What size solar battery do I need? Sizing from your evenings
The right size of battery is the one that is regularly filled and regularly emptied. That comes down to two numbers from your own home: the energy you use when the panels are idle, and the surplus there is to store.
In short
Start from the energy you use after dark (or outside your tariff's cheap hours), in usable kWh. If you have solar, check it against last year's average daily export, and remember winter surplus is a fraction of summer's. The smaller figure is a sensible starting size; much bigger is storage that rarely fills or empties.
The sizing helper
The helper starts from Ofgem's medium typical use for a multi-rate meter, with the Economy 7 day share, so it shows a sensible figure before you type anything. Replace each value with your own: yearly use from your bill or smart meter app, your best estimate of the share used outside the cheap or sunny hours, and, if you have panels, last year's export.
The result is a usable size, the energy you can actually draw out. Specification sheets give it separately from the nominal size on the label, which is usually a little larger. Compare quotes on usable kWh.
It is a starting point for a conversation, not a recommendation: an installer who has seen your home and your consumption data can refine it.
About 5.4 kWh usable
Starting values: Ofgem's medium use for a multi-rate meter (3,400 kWh a year, about 9.3 kWh a day) with 58% used outside the night window, Ofgem's Economy 7 split. That is about 5.4 kWh a day a battery could carry.
Replace them with your own figures. If you have solar, add last year's export: a battery can only store surplus you have.
Step 1: how much you use, and when
Your bill or smart meter shows yearly use. If you don't have it to hand, Ofgem's typical domestic consumption values, in use from 1 July 2026, are a reasonable stand-in. They are typical households, not a rule for a given number of bedrooms.
| Household | Single-rate meter, a year | A day | Multi-rate meter, a year | A day |
|---|---|---|---|---|
| Low | 1,600 kWh | 4.4 kWh | 1,900 kWh | 5.2 kWh |
| Medium | 2,500 kWh | 6.8 kWh | 3,400 kWh | 9.3 kWh |
| High | 3,800 kWh | 10.4 kWh | 6,100 kWh | 16.7 kWh |
Next comes the share of that use falling when a battery could help: after the panels stop, or outside a tariff's cheap window. Ofgem's figures assume Economy 7 homes use 58% of their electricity by day and 42% at night; for a single-rate home there is no standard split, so use your own sense of how much happens in the evening. Cooking, lighting, screens and washing in the evening all count.
How the typical figures relate to your own
Ofgem's values are medians: the household in the middle, not an average lifted by the heaviest users. They have also come down. The medium single-rate figure was 2,700 kWh a year in the 2023 review and is 2,500 kWh now; the medium multi-rate figure went from 3,900 to 3,400 kWh, because measured household use has fallen. Ofgem revisits them about every two years and means them to be broadly representative, not exact.
Which? puts the average home at 8 to 10 kWh a day, or roughly 2,900 to 3,650 kWh a year: between Ofgem's medium and high single-rate values (our arithmetic). Ofgem's decision records a supplier's worry that the gap between median and mean use is widening and could grow as more homes charge an electric car, and a consumer group's point that multi-rate meters span low-use homes with storage heaters and high-use homes with a car on charge. A home with either may sit far from any typical figure, so start from your own readings.
Step 2: how much surplus there is to store
With solar, a battery can only store what the house is not using at the time. Last year's export, from the export meter or Smart Export Guarantee statements, divided by 365, gives an average daily surplus. That average hides the seasons: PVGIS puts Leicester's output at about 3.88 kWh per kWp on a June day and 1.10 in December, so summer surplus is several times winter's.
On the same PVGIS settings (due south, 35 degrees, 14% losses), a kWp in Leicester averages about 1.19 kWh a day in January, 2.69 in March, 3.71 in April, 3.06 in September and 1.47 in November. April to August each average at least 3.5, and together those five months produce close to three-fifths of the year's output; November to February produce about a sixth (our arithmetic from PVGIS's monthly totals). A whole-year average therefore overstates the winter surplus and understates the summer one. If your export records go month by month, check a size against March or September: storage that fills in those months will fill on most summer days too.
Without solar, the "surplus" is whatever the cheap hours allow: a battery charged overnight can take up to its usable size each night. A battery without panels covers that case.
Step 3: take the smaller figure, then sense-check it
The smaller of your evening use and your typical surplus is a sensible usable size to start from. A battery much bigger than your evening use will rarely empty; one much bigger than your surplus will rarely fill. Either way, part of it sits unused. Then check three practical limits:
- Power, not just energy. The battery's kW rating limits how much it can supply at once. A big evening cooking load can exceed it even when the battery is full.
- Storage caps. PAS 63100 allows no more than 20 kWh in one enclosure, and 40 kWh per home in most locations (80 kWh outdoors, in a detached garage or outbuilding, or in a garage separated to the required fire rating).
- The estimate's own limits. The MCS self-use lookup method, according to a secondary source, does not cover batteries above 15.1 kWh.
What other households choose
DESNZ's statistics on batteries registered with MCS in existing homes show the sizes people chose in 2025/26. Of 42,903 batteries in the cleaned sample, 37.9% were under 6 kWh nominal (average 5.1 kWh), 33.7% were 6 to 11 kWh (average 9.8 kWh) and 28.4% were 11 kWh or more (average 15.5 kWh). Across all of them the average was about 9.6 kWh nominal (our arithmetic from DESNZ's totals).
Those are useful as a reality check, not as a target. A household with a small evening load and a modest array may be well served by the smallest band; one with high evening use and a large array, or a heat pump and an electric car, may sit at the other end. What households install has the full figures and how they changed.
Signs a proposed size may not fit
- It is chosen from the panels' kWp alone, with no question about your evening use.
- The quote gives only a nominal size, with no usable figure.
- The self-use share with the battery is above 90% of generation.
- No one has asked where the battery would go or whether you want backup in a power cut.
Once you have a size in mind, read how to compare specification sheets so you compare like with like, and the seasons guide for how that size behaves through the year.
What this page is based on
- Ofgem decision on typical domestic consumption values (May 2026, in use from 1 July 2026), PDF (read 3 Nov 2026)
- PVGIS, the EU Joint Research Centre solar estimator (annual and monthly yields) (read 1 Nov 2026)
- PAS 63100:2024, fire protection for battery energy storage in dwellings (BSI, sponsored by DESNZ), PDF (read 3 Nov 2026)
- Easy PV help article on the limits of the MCS self-consumption method (secondary) (read 3 Nov 2026)
- DESNZ statistics on home batteries registered with MCS in existing homes (workbook to March 2026) (read 3 Nov 2026)
- GOV.UK guide to the Smart Export Guarantee (read 3 Nov 2026)
- Which? guide to solar panel battery storage (secondary, updated 30 Oct 2026) (read 3 Nov 2026)
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