Battery storage · Guide
What size solar battery do I need?
What size solar battery do you need? Your yearly electricity use won’t tell you. That sizes your panels. A battery has a narrower job: it covers what you use when the panels aren’t generating, and the panels have to be able to fill it.
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Evening use
Start with your evening and overnight use.
Which? puts new home batteries at around 5 to 15 kWh. Here’s how to find where you sit in that range, and why a smaller battery, or none, is sometimes better.
Which? advises choosing a battery that covers your evening and night-time use, ready to charge again when the sun comes up. So what matters is what you use between the panels stopping and starting again, not your daily total.
- With a smart meter, your supplier’s app or account may show your use hour by hour.
- Without one, start from Ofgem’s typical yearly figures, used from 1 July 2026: 1,600, 2,500 and 3,800 kWh for low, medium and high use on a standard meter. Medium is about 6.8 kWh a day, and only part of that falls in the evening and overnight.
Filling it
Can your panels fill it? Summer against winter.
Which? warns there’s no point buying a battery bigger than your panels can fill, and spare solar varies a lot through the year. A solar diverter is another way to use spare solar, as hot water.
Government figures for feed-in tariff solar systems give a load factor of 13.6% for April to June 2024 and 4.7% for October to December. For a 3 kW system, that’s roughly 10 kWh on an average day in spring, and about 3.4 kWh a day from October to December. In winter much of that is used as it’s made, so the battery may barely charge from the panels.
Which? says a well-chosen battery should cover the evening for most of the year, but not always on the longest winter nights. See do solar panels work in winter?
Usable capacity
Usable capacity, not the headline figure.
- Depth of discharge. The Energy Saving Trust says the system stops drawing on the battery at a set level, perhaps 20% of its capacity, because running it to zero can shorten its life.
- Usable capacity. MCS tells installers to use the datasheet’s usable figure or, failing that, the total multiplied by the maximum depth of discharge (90% for lithium unless stated). So 10 kWh at 90% gives about 9 kWh.
- Backup reserve. Charge held back for a power cut isn’t available day to day. See do solar panels work in a power cut?
- Ageing. Batteries hold a little less each year. See how long do solar batteries last?
Power and capacity
Power (kW) as well as capacity (kWh).
MCS says to consider how much energy a battery can store (kWh) and how much it can deliver at once (kW).
- Big loads. If the oven, kettle and an electric shower are on together, the battery supplies what it can and the grid covers the rest.
- The inverter. One current modular battery’s datasheet lists 2.5 kW for one 5 kWh module, 5 kW for two and 6.2 kW at most, and says the inverter can limit it further. See hybrid inverters.
- The grid limit. A battery’s output counts towards the point where your network operator must approve the system first.
Time-of-use tariffs
Time-of-use tariffs change the sum.
On a time-of-use tariff you can charge the battery from the grid when electricity is cheaper and use it at peak times. Which? notes most people don’t have one yet.
This matters most in winter, when the panels rarely fill the battery. The question becomes how much peak-time use you want to move into the cheap overnight window. That can justify a bigger battery, if it charges fast enough: at 3 kW, 9 kWh takes about three hours. For costs and savings, see what a solar battery costs.
EVs and heat pumps
Electric cars and heat pumps.
MCS’s method for estimating how much solar a home uses leaves out both.
- An electric car. The Energy Saving Trust’s charging table covers car batteries of 25 to 100 kWh and chargepoints from 7.4 kW, far more than a home battery holds or delivers. MCS notes the car often isn’t home while the panels generate.
- A heat pump uses most in winter, when panels produce least. The Energy Saving Trust says a battery used with a smart time-of-use tariff saves more running a heat pump than just exporting extra solar. So winter grid charging, not spare solar, tends to set the size.
Adding more
Adding capacity later, and where it goes.
Many batteries are modular. The datasheet above stacks one to six 5 kWh modules and says new and old modules can be mixed. Not every battery can, so ask how far the model extends, whether new modules can join older ones, and what your inverter allows.
Where it goes matters too. PAS 63100:2024, the UK fire-safety standard for home batteries, rules out lofts, bedrooms and escape routes such as hallways and stairs, and caps total storage in a home at 40 kWh (80 kWh in places such as a detached garage or outdoors).
Worked example
A worked example: why bigger isn’t always better.
Take the home from how many solar panels do I need?: 2,500 kWh a year (Ofgem’s medium figure) and 3.15 kWp of south-facing panels near Manchester, making about 2,725 kWh a year. Say it’s empty for half the day on weekdays. MCS’s tables estimate how much of that solar it uses itself:
- No battery: about 23%, or 620 kWh a year.
- 5 kWh usable: about 64%, or 1,730 kWh.
- 9 kWh usable: about 71%, or 1,920 kWh.
- 15 kWh usable: about 72%, or 1,950 kWh.
The first 5 kWh does most of the work. Going to 9 kWh adds about 190 kWh a year; going on to 15 kWh adds about 30 kWh more, because the panels rarely have enough spare to fill it. Storing energy also loses some, the Energy Saving Trust notes.
MCS calls these tables a sense check, not a sizing tool, and they assume charging from the panels only. A time-of-use tariff, a car or a heat pump can push the answer up. A small system, or a household that already uses most of its solar as it’s made, can bring it down.
We survey your home, panels, inverter and where a battery can go, then recommend a size to suit how you use electricity. If a smaller battery, or none, would suit you better, we’ll say so. You get a clear written quote before any work starts, across England, Wales and southern Scotland. See solar battery storage.
Sources: Ofgem typical domestic consumption values (July 2026); Energy Saving Trust; Which?; MCS 032:2025 and its self-consumption tables; DESNZ feed-in tariff load factors; PAS 63100:2024.
Questions
The things people ask before they ring.
Not answered here? Call 0800 156 0191, Monday to Friday, 9:00am–5:00pm.
What size solar battery do I need for my house?
Enough to cover your evening and overnight use, and no more than your panels can fill. For a medium-use home with about 3 kWp of panels, MCS’s tables show most of the benefit comes from the first 5 kWh or so of usable storage. A time-of-use tariff, a car or a heat pump can justify more.
What size battery do I need for a 4 kW solar system?
For 4 kWp facing south near Manchester, MCS’s tables show gains flattening beyond about 5 to 7 kWh usable for a medium-use home, and 8 to 9 kWh for a high-use one (3,800 kWh a year), unless you also charge from the grid.
What size solar battery do I need to charge my electric car?
You don’t usually size a home battery for the car. Car batteries in the Energy Saving Trust’s table run from 25 to 100 kWh; home batteries are around 5 to 15 kWh. On an EV or time-of-use tariff, the car can charge straight from the grid at the cheapest time.
How much solar battery backup do I need?
If your battery can be set up for backup, some charge can be held in reserve for a power cut, and that part isn’t available day to day. MCS’s example reserves 20%. How much depends on what you’d keep running, such as the fridge, lights and router, and for how long. See do solar panels work in a power cut?
Ready when you are
Want a battery sized to how you really use electricity?
We’ll call you back to talk it through, and you get a clear written quote before any work starts. Mon–Fri 9:00am–5:00pm.