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How is solar payback calculated and what changes it?

Short answer

Payback is the installed cost divided by the yearly benefit. The benefit is self-used kWh × import price plus exported kWh × export rate, less upkeep. EST estimated 9–12 years with export payments across London, Manchester, Aberystwyth and Stirling (July 2026 prices). Cost, orientation, self-consumption, tariffs and inverter replacement move it most.

Last checked 24 September 2026 · 3 sources

In more detail

  • Inputs: kWp × Kk × SF (generation); MCS self-consumption fraction; import unit rate (26.32p Oct-Dec 2026 cap); export rate (4-15p); installed cost (DESNZ median £1,546-£1,595/kW TIC 2025/26); degradation ~0.5%/yr; inverter replacement ~£900 at ~year 12.
  • Standing charge (54.83p/day Oct-Dec 2026) is paid regardless, so it isn't a saving.
  • Which? (Sept 2026): rooftop 10.9-12.2 years; plug-in 3.6-7.1 years.
  • Payback falls when prices rise or export rates improve, and rises with shading, a north roof or low daytime use.

The facts, dated

  1. 01
    EST payback with export: London / Manchester / Aberystwyth / Stirling — 9 / 10-11 / 9-10 / 11-12 years

    market_data · GB · checked 24 September 2026 · Energy Saving Trust

  2. 02
    DESNZ 2025/26 median cost 0-4 kW (TIC) — 1,595 GBP/kW

    market_data · GB · checked 24 September 2026 · GOV.UK

  3. 03
    DESNZ 2025/26 median cost 4-10 kW (TIC) — 1,546 GBP/kW

    market_data · GB · checked 24 September 2026 · GOV.UK

  4. 04
    Ofgem cap electricity standing charge Oct-Dec 2026 — 54.83 p/day

    regulation · GB · checked 24 September 2026 · Ofgem

What we saw in the field

[QA review 2026-09-24] H-3 AGREES (plausible); H-8 AGREES; I-5 UNVERIFIABLE; I-6 UNVERIFIABLE for whole systems. | Earlier notes: Homeowner (late 2022, ~£19,000 for 5.6 kWp + 15 kWh battery, 7-10 year payback expected). DESNZ DNC 2022/23 median for 4-10 kW was £2,080/kW, so ~£11.6k for the PV. The remaining ~£7k for a 15 kWh battery is at the low end of published battery prices (EST ~£920/kWh for a 5 kWh system in 2026) but plausible for a hybrid package, so the total is PLAUSIBLE. A 7-10 year payback is at the optimistic end of EST's 9-12 years (2026). It was plausible with 2023's high import prices and a Flux export rate. 'Less favourable now as export rates fell' AGREES with Outgoing Octopus dropping 15p→12p on 1 Mar 2026. Installer 'panels cheaper than fence panels' is not testable against a primary source. 'Systems last ~30 years' is broadly consistent with EST's '25 years or more'.

Sources

Last checked: 24 September 2026