How many solar panels do I need, and what will they cost?
Size the system from your annual electricity use, not your bedroom count. A UK home using 2,900 kWh a year needs roughly 2.8 kWp — about 7 panels at 400 W — which generates around 2,615–2,807 kWh a year on an unshaded south-facing roof and costs about £4,980 installed at the DESNZ median. The arithmetic is: annual kWh ÷ yield per kWp = kWp, then ÷ panel wattage = panels. The calculator below does it for your postcode and shows every step.
Three questions gets you an answer. Everything else is optional and only makes it sharper. Nothing is sent anywhere and no email is required.
Showing West Midlands as a default
Sets how much you use rather than export
7
panels at 400 W
2.8 kWp
system size
2,615–2,807
kWh a year (typical range)
£4,980
typical installed cost
Year-one benefit
£386
£319 saved on bills + £67 exported at 4.5p
Simple payback
12.9 years
Excludes degradation, price changes and inverter replacement
Covers about
42%
of your 2,900 kWh — the rest still comes from the grid
Generation is a range because weather varies year to year. The spread shown is PVGIS’s own standard deviation for West Midlands (±4%), not a guess. Roof space needed: about 16 m².
7 panels at 400 W, about 2,711 kWh a year in West Midlands. This is the specification to ask installers to price.
Price This SystemFree and no obligation. Your calculation is carried over. Enquiry consent is separate from marketing consent.
A reference table for an unshaded south-facing roof at a standard pitch in the West Midlands, using 400 W panels. Your own postcode changes the yield, which is what the calculator adjusts for.
| Household | Annual use | Panels | System | Generation | Cost |
|---|---|---|---|---|---|
| 1–2 people, no electric heating or EV | 1,800 kWh | 5 | 2 kWp | 1,868–2,004 kWh | £3,560 |
| 3–4 people, typical appliances | 2,900 kWh | 7 | 2.8 kWp | 2,615–2,807 kWh | £4,980 |
| 5+ people, or high appliance use | 4,100 kWh | 11 | 4.4 kWp | 4,110–4,410 kWh | £7,470 |
| Plus an EV or a heat pump | 6,000 kWh | 15 | 6 kWp | 5,604–6,014 kWh | £10,180 |
Household bands are HSG planning figures, not official statistics. The annual kWh on your own bill beats every one of them.
Annual output per kWp installed, south-facing at a standard pitch, from PVGIS. The ± figure is PVGIS’s own year-to-year standard deviation — real weather variability, not an allowance we invented.
| Region | kWh per kWp per year | Year-to-year variation |
|---|---|---|
| East of England | 1,057 | ±44 (4%) |
| South West England | 1,044 | ±36 (3%) |
| Wales | 1,042 | ±36 (3%) |
| South East England | 1,024 | ±38 (4%) |
| London | 1,019 | ±42 (4%) |
| East Midlands | 970 | ±30 (3%) |
| West Midlands | 968 | ±34 (4%) |
| North East England | 948 | ±33 (4%) |
| Yorkshire and the Humber | 947 | ±31 (3%) |
| Northern Ireland | 919 | ±28 (3%) |
| Southern Scotland | 903 | ±33 (4%) |
| North West England | 886 | ±30 (3%) |
| Northern Scotland | 811 | ±37 (5%) |
Both tables are for London and are indexed against a south-facing standard-pitch roof. Pitch is the input most calculators leave out entirely — a flat roof loses about 13% against a standard pitch, which is more than many people expect.
| Direction | kWh/kWp | vs south |
|---|---|---|
| south | 1,019 | 100% |
| south-east | 968 | 95% |
| south-west | 947 | 93% |
| east | 817 | 80% |
| west | 787 | 77% |
| north | 541 | 53% |
| Pitch | kWh/kWp | vs standard |
|---|---|---|
| Flat (about 5°) | 885 | 87% |
| Shallow (about 20°) | 976 | 96% |
| Standard (about 35°) | 1,019 | 100% |
| Steep (about 50°) | 1,011 | 99% |
| Input | Value | Source and basis |
|---|---|---|
| Solar yield | 811–1,057 kWh/kWp | European Commission JRC, PVGIS v5.3, PVcalc. PVGIS-SARAH3, 2005-2023, 14% system losses. 116 postcode areas × 6 orientations × 4 pitches. |
| Installed cost, up to 4 kW | £1,780.25/kW | DESNZ Solar PV cost data, 2025/26 financial year, 265,682 installations analysed. Declared Net Capacity basis. Median, not a cap: half of installations cost more. England, Scotland and Wales only. |
| Installed cost, 4–10 kW | £1,696.97/kW | DESNZ Solar PV cost data, 2025/26 summary table, 4-10 kW band. Declared Net Capacity basis. Lower per kW than the 0-4 kW band because scaffolding, the DNO application, the inverter and the labour day do not scale with panel count. |
| Electricity price | 26.11p/kWh | Ofgem energy price cap — unit rates and standing charges, 1 July 2026 to 30 September 2026 (Direct Debit, Great Britain (England, Scotland, Wales) average) |
| Export price | 4.5p / 12p / 15p | Market observation of published SEG tariffs — not a regulated rate. Ofgem obliges larger suppliers to offer one but sets no minimum, so the spread is wide. Yours to choose in the tool; it defaults to the conservative end. |
| Self-consumption | 35–55%, more with a battery | HSG planning model, set by daytime occupancy and then scaled down where the system would generate more than the household uses. A planning assumption, not a measurement of your home. |
| Shading | 1 / 0.92 / 0.78 / 0.6 | HSG planning model. The weakest input here — only a site survey settles real shading loss. |
| VAT | 0% | Reverts to 5% from 1 April 2027 unless extended. Verify before Q1 2027 content. |
Model version 2.0.0. Energy prices last verified 2026-08-07, next check 2026-09-15, because the Ofgem cap changes quarterly. Yield matrix generated 2026-08-08.
Generation uncertainty and financial uncertainty are different things, and collapsing them into a single “±10%” would be misleading. Each output below has its own source of error.
| Output | Main source of error | How we show it |
|---|---|---|
| Annual generation | Weather varies year to year. Separately, we place you at your postcode area’s reference point rather than your exact coordinates — which we have measured against location-specific PVGIS results rather than assumed. | A published range using PVGIS’s own year-to-year standard deviation for your postcode area, orientation and pitch — typically 3–5%. Our location approximation adds a measured median of 0.3%, a 90th percentile of 1.7% and a worst case of 3.0%across 20 UK towns, so weather remains the larger uncertainty. |
| Installed cost | It is a national median, so half of real installations cost more. Scaffolding, roof access and equipment choice move individual quotes a long way. | A single median figure with the band and basis named. We deliberately do not invent a ± range around it — the honest test is to compare your own quote, which the tool lets you do. |
| Year-one saving | Dominated by two assumptions, not by the physics: what share of generation you use yourself, and what your export tariff pays. Both are larger sources of error than the generation estimate. | Self-use and export are shown separately so you can see which is doing the work, and the export tariff is yours to change. |
| Simple payback | Inherits every error above, then ignores degradation, future electricity prices and an eventual inverter replacement. | Presented as simple payback only, with the exclusions stated. Treat it as a comparison tool between options, not a forecast. |
The single largest lever is not the panels. It is what share of your generation you use yourself and what you are paid for the rest — which is why those are inputs here rather than fixed constants.
Being straight about the limits is more useful than a confident number:
We do not sell solar panels and we are not owned by an installer. If the numbers do not work for your property, this calculator will say so.