Home batteries: how they work and what size you need

Two ways a battery saves you money

1. Storing spare solar power. Without a battery, spare solar electricity is exported for around 12p per kWh (the typical figure the Energy Saving Trust gives), and in the evening you buy electricity back at around 26p (Ofgem’s average cap for October to December 2026, which excludes VAT while VAT is removed from electricity bills until 31 March 2027). Storing it saves the difference: roughly 14p for every kWh you move, before losses.

2. Charging at a cheap rate. Time-of-use tariffs charge less at certain hours, usually overnight. A battery can fill up then and run your home at dearer times. The saving for each kWh is the gap between the cheap rate and the normal rate. This works without solar panels too, and in winter, when panels make little, it is how most batteries earn their keep.

Rates vary between suppliers and tariffs, so use your own figures. See Tariffs and export payments.

What size battery do you need?

A battery’s size, or capacity, is measured in kWh. The Energy Saving Trust says there’s no exact formula and that installers will advise, but you can get a sensible starting figure in four steps.

1. Find how much electricity you use in a day. Your annual statement or supplier app shows your yearly use in kWh: divide it by 365. Ofgem’s figure for a typical home is 2,500 kWh a year, which is about 6.8 kWh a day.
2. Take off what you use during cheap hours, if you have or plan to get a time-of-use tariff, because you will buy that at the cheap rate anyway. What is left is what the battery can usefully supply.
3. Allow for the reserve. Most batteries can’t be emptied completely, so the usable capacity is less than the headline figure. Check the “usable capacity” on the specification sheet.
4. Check the power (kW) as well as the capacity (kWh). The power rating tells you how much the battery can supply at once, and how quickly it can refill during your cheap hours.

Bigger isn’t always better

• Capacity that rarely gets filled or emptied costs money but earns nothing.
• In summer, a battery bigger than your typical spare solar power will only fill up from the grid.
• In winter, panels make little, so plan on charging mainly at the cheap rate.
• Homes with a heat pump or electric car use much more electricity. The same four steps apply, but the numbers are bigger, and cars are often charged directly at the cheap rate rather than from the battery.
• Many batteries can be extended later with extra modules. Ask whether the one you are offered can.

Will it pay for itself?

A rough check: the kWh the battery shifts each day, multiplied by the saving per kWh, multiplied by 365, gives a yearly saving. Compare that with the price and the battery’s expected life.

That suits a home using about 3,650 kWh a year or more, for example one with an electric car or a heat pump. For a home using Ofgem’s typical 2,500 kWh, a 10 kWh battery can only shift about 5.8 kWh a day: the saving falls to about £339 a year and the payback rises to about 15.5 years, so a smaller battery may suit it better. At the Energy Saving Trust’s figure of £4,600 for a 5 kWh system, the same sums give about 17.5 years.

The Energy Saving Trust gives about 10 to 12 years as a battery’s typical life, so the price you pay, the gap between your cheap and normal rates, and using the battery fully every day all make a big difference. Pairing a battery with solar panels can shorten the payback too. Our battery size and payback calculator does the sums for you, and Guide 7 explains 2026 battery prices in more detail.

AC-coupled or DC-coupled?

AC-coupled: the battery has its own inverter and connects to your home’s normal wiring. This is usually the simplest way to add a battery to panels you already have.

DC-coupled: the panels and the battery share one “hybrid” inverter. There is one less conversion, so it can be slightly more efficient, and it is common when panels and battery are fitted together.

Either can work well. The installer should explain which suits your system and why.

Where it can go

Batteries are usually fitted on an outside wall, in a garage or in a utility room. The British Standards Institution’s fire-safety specification for home batteries, PAS 63100:2024, says they should not go in lofts, roof spaces or other voids, in bedrooms, or in escape routes such as the hallway or stairs you would use to get out in a fire. Your installer will check the location, and you should tell your home insurer.

Cold matters too: lithium batteries cannot charge below about 0 °C, so one on an outside wall or in a cold outbuilding without a built-in heater may not charge on frosty nights. Battery types, cold weather and safety explains the location rules, cold weather and the different battery types in full.

Rules and paperwork

• Installer: the Energy Saving Trust recommends getting at least three quotes from MCS-certified installers. MCS has a battery installation standard (MIS 3012).
• Network operator: a battery must be notified. Your installer submits a G98 notification within 28 days of installation, or applies for approval before installing under G99 if your solar and battery together exceed 16 amps per phase (about 3.68 kW on a normal supply). See G98 and G99 explained.
• VAT: installing a battery in a home has 0% VAT until 31 March 2027. Since 1 February 2024 this also covers batteries added to an existing system. It applies to installation, not to buying the materials alone.
• Plug-in solar: in Great Britain a plug-in solar kit must not be used with a battery. A home battery is a separate, professionally installed system. See Plug-in solar explained.
• Portable power stations: a portable battery with its own sockets needs no installer or notification, but powers only what is plugged into it and pays the normal 20% VAT. See Portable power stations and solar panels.
• Renting or leasehold: get written permission first. See Renting and solar panels.
• Insurance: tell your home insurer.
• Warranty: check how many years it lasts, how much of the original capacity is guaranteed to remain, and any limit on the number of cycles or the total energy put through it.