How we calculated

On our calculation, a 4 kWp home solar system is worth around £523 to £603 a year with a south-facing roof and around £435 to £495 with an east-facing roof, if you also buy your electricity from the supplier that pays you for exports. That counts bill savings at the Ofgem price cap for October to December 2026 (26.32p per kWh, with no VAT on electricity in that period) and Smart Export Guarantee payments at 12p per kWh, the typical rate for a supplier's own customers. If you sell your exports at a rate open to anyone (around 4p per kWh), a south-facing roof is worth around £299 to £333. We assume a household using 2,500 kWh a year (Ofgem's medium typical consumption) that is at home for half the day (the middle of three occupancy profiles) and uses about 18% to 23% of its solar electricity itself, the share in the self-consumption tables certified installers use, across London, Manchester, Edinburgh and Cardiff (European Commission PVGIS). The full working is below.

How the calculation works

There is no official figure for what solar panels are worth to a UK household, so we show the full working:

Generation: the European Commission's PVGIS (version 5.3, SARAH3 data, crystalline silicon, 14% system losses, 30 degree slope) gives, per kWp a year, 1,010.28 kWh in London, 879.60 in Manchester, 892.96 in Edinburgh and 1,034.04 in Cardiff on a south-facing roof, and 826.74, 726.24, 713.04 and 823.24 kWh on an east-facing roof. A 4 kWp system in London facing south generates 1,010.28 × 4 = 4,041 kWh a year. We leave out Northern Ireland because the Ofgem price cap covers Great Britain only.
Bill savings: every kWh you use from your panels is a kWh you do not buy. We value it at the Ofgem price cap unit rate for 1 October to 31 December 2026, 26.32p per kWh, with no VAT on electricity from 1 October 2026 to 31 March 2027. The standing charge does not change with solar, so we leave it out. We use the current quarter because the unit rate is what a capped household pays today.
Household: 2,500 kWh of electricity a year, Ofgem's medium typical domestic consumption value in its summary of the October to December 2026 price cap.
Self-consumption: the share of solar electricity the household uses itself comes from MCS's look-up tables (MGD 003) for a home using 2,500 to 2,999 kWh a year with panels only, no battery. For a household at home for half the day the share is 22.6% for 2,700 to 2,999 kWh of generation, 21.2% for 3,000 to 3,299 kWh, 20.0% for 3,300 to 3,599 kWh and 18.0% for 3,900 to 4,199 kWh. MCS's installation standard (MIS 3002 Issue 6.0, §4.1.2) requires certified installers to estimate self-consumption with these tables. The standard sets no default when occupancy is unknown, so choosing "in half the day", the middle of the three profiles, is our assumption.
Export payments: everything you do not use is exported. On 7 October 2026 we read the Smart Export Guarantee rates of eight suppliers on their own websites. The six rates offered only to customers who also buy their electricity from that supplier are 12p, 12p, 12p, 12p, 13p and 13p, a median of 12p. The eight rates open to anyone are 2p, 3p, 3p, 4p, 4.1p, 4.5p, 6p and 6p, a median of 4.05p.
The sum: annual benefit = generation × (self-consumption share × 26.32p + the rest × export rate). London, south-facing, at home half the day: 4,041 kWh generated; 18.0% self-used is 726 kWh × £0.2632 = £191; 3,315 kWh exported × £0.12 = £398; total £589 a year.
What moves the result: the occupancy profile moves a south-facing result by about £25 a year either way, while the export rate (12p against 4.05p) moves it by about £225 to £270.
Payback is the installed cost divided by the annual benefit. For the cost we use the Department for Energy Security and Net Zero's solar PV cost data for 2025/26, average installed cost for retrofitted home systems: £1,651.40 per kW for 4 to 10 kW and £2,108.65 per kW for 0 to 4 kW. A 4 kWp system sits on the boundary, so we use both: 4 × £1,651.40 = £6,606 and 4 × £2,108.65 = £8,435.
Cross-checks: the DESNZ median for all 0 to 4 kW installations, £1,595.06 per kW, is lower because it includes non-domestic and new-build installations, which cost less per kW. The Energy Saving Trust gives a payback of 9 to 12 years on July 2026 fuel prices for a system of around 4.5 kWp costing around £7,600; it does not publish its self-consumption assumption, and its cost per kW is lower than the DESNZ retrofit averages.

The full figures

4 kWp, no battery, a household using 2,500 kWh a year, Ofgem price cap 26.32p per kWh. Figures are rounded; the central case is "at home for half the day".

London, south-facing: 4,041 kWh generated, 18.0% self-used (726 kWh). £589 a year at 12p (£611 if at home all day, £562 if out during the day); £325 at 4.05p.
London, east-facing: 3,307 kWh generated, 20.0% self-used (660 kWh). £491 a year at 12p; £281 at 4.05p.
Manchester, south-facing: 3,518 kWh generated, 20.0% self-used (702 kWh). £523 a year at 12p (£544 at home all day, £497 out during the day); £299 at 4.05p.
Manchester, east-facing: 2,905 kWh generated, 22.6% self-used (658 kWh). £443 a year at 12p; £264 at 4.05p.
Edinburgh, south-facing: 3,572 kWh generated, 20.0% self-used (713 kWh). £531 a year at 12p (£552 at home all day, £505 out during the day); £303 at 4.05p.
Edinburgh, east-facing: 2,852 kWh generated, 22.6% self-used (646 kWh). £435 a year at 12p; £259 at 4.05p.
Cardiff, south-facing: 4,136 kWh generated, 18.0% self-used (743 kWh). £603 a year at 12p (£626 at home all day, £575 out during the day); £333 at 4.05p.
Cardiff, east-facing: 3,293 kWh generated, 21.2% self-used (697 kWh). £495 a year at 12p; £289 at 4.05p.
Export rates for a supplier's own electricity customers, read 7 October 2026: Octopus Energy Outgoing Octopus 12p, ScottishPower SmartGen Premium 12p, British Gas Export Premium 12p (systems up to 15 kW), OVO SEG Beyond Exclusive 12p, EDF Export 12m V2 13p, E.ON Next Export Exclusive v3 13p.
Export rates open to anyone, read 7 October 2026: Utility Warehouse 2p, British Gas Export SEG 3p, EDF SEG Export Variable 3.0p, OVO SEG 4p, Octopus SEG 4.1p, So Energy So Export Flex 4.5p, ScottishPower SmartGen 6p, E.ON Next Flex Export v1 6p. Deals tied to buying the panels or a bundle from the supplier are left out.
Payback at 12p, at home half the day, on £6,606 to £8,435: south-facing 11.0 to 16.1 years (Cardiff 11.0 to 14.0, London 11.2 to 14.3, Edinburgh 12.4 to 15.9, Manchester 12.6 to 16.1); east-facing 13.3 to 19.4 years. For London south-facing: £6,606 ÷ £589 ≈ 11.2 years and £8,435 ÷ £589 ≈ 14.3 years.
Payback at 12p, south-facing, by occupancy: 10.6 to 15.5 years at home all day, 11.5 to 17.0 years out during the day.
Payback at 4.05p, at home half the day: south-facing 19.8 to 28.2 years, east-facing 22.9 to 32.6 years.
Cross-check at the DESNZ overall median (£1,595.06 per kW, £6,380 for 4 kWp): south-facing 10.6 to 12.2 years at 12p.

Caveats

The 12p export rate is only for customers who also buy their electricity from that supplier, so you are tied to that supplier's import tariffs (some smart tariffs are excluded) and may not be on the cheapest import deal. Rates open to anyone are much lower.
At 12p, payback is around 11 to 16 years with a south-facing roof and 13 to 19 years east-facing, on DESNZ's 2025/26 average installed costs for retrofitted home systems (£1,651.40 to £2,108.65 per kW). At an export rate open to anyone (around 4p) it is around 20 to 28 years south-facing, which approaches or exceeds the working life of the panels (Energy Saving Trust: 25 years or more). Payback is not guaranteed.
Self-consumption comes from the industry look-up tables for a home without a battery. Being at home all day adds about £20 to £25 a year on a south-facing roof, and being out during the day takes about £25 to £30 off. A battery changes both the cost and the self-consumption share, so quotes with storage need their own comparison.
Roof orientation matters more than location, and shading lowers the result further; we do not model shading.
Electricity prices change every quarter with the Ofgem price cap, and the benefit moves with them. VAT on electricity returns from April 2027.
Smart Export Guarantee rates are set by each supplier and can change without notice; they must stay above zero, but choosing a good export tariff is a large part of the return.
VAT on supply-and-install of solar panels is zero-rated from 1 May 2023 until 31 March 2027, then reverts to the reduced rate of 5%. The zero rate covers homes and buildings used solely for a relevant charitable purpose, not ordinary commercial buildings. Supply-only purchases are standard-rated.
The figures cover Great Britain; the Ofgem price cap does not apply in Northern Ireland.
All figures on this page are for a domestic system. We publish no benefit, payback or cost figure for commercial installations, whose size, tariffs and VAT treatment differ.

Sources

Last updated: 8 October 2026. Next planned update: quarterly, alongside Ofgem price-cap changes and a fresh read of the export rates; the DESNZ cost data is refreshed yearly, and VAT changes on 1 April 2027.