Last reviewed: 3 September 2026. Default electricity price is the Ofgem price cap for 1 October to 31 December 2026 (26.32p/kWh, direct debit, GB average). Every assumption used is listed below the result — nothing here is hidden.
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| Generation before losses | — |
|---|---|
| After orientation, mounting and shading | — |
| After inverter clipping (800 W AC ceiling) | — |
| Used in the home (the only part worth money) | — |
| Exported and not paid for | — |
You are not paid for exported electricity. A self-installed plug-in kit has no MCS
certificate and no export meter point, so it cannot be registered for the Smart Export Guarantee.
Everything in the “exported” row above is given to the network for nothing. That is why the
single most valuable change you can make is free: run the washing machine, dishwasher or hot water
in the middle of the day.
The assumptions this calculator uses
All of them, in one place, so you can disagree with any of them.
- Regional yield (kWh per kWp per year). Scotland north 780 · Scotland central and south 830 · North East and North West England 850 · Yorkshire and the Humber 870 · Northern Ireland 870 · Midlands 900 · Wales 900 · East of England 950 · London and South East 970 · South West 1,000. These are our own rounded estimates, derived from the published range for Great Britain of roughly 750 to 1,050 kWh/kWp for a well-oriented array. They are not the MCS irradiance tables, and we do not present them as such.
- Orientation factor. South 1.00 · SE/SW 0.95 · E/W 0.80 · NE/NW 0.60 · North 0.45. Our own estimates.
- Mounting factor. Angled frame 1.00 · fence or ground at a shallow angle 0.95 · vertical balcony rail 0.70. Our own estimates. Vertical mounting is the hidden cost of the classic balcony install.
- Shading factor. None 1.00 · light 0.90 · moderate 0.75 · heavy 0.55. Your judgement, applied as a simple multiplier.
- Inverter clipping. Where panel capacity exceeds 1.0 kWp behind an 800 W AC inverter we deduct 3% for each additional 0.25 kWp, capped at 12%. Our own estimate of a real effect that only bites on sunny summer days.
- Self-consumption. Your selection. Exported units are valued at zero.
- Electricity price. Default 26.32p/kWh — Ofgem price cap, 1 October to 31 December 2026, direct debit, GB average. There is no VAT on domestic electricity in Great Britain from 1 October 2026 to 31 March 2027. Replace it with your own rate if you are on a fixed or time-of-use tariff.
- Not included: the daily standing charge (payable regardless), future energy price changes (we do not forecast), panel degradation, and any export income (there is none).
Background and worked examples: savings and payback · the complete plug-in solar guide · the rules · how we produce our figures. Sources: Ofgem price cap · Ofgem, Smart Export Guarantee · SI 2026/848.