Russell Andrews
Choosing a home battery size comes down to matching storage to the way your household uses electricity. A battery that is too small runs out early in the evening. One that is too large may rarely fill, so you pay for capacity you do not use. This guide covers what to work out before you ask for a quote.
Capacity and power are different things
Two numbers describe a home battery, and they do different jobs.
- Capacity, in kilowatt hours (kWh), is how much electricity the battery can store. Think of it as the size of the tank.
- Power, in kilowatts (kW), is how fast the battery can charge or discharge. Think of it as the width of the tap.
A large battery with a low power rating can struggle to run several heavy appliances at once, such as an oven and a kettle together. Ask for both figures, and ask which one limits the battery in practice.
Nominal capacity versus usable capacity
Product listings often quote the total capacity of the battery cells. Usable capacity is the part you can actually draw on, and the gap between the two is held back to protect the battery's lifespan. Make sure you compare usable capacity with usable capacity. Our guide to solar batteries looks at some of the products available.
Work out your evening and overnight use
The most useful starting point is how much electricity you use when your panels are not producing, which is mainly the evening and overnight. Your bill shows your annual use in kWh. For context, Ofgem's typical medium figure for a household, applied from 1 July 2026, is 2,500 kWh a year. That averages under 7 kWh a day, but real homes vary widely. A household with an electric vehicle, a heat pump or someone working from home most days will use more.
A smart meter or your supplier's app can show your half-hourly usage. Look at the hours between early evening and the morning. That figure is a better guide to battery size than the daily total, because the battery is there to cover the hours when the sun is not.
Match the battery to your solar system
A battery is charged from surplus solar generation, so it needs enough spare generation to fill it. If your panels barely exceed what you use during the day, a large battery will rarely be full. Ask the installer how much surplus they expect across the seasons, and remember that winter generation is much lower than summer.
This is why battery size is best decided as part of the whole design. The solar battery storage guide covers how storage fits into the first conversation about a system, and our residential solar page shows how REST approaches home systems.
Think about what else the battery might do
- Time-of-use tariffs. If you plan to charge from the grid at cheaper times, you will want a battery that can handle that cycle every day. See our guide to tariffs and batteries for how this works.
- Power cuts. Energy Saving Trust points out that a battery does not normally power your home in an outage unless extra equipment is included in the installation. If backup matters to you, size and power rating decide what can run and for how long. Our guide to home battery backup explains partial and whole-home backup.
- Future changes. An electric vehicle, a heat pump or a growing family will raise your use. Ask whether more battery modules can be added later.
- Lifespan. Energy Saving Trust notes that batteries generally have a shorter life than solar panels, so check the warranty term, and what it says about capacity over time.
Sizing a battery without solar panels
A battery can be installed without panels and charged from the grid, usually on a tariff with cheaper off-peak rates. The sizing logic is the same, but the benefit depends on the gap between cheap and expensive periods on your tariff, so check this with your supplier first. Our home battery storage page covers this option in more detail.
Questions to ask us, or any installer
- What is the usable capacity, and what are the maximum charge and discharge rates?
- How did you work out the size recommended for my home?
- What happens to performance over time, and what does the warranty say?
- Can more storage be added later, and what would that involve?
- Is the battery compatible with my existing inverter, if I already have panels?
Once you have answers, our guide on what to look for in a solar and battery quote helps you check the detail. REST UK is MCS certified and HIES protected, and our team can size a battery around your own usage, so request a free quote to get started. If you already hold a quote elsewhere, send it in and we will review it, subject to assessment and approval.
Figures are indicative and depend on your home, your tariff and your habits. Benefits are not guaranteed. This guide is general information and is not financial advice.