The sunshine record kept at Rothamsted, just west of Hatfield, says something simple about the year. Over 1991–2020 the Met Office station there averaged around 209 hours of sunshine in July and around 56 in December, so the darkest month gets roughly a quarter of the brightest (our arithmetic). PVGIS, the European Commission’s solar tool, gives a 4 kWp array in Hatfield about 484 kWh in July and about 135 kWh in December, a similar ratio. Two separate records, one shape, and it is the shape any battery in AL9 or AL10 has to live with.
This guide turns those months into daily figures and shows what they mean for battery size. For where a battery goes in a Hatfield home and how it connects to the network, see our page on battery storage in Hatfield.
Four months on one roof
PVGIS 5.3 estimates about 3,965 kWh a year from a 4 kWp array facing due south at 35° in Hatfield, with 14% system losses, using the satellite record for 2005 to 2023. Spread across the days of each month, the high and low points look like this (our arithmetic, unshaded roof):
| Month | PVGIS output | Average per day |
|---|---|---|
| June | about 478 kWh | about 15.9 kWh |
| July | about 484 kWh | about 15.6 kWh |
| December | about 135 kWh | about 4.4 kWh |
| January | about 154 kWh | about 5.0 kWh |
December’s daily figure is about 28% of June’s. Panels generate from daylight rather than direct sunshine alone, which is why the output gap is a little gentler than the sunshine gap, but the winter trough is real on every roof in the town.
Where a household’s own use sits
Ofgem’s typical domestic consumption value from 1 July 2026 is 2,500 kWh a year for a medium, single-rate home, about 6.8 kWh a day (our arithmetic). Your own figure may be higher with an electric car or electric heating, or lower in a one-bedroom flat. Either way, the comparison is the point. On an average June day the 4 kWp roof makes more than twice the typical household’s use. On an average December day it makes less than that household’s use, and most of it arrives while people are out.
June: more than the house can take
In June the panels produce most of their output between mid-morning and late afternoon. A household out at work or study uses only a slice of it then. The rest either goes to the grid or into a battery. On paper, an average June day in Hatfield can fill a 10 kWh battery with output to spare; on a cloudy June day, or under a neighbour’s tree, it will not. Treat the June figure as a ceiling, not a promise.
December: a different job
In December the same roof averages about 4.4 kWh a day and the house uses most of it as it is generated. A battery sized to catch June’s surplus will often start a December evening close to empty. That is not a fault in the battery; it is the season. If your tariff has a cheaper overnight rate, a battery can still charge from the grid then and cover the dearer evening, which is the same way a battery works with no panels at all. Our guide to battery storage without solar panels explains when that arithmetic works.
A smaller roof shrinks both seasons
Hatfield is not a town of big roofs. Across its four wards, 33.4% of the 12,793 households live in terraced houses, 27.5% in purpose-built flats, 25.1% in semis and 11.8% in detached houses (our sum of the ONS Census 2021 ward figures). A terrace roof often takes fewer panels than a 4 kWp array needs. Scaling PVGIS in proportion, a 3 kWp array would make roughly 359 kWh in June and 101 kWh in December, about 12.0 and 3.3 kWh a day (our arithmetic). A smaller array cuts the summer surplus a battery feeds on far more, in kWh, than it cuts the winter output. That is an argument for a smaller battery on a smaller roof, not the same battery as a detached house.
Size for the evening
The rule that follows is plain. Size a battery to what your home uses between late afternoon and bedtime, and to what the array can spare on an ordinary spring or summer day, not to winter self-sufficiency. The Energy Saving Trust says there is no exact formula, puts a typical home system at 10 kWh and a 5 kWh system at around £4,600, and gives a typical lifespan of about 10 to 12 years. Our guide on what size home battery you need shows how to read the evening figure from your own bills or smart-meter data.
Cars change that evening figure. DVLA records 3,464 privately kept plug-in vehicles across Welwyn Hatfield at the end of June 2026, up from 2,541 a year earlier. A car charged overnight adds to overnight use rather than to the evening peak a house battery covers, so mention the car at the survey and we size around it.
Before the numbers: who decides
Only 44.0% of households across Hatfield’s four wards own their home, while 27.5% rent from a social landlord and 26.8% rent privately (our sum of the ward figures). For more than half the town, the landlord or freeholder decides whether a battery goes in, so that conversation comes before any sizing.
On the network side, Hatfield’s town centre sits in UK Power Networks’ Eastern Power Networks licence area. A system up to 16 A per phase (3.68 kW) is notified under G98; anything larger needs a G99 approval before work starts, and a 4 kW inverter, at 17.39 A, is just over. A standard grid-tied battery also shuts down in a power cut unless backup has been designed in. We are a NAPIT member (73495), MCS-certified for solar PV and battery storage through NAPIT. The roof side is on our page for solar panels in Hatfield, and the wider county on our Hertfordshire solar page.
Sources: European Commission JRC, PVGIS 5.3, PVGIS-SARAH3 2005–2023, 4 kWp, 35°, due south, 14% losses, building-mounted (run 15 October 2026); Met Office, Rothamsted 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, accommodation type and tenure (TS044 and TS054 categories), May 2022 wards Hatfield Central, East, South West and Villages, via the ONS Census API, sums ours; DfT and DVLA, VEH0142 licensed plug-in vehicles, private keepers, Welwyn Hatfield (published 23 September 2026); UK Power Networks Open Data, licence boundaries; Energy Networks Association, EREC G98 Issue 2 (10 March 2025).