In an average June, a 4 kWp south-facing roof in Egham generates about 491 kWh. In an average December, the same roof makes about 138 kWh. Those two numbers, from our run of the European Commission’s PVGIS 5.3 tool at the Egham town-centre point, explain more about home batteries than any brochure does. This guide walks through what a battery is doing in each of those months and what that means for choosing its size. For the full local picture, including Egham’s housing and the network notification, see our page on battery storage in Egham.
The two months side by side
The PVGIS run assumes a 4 kWp array at 35 degrees, facing due south, unshaded, with 14% system losses, using satellite data from 2005 to 2023. It gives about 4,021 kWh a year in total, about 491 kWh in June and 494 in July, and about 153 kWh in January and 138 in December. On our arithmetic, December is about 28% of June. Per day, that is roughly 16 kWh in June against about 4 kWh in December. The Met Office’s Heathrow station, about five miles north-east of Egham, tells the same story in sunshine: around 1,675 hours a year on the 1991–2020 averages, peaking at close to 218 hours in July.
For scale, Ofgem’s typical domestic consumption value from 1 July 2026 is 2,500 kWh a year, about 6.8 kWh a day on our arithmetic. Over a year, an Egham roof of this size makes more than a typical home uses. Month by month, it does not.
June in Egham: the battery’s busy season
On a June day the panels produce far more than most households use while the sun is up, so the surplus either goes to the grid or into a battery. This is when storage earns most of its keep: daytime generation is held back for the evening, when the household would otherwise be buying from the grid. In Egham, where semi-detached homes are the largest single group (35.8% of households at the 2021 Census) and a typical two-slope semi takes something like 6 to 10 panels, the summer surplus is real but finite. The battery that captures it fully on an ordinary June day is the one worth paying for.
December in Egham: little to store
At about 4 kWh a day, a December roof of this size generates less than a typical household uses, and most of it is used as it is made. There is usually little or nothing left over to put into a battery. That is not a fault in the system; it is the physics of a short, low winter sun over the Thames valley. It is also why any claim that a battery makes an Egham home self-sufficient in winter should be treated as a red flag in a quote.
A battery can still work in winter, but on a different job. Paired with a time-of-use tariff, it can charge from the grid overnight at a cheaper rate and cover the expensive early-evening hours. Whether that pays depends entirely on the gap between your tariff’s rates, which is why we do not print figures for it. Our guide to battery storage without solar panels explains how that tariff arithmetic works.
What the gap means for battery size
Put the two months together and the sizing rule follows: size a solar battery to your summer evening use, not to winter. The Energy Saving Trust, in battery advice updated 19 August 2026, says there is no exact formula and gives a typical home system of perhaps 10 kWh, with a 5 kWh system at around £4,600. In June an Egham roof can usually fill a modest battery; in December it rarely can. A battery much larger than your June surplus spends most of the year part-empty, and you pay for every kilowatt-hour of capacity, used or not. Our guide on what size home battery you need goes through the method.
Two Egham details are worth adding. A quarter of households (25.7% at the 2021 Census) rent privately, so the landlord’s agreement comes before any of this. And the town-centre point sits in SSEN’s Southern Electric Power Distribution area, 3.2 km from UK Power Networks’ South Eastern boundary, so check which network your postcode is on; the installer notifies it either way.
Checking the seasons for your own roof
The PVGIS figures are for an ideal roof. Your own will differ with its direction, pitch and shading, and the year-to-year swing in UK weather is real. A good survey replaces the town estimate with one for your roof, then sets your summer surplus against your evening use to size the battery. Our solar panels Egham page covers the conservation areas, the housing mix and Solar Together Surrey, and the solar panels Surrey hub covers the county picture. If the seasonal arithmetic says a battery will not pay its way for you, we will say so.
Sources: European Commission JRC, PVGIS 5.3 (PVGIS-SARAH3, 2005–2023, run 15 October 2026); Met Office, Heathrow location-specific long-term averages (1991–2020); Ofgem, Typical Domestic Consumption Values (from 1 July 2026); Energy Saving Trust, Battery storage (updated 19 August 2026); ONS Census 2021, TS044 and TS054, Egham built-up area; NESO GB DNO licence area boundaries (May 2024) and UK Power Networks licence boundaries (December 2024).