Put a year of sunshine on one side of the scales and a year of household use on the other, and you get the most a Watford roof could ever give your car. On PVGIS figures, a 4 kWp south-facing array in Watford town centre makes about 3,938 kWh a year. Ofgem’s typical household uses 2,500 kWh. The difference, about 1,438 kWh, is the ceiling (our arithmetic). This guide explains why the real figure is lower, which months carry it, and which Watford households can use it at all.
The charger side, from the fuse to Hertfordshire’s rules on pavements, is on our page about EV chargers for Watford homes.
The year in one sum
PVGIS, the European Commission’s solar model, is run here for a 4 kWp array at a 35° tilt facing due south, with 14% system losses, at the Watford town-centre point. It gives about 3,938 kWh a year. Ofgem’s typical domestic consumption value from 1 July 2026 is 2,500 kWh a year for a medium household. Subtract one from the other and about 1,438 kWh is left, which on paper is about 24 charges of a 60 kWh car battery (our arithmetic).
That number is useful because it is an upper limit. Nothing a Watford household does with a 4 kWp roof can send more than that to the car, and two things pull the real figure down.
Why the ceiling is not the answer
First, timing. Panels produce in daylight; much of a household’s use comes in the evening, and the car is often out during the working day. The 9 kWh or so that a Watford roof can spare on an average June day arrives while the sun is up, and a surplus that comes at 1pm on a Tuesday only reaches the car if it is at home and plugged in, and if the charger setup can match its current to what the roof is producing. That needs a compatible combination of charger, settings and metering, which we confirm at the survey. Second, the seasons. The surplus does not arrive evenly across Watford’s year: it is concentrated in a few months, and in winter there is none.
June and July: where the surplus lives
PVGIS gives the Watford array about 476 kWh in June and 478 kWh in July, roughly 15.9 and 15.4 kWh a day. After the 6.8 kWh a day a typical household uses, about 9 kWh a day is left in June and about 8.6 kWh in July (our arithmetic). Nine kilowatt-hours is roughly an hour and a quarter of charging at 7 kW. The Met Office’s Northwood station, just south of Watford, averaged about 209 hours of sunshine in July over 1991 to 2020, making it the sunniest month there. If the car is at home on summer weekends, or you work from home, these are the Watford months where solar charging is real.
December and January: none to spare
The same array makes about 137 kWh in December and 156 kWh in January, roughly 4.4 and 5.0 kWh a day. Both fall below the house’s own 6.8 kWh, so nothing is left for the car (our arithmetic). Together, the two months give about 293 kWh, against about 420 kWh of typical household use over the same 62 days. Northwood averaged around 51 hours of sunshine in December. In those months, every kilowatt-hour in a Watford car comes from the grid. Every new home charger sold in Great Britain has had to be smart since 30 June 2022, so it can be set to charge in an overnight off-peak window, which is what does the work through the winter months.
Who in Watford can use a roof
The sums assume you control a roof and a parking space. Many Watford households do not. Of the borough’s 39,628 households, 28.8% live in purpose-built flats and 5.5% in converted or shared houses, and 28.2% rent privately (Census 2021). For them, panels are a decision for the freeholder or landlord, and a home charger needs a private or allocated space and their agreement. Renters and flat owner-occupiers with that parking may qualify for an OZEV grant towards the charger; our guide to who qualifies for the EV chargepoint grant explains it, and claims go through an OZEV-authorised installer, so tell us your situation at the quote stage.
For the 28.1% in semis and 10.5% in detached houses, a drive and a roof often come together. That is where the summer surplus is worth designing for. If your house is in one of Watford’s ten conservation areas, our page on solar panels in Watford covers the planning checks for the roof.
What to decide before a quote
For the semis and detached houses that make up 38.6% of Watford households (our sum of the Census 2021 shares), two questions settle most of it. Is the car usually at home in daylight from May to August? And would you rather the summer surplus went to the car, to the house in the evening through a battery, or out to the grid on an export tariff, whose rates are set by each supplier? If the car is mostly out by day, overnight off-peak charging will carry nearly all of your driving, and the roof’s job is the house. Our home EV charger installation page explains the charger we fit, the EnSmart SmartBee-M.
Sources: European Commission Joint Research Centre, PVGIS 5.3 (PVGIS-SARAH3 2005–2023; 4 kWp, 35° tilt, due south, 14% losses, building-mounted), Watford town-centre point, run 15 October 2026, annual, monthly and daily arithmetic ours; Ofgem, Typical Domestic Consumption Values from 1 July 2026; Met Office, Northwood location-specific long-term averages 1991–2020; ONS Census 2021, TS044 accommodation type and TS054 tenure, Watford (E07000103), via the Nomis API; The Electric Vehicles (Smart Charge Points) Regulations 2021; GOV.UK, Changes to electric vehicle chargepoint grant schemes from 1 April 2026 (updated 7 July 2026); Ofgem, Smart Export Guarantee.