Data sources
| Input | Value | Source | Updated |
|---|---|---|---|
| Electricity import rate | 26.32p/kWh (GB average, direct debit) | Ofgem default-tariff price cap Q4 2026 | Quarterly (each cap announcement) |
| SEG export rate | 6p/kWh default (3–15p typical range) | Supplier SEG tariffs (open rates) | As tariffs change |
| Regional sun hours | 840–1,113 kWh/kWp per year (12 UK regions) | PVGIS 5.2 (European Commission) | Annually |
| System cost | £1,600/kW installed | MCS installer data / market quotes | Annually |
| Battery cost | £4,000 for a typical 5 kWh home battery | Market quotes | Annually |
| VAT on solar installs | 0% (GB, to 31 March 2027) | HMRC Notice 708/6 | As policy changes |
Core formulas
annual generation (kWh) = system size (kW) × sun hours
Regional sun hours are PVGIS net generation figures (kWh per kWp per year), so they already cover inverter, temperature, cabling and soiling losses. No separate derate is applied.
self-consumed = generation × self-use ratio, where the ratio is 45% without a battery and 75% with one (capped at your actual annual usage).
annual savings = (self-consumed × import rate) + (exported × SEG rate)
simple payback = system cost ÷ year-one savings
For 25-year projections we apply 0.5% annual panel degradation and 3% annual rate inflation (conservative vs the historical 3–5%).
Calculator-specific notes
Savings calculator
Uses your region's PVGIS sun hours, the Ofgem import rate and a 6p SEG default. 25-year profit = Σ annual savings (degraded and inflated) − system cost.
ROI & payback calculator
Payback is simple payback (year-one savings). IRR is approximated by solving for the discount rate where 25-year NPV = 0.
How many panels
Sizes the array so annual generation covers a target share of your usage, using the same PVGIS net generation data.
Panel cost calculator
Price = kW × £1,600, with 0% VAT (GB) baked in.
Battery ROI
Compares self-consumption at 45% vs 75% to value each extra stored kWh at the import rate, against the battery's upfront cost and warranty life.
Solar EV charging
Uses real-world EV consumption (EV Database) against PVGIS generation to estimate solar coverage, comparing with petrol and grid charging.
Compare solar systems
Runs the same savings model for two configurable options side by side, highlighting the winner on cost, payback and 25-year profit.
What these estimates are not
- Not a quote. Installed prices vary by roof, scaffolding, region and installer — always get 3+ MCS-certified quotes.
- Not a guarantee. Generation depends on weather, shading and system health; savings depend on future electricity prices, which can fall as well as rise.
- Not financial advice. Figures are illustrative estimates, not regulated financial, tax or investment advice under FSMA.
How we stay honest
We update every calculator when Ofgem announces a new price cap (every three months) and re-verify sun hours, costs and tariffs at least annually. If you spot a number that looks wrong, email [email protected] — we check and correct promptly.
Sources: Ofgem (price cap), PVGIS 5.2 (European Commission), MCS, Energy Saving Trust, HMRC.
Ofgem price-cap figures are public sector information licensed under the Open Government Licence v3.0.