Solar Panel Calculator

Generation = system size in kWp × yield per kWp × orientation factor × shading factor. A UK roof produces around 950 kWh per kWp a year on average, more in the south and less in Scotland. A 4kWp system on an unshaded south-facing roof therefore makes about 3,800 kWh a year.

Every kWh used in the house saves 26.32p at the price-cap rate. Every kWh exported earns about 13p under a Smart Export Guarantee tariff. So a unit you use is worth roughly double a unit you sell, which makes self-consumption the biggest lever on payback after the roof itself.

The calculator scales output against a due-south roof: south-east or south-west keeps 95%, east-west 85% and north only 62%. For the mixed household above, an east-west roof stretches payback to about 11.4 years and a north roof to about 15.6 years. Direction is fixed before you get a quote, so check it first.

Panels are wired in strings, so a chimney or tree shading part of the array can drag down more than its share. Light shading costs about 5% here, some shading 15% and heavy shading 30%. Microinverters or optimisers reduce the effect, at extra cost.

Payback is install cost divided by yearly benefit, so a quote £1,000 lower takes about a year and a half off for the mixed household. Get at least three MCS-certified quotes, and compare them per kWp rather than as totals, because system sizes differ.

A domestic quote covers the panels, an inverter, mounting rails, scaffolding, wiring, the generation meter and registration with your network operator. Scaffolding is a fixed cost whatever the system size, which is one reason larger systems usually cost less per kWp.

The calculator starts at £6,500 as a typical figure. Replace it with your actual quote — it is the input that matters most for the answer.

A battery stores midday surplus for the evening. The calculator adds about 20 percentage points of self-consumption for one, capped at 85%. For the mixed household that raises the yearly benefit from £671.16 to £772.39 — an extra £101.23. So the battery has to cost less than about £1,000 for every ten years it lasts just to break even. Switch it on and off above and compare.

Run the dishwasher, washing machine and immersion heater in the middle of the day. Each kWh moved from the evening to midday is worth the gap between the unit rate and the export rate — 13.32p at these rates. The appliance running cost calculator shows which appliances use enough to be worth moving.

An electric car charged at home during the day, or a heat pump running on sunny spring days, can soak up surplus that would otherwise be exported for 13p. See the EV charging cost calculator and our guide to whether solar is worth it for the four numbers that decide.

A 4kWp system makes about 3,800 kWh a year, more than the 2,700 kWh a typical home uses. But most of it is generated at midday in summer, when usage is low. For the mixed household, the 1,330 kWh used at home covers about 49% of annual demand — the rest still comes from the grid in the evenings and in winter.

That is why a bigger system does not always pay back faster: the extra output is mostly exported at the lower rate.

Modern domestic panels are typically around 400W each, so 4kWp is about ten panels. Each is roughly 1.7 m², so allow about 18 m² of unshaded roof, plus clearance at the edges. Split roofs work, but each face should be entered with its own direction.

The Smart Export Guarantee requires larger suppliers to pay for what you export, but they set their own rates, and the best ones often require you to buy your electricity from them too. Compare the export rate and the import tariff together before choosing.