Solar battery storage: what a home battery does, in kWh
A battery moves electricity in time: from the middle of the day, or the cheap hours, to the evening when your home wants it. This guide works out how much that is for your home, season by season, and covers backup in a power cut, the fire-safety rules on where a battery can go, and the network paperwork. No prices, no sales pitch: energy, power and hours.
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Solar generationWhat the house usesBattery charge level, hour by hourDark hours
An illustration of one sunny day, not measured data. The battery fills from midday surplus, then carries the evening peak until it reaches the level it keeps in reserve.
9.6 kWh
average nominal size of a battery registered with MCS in an existing home, 2025/26 (DESNZ, our arithmetic)
2,500 kWh
a year: Ofgem's medium typical use on a single-rate meter, from 1 July 2026
3.5×
as much solar on a June day as a December day (PVGIS, Leicester)
01 The short answer
What home battery storage is for
In one paragraph: solar battery storage keeps electricity your panels make while you are out, or that a time-of-use tariff sells cheaply overnight, and gives it back when your home needs it, usually in the evening. It helps only with energy the home would otherwise export or buy at a dearer time, so its useful size is set by your evening and night use and by how much surplus there is to store, which changes a great deal between summer and winter.
Three setups cover almost every home. A battery added to panels you already have stores midday surplus. A battery fitted with new panels at the same time is designed with them, often around one hybrid inverter. A battery with no panels at all charges from the grid in cheaper hours. Any of the three can have backup for power cuts, but most do not by default.
Words that matter on every page
kWh: an amount of energy, the size of the battery's "tank".
kW: a rate of power, how fast energy can flow in or out.
Usable capacity is what you can draw out; nominal is the label size, which official statistics use.
Scroll through one sunny day. The same four moments happen in winter, but the midday surplus is much smaller and some days there is none.
Morning
The house takes the first solar
As the panels wake up, everything they make goes straight into the kettle, the fridge and whatever else is running. Nothing is left over yet, so the battery waits at the level it kept back overnight.
Midday
Surplus goes into the battery
Once the panels make more than the house is using, the extra charges the battery instead of flowing out to the grid. When the battery is full, the rest is exported, exactly as it would be without storage.
Evening
The battery covers the peak
Cooking, lighting and screens push use up just as the panels fade. The battery supplies the house until it reaches its reserve, which is the part of the evening a battery is really for.
Night
The grid takes over
With the battery at its reserve, the house buys from the grid as it would anyway. On a time-of-use tariff, some setups refill the battery from the grid in the cheap hours, ready for tomorrow.
Where the energy goes through one sunny day. Our illustration, not measured data. Full size
03 Your own figures
How much of your solar would stay at home?
Installers registered with MCS estimate the share of a year's solar a home uses itself, with and without a battery, using a lookup-table method. Put the two shares from your quote into the explorer, with your panel size and yearly use, and it shows what the battery would add in kWh.
Two limits apply whatever a quote says. We never show more than 75% of generation used at home without a battery, or 90% with one, and a home can never use more solar than it uses in total. A secondary source notes the lookup method does not cover generation above 6,000 kWh a year, use outside 1,500 to 6,000 kWh, or batteries above 15.1 kWh, so treat a quote's figures with care at those edges.
The surplus the battery cannot hold is still exported, and you can be paid for it under the Smart Export Guarantee if your installation and meter qualify. Exporting from a battery explains how storage changes that.
About 3,860 kWh of solar a year
A 4 kWp array near Liverpool makes about 3,860 kWh in a typical year on PVGIS's figures. On a June day that is roughly 15.5 kWh; on a December day roughly 4.4 kWh (Leicester's monthly profile, used as the example).
Add the two self-use shares from your installer's quote to see how much more of it a battery would keep in the home in year one.
04 The seasons
Why a battery that fills every summer day may not fill in winter
The European Commission's PVGIS tool puts Leicester's output at about 3.88 kWh per kWp of panels on a June day and 1.1 kWh in December, roughly 3.5 times more in midsummer. A 4 kWp array goes from about 15.5 kWh a day to about 4.4 kWh, while a medium-use home still uses about 6.8 kWh a day in both months, and usually more in winter.
So in December most of what the panels make is used as it is made, and there is little or nothing left to store. A battery sized to soak up every kWh of a June afternoon will sit partly empty for months. That is why we size from evening use first and surplus second. The winter and summer guide goes month by month, and what size battery turns your own figures into a usable size.
June and December days for 4 kWp of panels, against typical use. PVGIS and Ofgem figures; our illustration. Full size
05 Power cuts
Most home batteries switch off when the grid does
The Electricity Safety, Quality and Continuity Regulations 2002 require a source running in parallel with the network to disconnect itself when the network supply is lost (regulation 22). That protects people working on the lines. A battery with no backup function simply stops, however full it is.
Backup adds switching that first separates your home from the grid, which regulation 21 requires of a switched alternative source, and then lets the battery run chosen circuits until it reaches its reserve. How long that lasts depends on the usable energy above the reserve and the power of what you keep on. Backup in a power cut has a runtime estimator.
What happens in a power cut, with and without backup. Our illustration of ESQCR 2002 regulations 21 and 22. Full size
06 Safety and paperwork
Where it can go, and who needs to be told
PAS 63100:2024, the BSI specification sponsored by DESNZ for protecting homes from battery fires, prefers an outdoor spot or a separate outbuilding. It bars bedrooms, escape routes such as halls, landings and stairs, cupboards opening into bedrooms, lofts, roof spaces and voids. Indoors elsewhere, it asks for fire-resisting separation, ventilation to the outside and, in rarely visited spots, an alarm linked to the rest. It also caps storage at 20 kWh per enclosure and 40 kWh per home in most places (80 kWh outdoors or in a separate building). Where to put a home battery takes it clause by clause.
Every battery has to be notified to the local network operator. The law asks for the operator to be told before or at commissioning; in practice small systems use G98 within 28 days, and storage over 3.68 kW goes through G99 before installation. See G98 and G99 for a home battery. At handover you should receive the user instructions, including how to shut the battery down and what to do in a battery fire, and a statement that the installation conforms to PAS 63100.
07 Before you ask for quotes
What to know about your own home first
Your yearly electricity use, from a bill or the smart meter app, and roughly how much of it falls after dark.
If you have panels: their size in kWp and last year's export, from the export meter or Smart Export Guarantee statements.
Whether you want backup, and for which circuits.
Where a battery could physically go, judged against the rules above.
With those, you can read the technical side of any quote: usable kWh, power in kW, round-trip efficiency, coupling, warranty energy and the network route. Comparing battery specifications shows how. Questions about cost are covered by a separate price guide; this one stays with energy and fit.
Paying for storage, in three sentences: there is no national grant for a home battery anywhere in the UK. The Warm Homes Loan Scheme is borrowing that must be repaid. Warm Homes: Local Grant can fund a battery for some lower-income homes in England, but only alongside solar panels.
08 What households fit
The sizes people actually choose
DESNZ's statistics on batteries registered with MCS in existing homes give a picture of real choices. In 2025/26, 37.9% of the batteries in the cleaned sample were under 6 kWh nominal, averaging about 5.1 kWh; 33.7% were 6 to 11 kWh, averaging 9.8 kWh; and 28.4% were 11 kWh or more, averaging 15.5 kWh. The share of the biggest band rose from 20.6% the year before. Those are nominal sizes; the usable figure on the specification sheet is usually a little lower.
Popular is not the same as right for your home. A large battery that rarely empties, or rarely fills, is storage that does little. The guide to what households install sets the figures out in full, and the guide to how a solar battery works explains the energy flows behind them.
Asked a lot
Home battery storage: common questions
What does a solar battery actually store?
Electricity your panels make that the house is not using at that moment. Without a battery that surplus is exported; with one, it is kept for later in the day, usually the evening. A battery can also be filled from the grid on a tariff with cheaper hours.
Will a battery store all of my solar?
Rarely. It only stores surplus, only up to its usable capacity, and only after the house has taken what it needs. On long summer days a full battery means the rest is exported anyway. In midwinter there may be little or no surplus to store.
What size battery suits a typical home?
There is no single answer, but a useful starting point is the energy you use after dark. Ofgem puts a medium-use home on a single-rate meter at 2,500 kWh a year, about 6.8 kWh a day. The average battery registered with MCS in an existing home in 2025/26 was about 9.6 kWh nominal (DESNZ figures, our arithmetic).
Will a home battery keep my lights on in a power cut?
Only if it has a backup function. By law, a source that runs alongside the grid must disconnect when the grid supply is lost, so a standard battery switches off in a cut, however full it is. Backup adds switching that separates the home first, then runs chosen circuits.
Can a battery go in the loft?
No. PAS 63100, the fire-protection specification for batteries in homes, lists lofts, roof spaces and voids among the places a battery should not go, along with bedrooms and escape routes.
Does the network operator need to know?
Yes, always. Small systems are notified within 28 days of installation under G98; MCS says storage over 3.68 kW needs a G99 application before it is fitted. Your installer normally handles it.
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