do not use every last bit of their rated capacity, to protect themselves. Many current lithium batteries quote a usable share of about 90 per cent.

Step 1: Know your electricity use

The most useful starting point is your annual electricity use from your bills, and ideally half-hourly data from your smart meter.

For reference, Ofgem's typical domestic electricity use figures for a single-rate customer, updated in July 2026, are:

• Low: 1,600 kWh a year

• Medium: 2,500 kWh a year

• High: 3,800 kWh a year

Ofgem lowered these from July 2026, citing long-term efficiency improvements, climate and changes in behaviour. If your own use is well above them, a larger battery is more likely to be kept busy.

Step 2: Look at when you use it

This is the part a single annual number hides.

A household out all day uses little of its solar as it is generated, so a battery has a lot of surplus to capture. A household at home all day already uses much of its solar directly, so a battery has less to do. MCS's own method for estimating how much solar you will use yourself treats occupancy as a primary input for exactly this reason.

What you want is a battery roughly big enough to cover your usual evening and overnight use, without buying capacity that sits empty most of the year.

Step 3: Think about your tariff

If you are on, or plan to move to, a tariff with a cheap overnight window, your battery can be filled from the grid at the cheap rate and used later. That changes the sizing question: it is no longer only about solar surplus, it is about how much of your daytime use you can shift onto cheap electricity.

Here, charging power matters as much as capacity. There is little point in a battery bigger than your inverter can fill in the cheap window. And tariff eligibility often depends on the battery brand and the supplier, which changes often, so check before you commit.

Step 4: Plan for winter

In winter your panels produce much less, so a battery sized around summer surplus will often sit partly empty from November to February. That is normal. If you want the battery working hard all year, a cheap overnight tariff does that job in winter, which is another reason to consider the tariff alongside the size.

Step 5: Electric cars and heat pumps

MCS's self-consumption method specifically excludes electric vehicles, heat pumps, electric heating and diverters, because they change household demand so much. If you have one, or plan to, size from your real use including it, not from a standard figure.

An electric car is a large load, and it can be charged directly from a cheap overnight tariff rather than from the home battery. The two are best planned together.

Step 6: Size for year ten, not day one

Home batteries lose capacity over time. FoxESS, for example, warrants its EP6 and EP12 batteries to keep at least 70 per cent of their nominal capacity for ten years, subject to a limit on total energy throughput. So a battery sized exactly to today's evening use will cover less of it later in its life.

A little headroom now is usually better than a battery that is too small by year eight.

Why bigger is not always better

There is a ceiling on how much of your solar you can use yourself. MCS's estimates cap self-consumption at 95 per cent of annual generation, and in practice you reach diminishing returns well before that. Beyond a certain size, extra capacity sits unused most days.

Location can limit size too. Fire-safety best practice in the specification PAS 63100 sets limits on how much battery energy should be installed in different parts of a home, and rules out places like lofts, bedrooms and escape routes altogether. On a London terrace, where the battery can go can matter as much as how big it is.

A quick way to think about it

• Mostly out in the day, modest evening use: a battery sized to your typical evening use captures most of your surplus.

• Home most of the day: you already use much of your solar. A smaller battery, or one sized for tariff shifting, may make more sense.

• Electric car or heat pump, or planning one: size from real data including those loads, and plan the tariff and charging together.

A survey with your real consumption data will give you a far better answer than any rule of thumb.

Frequently asked questions

Is a 10 kWh battery the right size?

It is a common size, and the Energy Saving Trust describes it as typical, but whether it suits you depends on your evening use and tariff.

Can I add more battery later?

Many modular systems can be extended, but not all, and rules on mixing battery models vary by manufacturer. Worth asking about at quote stage.

Does battery size affect my DNO application?

The inverter power counts towards the network operator's limits, not the battery's capacity in kWh.

Should I size for power cuts?

Backup is a separate decision. Most systems back up only selected circuits. Our guide to what keeps working in a power cut explains the options.

Request a battery storage assessment. We will size it from your real electricity use and tariff, not from a standard figure.