Off-grid solar calculator
How big a solar array and battery a UK home would need to run without the grid, month by month, and how much a generator or other source would still have to cover in winter.
Why winter decides everything
Solar output in the UK swings hugely through the year. Modelled yields per kWp of south-facing panels from the European Commission’s PVGIS tool:
| Area | June | December | December as % of June |
|---|---|---|---|
| London (E) | 123 kWh | 37 kWh | 30% |
| Cardiff (CF) | 131 kWh | 32 kWh | 25% |
| Birmingham (B) | 117 kWh | 33 kWh | 28% |
| Manchester (M) | 111 kWh | 28 kWh | 25% |
| Belfast (BT) | 116 kWh | 26 kWh | 23% |
| Edinburgh (EH) | 111 kWh | 22 kWh | 20% |
| Inverness (IV) | 107 kWh | 14 kWh | 13% |
Heating demand moves the other way: the government’s long-term average puts January at about 17 times July in heating degree days. That is why an off-grid home can run on spare solar for half the year and still come up short from November to February. In practice off-grid homes cover the gap with a generator, a wood stove or, where the site allows, wind or a small hydro scheme. Our guide to off-grid living in the UK compares the options and their costs.
How the calculator works
- Solar output for each month = array size × PVGIS monthly yield for your postcode area (south-facing, 35° tilt, 14% system losses).
- Demand for each month = your electricity use spread evenly, plus, with a heat pump, heat demand ÷ SCOP. We put 22.5% of heat demand down to hot water spread evenly, and spread space heating by the government’s 30-year average heating degree days.
- Covered by solar counts, month by month, the smaller of output and demand, less 10% for battery and inverter losses. Summer surplus cannot be carried into winter.
- Battery = average December daily demand × days of autonomy ÷ 0.9.
- Cost = £1,546 per kWp (the 2025/26 median for 4–10 kW systems in government MCS data) plus £400–£800 per usable kWh of battery, an indicative range. Generators, off-grid inverters, scaffolding and wiring are not included.
Monthly averages hide dull spells, so the real winter gap is larger than shown: our hourly modelling found that even 12 kWp with a 30 kWh battery left an all-electric home short on dozens of days a year. Treat the results as a best case. For a grid-connected home, use the solar panel calculator; for batteries, see home battery storage.
Frequently asked questions
Can solar panels and a battery run a UK home off grid?
For electricity alone, mostly yes, with a large array, a big battery and a backup generator for the darkest weeks. Heating a home with a heat pump off grid is much harder, because winter heat demand peaks just as solar output falls to a fraction of its summer level.
How many solar panels do I need to go off grid in the UK?
Enough to cover your worst months, not your yearly total. Use the calculator: in most of the UK, a home using 2,500 kWh of electricity a year needs several times more panel capacity to cover December than it does to cover the year on average.
How big a battery do I need for off-grid living?
Size it to carry you through a run of dull days. The calculator multiplies your average winter daily use by the number of days of autonomy you choose. Longer runs of dull winter weather are common, which is why off-grid systems keep a generator.
Why does the calculator show a winter shortfall?
Because December solar output in the UK is roughly a fifth to a third of June’s, while demand, especially for heating, peaks in winter. A month-by-month balance makes that visible; covering it with panels alone would need a very large array.
Does this include the cost of a generator and off-grid inverter?
No. We could not confirm typical UK prices for those, so the cost estimate covers panels and batteries only. Get quotes from MCS-certified installers.
General information, not advice; get several quotes from MCS-certified installers. Sources: PVGIS (European Commission JRC), DESNZ Energy Trends section 7 (weather), DESNZ solar PV cost data.