A south-facing 4 kWp array on a Romford roof makes about 489 kWh in June and about 143 kWh in December. That is the whole of this guide in one line: the same panels, on the same roof, produce roughly three and a half times as much in midsummer as in midwinter. The figures come from our run of PVGIS 5.3, the European Commission’s solar calculator, for Romford’s town centre. What follows is what that gap means for the size of the array and for a battery. For planning, Gidea Park’s Article 4 direction and the grant position, see our page on solar panels in Romford.
Every figure here is for an unshaded roof facing due south at 35°. Your roof will differ, which is what a survey measures.
Four months side by side
| Month | PVGIS output, 4 kWp | Share of the year (our arithmetic) |
|---|---|---|
| June | about 489 kWh | about 12% |
| July | about 496 kWh | about 12% |
| December | about 143 kWh | about 4% |
| January | about 159 kWh | about 4% |
| Whole year | about 4,055 kWh | 100% |
On our arithmetic, December is about 29% of June and January about 32% of July. For scale, Ofgem’s typical domestic consumption value from 1 July 2026 is 2,500 kWh a year, an average of about 208 kWh a month. In June the array makes more than twice that average; in December, well under it.
Why winter falls so far, but not to nothing
Romford’s nearest Met Office climate station is at Greenwich Observatory. Over 1991–2020 it averaged about 1,526 hours of sunshine a year, against about 1,403 for the UK as a whole, with July the sunniest month at about 209 hours and January the dullest at about 44. Sunshine hours in January are about a fifth of July’s, yet PVGIS output in January is about a third of July’s (our arithmetic on both sources). The difference is that panels keep generating from daylight on overcast days, at a lower rate. Our guide to whether solar panels work on cloudy days explains why.
Winter output also suffers from a low sun. A chimney, a neighbour’s extension or a tree that shades nothing in June can shade a panel for much of a December day. In a borough of semis and terraces, that is often the single biggest difference between the PVGIS figure and a real roof.
What the gap means for the size of the array
The tempting answer is to size panels to cover winter. It does not work on a house roof. To make about 208 kWh in December, the average monthly use above, an array would need to be around 5.8 kWp at the PVGIS rate for Romford, our arithmetic, and that assumes an ideal south-facing roof with no shade. In practice the roof sets the limit. At the 2021 Census, 41.2% of Havering’s households lived in semi-detached houses, more than double London’s share, and a semi’s main slope usually decides how many panels fit.
So the sensible approach runs the other way: fit what the usable roof sensibly takes, then judge it across the year. On these figures a 4 kWp array makes more than a typical household uses over twelve months, with the surplus concentrated between spring and autumn. That surplus is where export payments and a battery come in.
What the gap means for a battery
A battery moves daytime surplus into the evening. In June a Romford array makes far more around midday than most homes use, so there is plenty to store. In December there is little surplus at all. That is the honest reason a battery is sized to the evening load on a summer day, not to winter self-sufficiency, which no home battery delivers. The Energy Saving Trust says there is no exact formula for working out the size, and that a typical home system might be 10 kWh. Our guide to what size home battery you need sets out the method, and our page on battery storage in Romford covers how a battery fits a Havering home and its paperwork.
One more limit is worth stating: a standard grid-tied battery shuts down in a power cut. Backup is possible only where the system is designed and installed for it.
Winter is also when insulation earns its keep
Havering’s median EPC score is 67, band D, and the median semi-detached house scores just 61 (ONS, records to March 2025). Panels make the least electricity in the months when a leaky house loses the most heat. That does not argue against solar, but it does argue for looking at the loft and walls in the same conversation, because the season when panels help least is the season when insulation helps most.
Reading a generation figure on a quote
A quote should show its expected annual output and say how it was worked out: orientation, pitch, shading and the tool used. If it shows a monthly breakdown too, check that December looks like a fraction of June, as it does above. A winter figure close to summer’s is a reason to ask questions. For the wider picture across the area, see our solar panels in North London hub.
Sources: PVGIS 5.3, European Commission Joint Research Centre, PVcalc for Romford’s town-centre point (4 kWp, 35°, due south, 14% losses, building-mounted), PVGIS-SARAH3 2005–2023, run 15 October 2026; Met Office, Greenwich Observatory location-specific long-term averages, 1991–2020; Ofgem, typical domestic consumption values (from 1 July 2026); ONS Census 2021, TS044 Accommodation type (Havering; London); ONS, Median energy efficiency score, England and Wales (records to March 2025); Energy Saving Trust, Battery storage (updated 19 August 2026).