How much home battery storage do you need? There is no single kWh size that suits every UK home. Estimate the energy you want the battery to provide, then check whether your solar panels or tariff can charge it and whether its output can run the equipment you have in mind.
The Energy Saving Trust says a typical home battery system might be 10 kWh, but also notes that there is no exact formula for choosing a size. Treat that figure as a reference point, not a recommendation for your home.
Choose the job before choosing a capacity
A battery sized for using more solar after sunset may not be the right size for shifting cheap electricity into an expensive tariff period or keeping selected circuits running in a power cut. Start with the main purpose, then calculate the energy needed during the relevant hours.
| What you want the battery to do | What to estimate | What can limit the result |
|---|---|---|
| Use more solar at home | Evening and overnight use, alongside the solar electricity usually left over during the day | How much surplus solar is available to charge the battery, especially in winter |
| Shift electricity between tariff periods | Use during the higher-priced period that you want the battery to cover | How much the battery can charge during the cheaper period, and energy losses |
| Provide backup power | Energy used by selected loads over the outage period you are planning for | Usable energy, inverter output, starting power and any protected reserve |
| Combine everyday use with backup | Daily energy shifting plus the energy you want held back for an outage | Whether the reserve is actually protected from everyday battery use |
For solar storage, compare the electricity you use after solar generation falls with the surplus your panels can realistically put into the battery. If you are mainly using a time-of-use tariff, focus on consumption during the more expensive hours. If backup matters, calculate the selected loads and outage duration rather than using your whole household’s daily total.
Know which battery figure you are comparing
Kilowatt-hours (kWh) measure stored energy. They help indicate how long a battery can supply a load. Kilowatts (kW) measure power at a given moment, so they help determine which appliances can run at the same time. A battery can have enough kWh for a long period but still lack the output to run several appliances together or start a motor.
Check whether a quote gives nominal capacity or usable capacity. Battery specifications and quotes may use “usable” differently, so ask how much energy can actually reach your home’s AC circuits under the quoted assumptions. Stored energy is not all delivered to appliances: conversion losses and any operating reserve affect what is available. The UK Prepared home battery size calculator explains the figures to ask an installer to confirm.
Ask about maximum continuous output and short starting or surge output as well as kWh capacity. Fridges, pumps and other motor-driven equipment can have a higher starting demand than their normal running demand. The inverter and battery system must be designed for the loads you want to run together.
Estimate the amount your household needs
- Collect your electricity data. Look at bills or supplier records covering a full year, and use half-hourly smart meter data if available. An annual total shows how much electricity you use, but not when you use it. If you already have solar panels, separate electricity imported from the grid from electricity generated, used at home or exported.
- Choose the period the battery needs to cover. For solar or tariff shifting, look at the hours after your solar generation drops or your tariff becomes more expensive. For backup, choose the circuits and the number of hours or days you want to support. Include seasonal differences where heating or solar generation changes substantially.
- Calculate the energy used in that period. For a steady load, use: watts × operating hours ÷ 1,000 = kWh. For example, an average 400 W load running for 8 hours uses 3.2 kWh before battery and inverter losses or any reserve. This is an illustration of the calculation, not a recommended battery size. For cycling equipment such as a fridge, measured consumption over a representative period is more useful than assuming it runs continuously at its stated wattage.
- Compare the estimate with what can charge the battery. Solar panels cannot fill a battery with electricity they do not generate as surplus. For tariff shifting, check the lower-priced charging window and the higher-priced period you want to cover. If the financial case depends on exporting solar electricity, compare it with the export payment you would give up. Ofgem explains the Smart Export Guarantee and how its suppliers set their tariff offers.
Battery systems are not perfectly efficient, and the energy available to use will vary with the product and its settings. Do not apply a generic loss or reserve percentage on top of a figure that already means usable AC energy. Ask the installer to show the calculation and state which assumptions they have included.
If you want backup, account for the reserve and the system design
A battery used every day for solar or tariff shifting may not be full when a power cut starts. If you want energy held for emergencies, ask whether the system can keep a chosen reserve aside and how that setting affects everyday use.
Do not assume that a standard grid-connected battery will power your home during an outage. The Energy Saving Trust notes that additional equipment and configuration are normally needed for a battery to provide power in a power cut. Ask the installer which circuits will remain live, how the system changes over and whether solar panels can recharge it while it is operating separately from the grid. Our guide to backup power for a UK home covers the wider options.
If anyone in the home relies on medical or care equipment, arrange an outage plan with the care team or equipment supplier. A general battery-size estimate should not be the only plan for equipment that someone depends on.
What to ask an installer before accepting a size
A useful proposal should show why the suggested size fits your home, rather than relying on a headline capacity alone. Ask for:
- The nominal capacity in kWh and the usable energy available to your intended loads.
- The maximum continuous output and supported starting or surge loads in kW.
- The household use, solar surplus, tariff periods and seasons used in the estimate.
- Any reserve setting, assumed starting charge and losses included in the calculation.
- How much capacity is covered by the warranty and whether backup operation is included.
- Whether the system can be expanded later, and what compatibility or installation limits apply.
Use the solar and battery planning checklist to gather bills, meter data and household plans before requesting quotes. If you rent or live in a flat, check who can authorise an installation and where equipment could be placed before paying for detailed proposals. There is no need to buy extra capacity for a possible future heat pump or electric vehicle unless the likely timing and electricity use have been included in the design.
Home battery sizing resources
- Energy Saving Trust battery storage guidance explains common uses, capacity in kWh, efficiency losses and why sizing depends on the household.
- Energy Saving Trust guidance to energy storage options describes how household use, generation and planned technologies can affect storage needs.
- Energy Saving Trust’s solar power fact check covers seasonal generation, export payments and the additional equipment needed for solar or batteries to work during a power cut.
- Ofgem’s Smart Export Guarantee information explains the export scheme and supplier tariff offers.
- UK Prepared home battery size calculator helps estimate energy needs and prepare questions about usable capacity, output and backup.
- UK Prepared solar and battery planning checklist helps organise household electricity data and installation questions before comparing quotes.
