Sources & Methodology

Ofgem default-tariff price cap, Direct Debit, average across England, Scotland and Wales, incl. 5% VAT: 24.67p/kWh for the cap period 1 Apr–30 Jun 2026. Standing charge 57.21p/day. Updates quarterly.

Source: Ofgem (verified 4 June 2026)

SEG rates are supplier-set, not regulated, so there is no single official figure. Typical 2026 fixed rates run ~3–15p/kWh; the best widely-available fixed rates (Octopus Outgoing, OVO, E.ON Next) reach ~12–15p/kWh. 13p/kWh is a reasonable typical/midpoint for a representative offer. Always quote it as supplier-dependent.

Source: Energy Saving Trust (verified 4 June 2026)

Rate used to convert an annual/monthly electricity bill to kWh. DECISION: set equal to electricityUnitPrice (the sourced Ofgem unit cap, 24.67p/kWh) rather than the legacy distinct 28p figure in lib/planner/estimate.ts. Using one sourced rate keeps the bill→kWh conversion consistent with the import rate the estimator values self-consumption at, and removes an unsourced constant. (A bill also includes a standing charge, so converting the WHOLE bill at the unit rate slightly underestimates kWh; acceptable for an indicative estimate and erring conservative. The wiring task may choose to net off standing charge first.)

Source: Ofgem (verified 4 June 2026)

0% VAT on installing solar PV and battery storage (energy-saving materials) in residential properties to 31 March 2027, Great Britain only; Northern Ireland differs. Reverts to the 5% reduced rate from 1 April 2027.

Source: HMRC / GOV.UK (verified 4 June 2026)

ECO4 (the supplier-obligation scheme for fuel-poor households) runs to 31 December 2026 after a nine-month extension. Most funding is allocated and new applications are limited late in 2026. No ECO5 successor; the Warm Homes Plan takes over.

Source: GOV.UK (verified 4 June 2026)

Government grant-funded programme (published 21 Jan 2026) replacing ECO4 from 2027. gov.uk headlines £15bn of public investment (the £13.2bn Spending Review commitment plus £1.5bn announced at the Budget) to upgrade up to 5 million homes and lift up to 1 million families out of fuel poverty by 2030, via insulation, clean heating and home upgrades. Funded by government grants/loans, not a supplier obligation.

Source: GOV.UK / DESNZ (verified 4 June 2026)

Solar PV funding cap is £6,000. CRITICAL: since 27 June 2023, solar PV (and battery storage) funding is available ONLY as part of a package with a heat pump or high heat retention storage heaters — solar-PV-only applications are no longer eligible. Of the £6,000, up to £1,250 can be a cashback grant when taken as part of a package; the remainder is an optional interest-free loan (not a standalone solar-PV cashback grant). Requires an MCS installer + MCS/Solar-Keymark product. Verify current caps via the Home Energy Scotland funding finder before quoting.

Source: Home Energy Scotland (Scottish Government) (verified 4 June 2026)

Welsh Government Warm Homes Nest scheme: a free package of energy-efficiency improvements (which can include solar PV, insulation, heating, heat pump) for eligible households. Means-tested: must own or privately rent, receive a means-tested benefit or be on a low income, and have an EPC of E(54) or worse (D(68) or worse with an eligible chronic health condition).

Source: GOV.WALES (Welsh Government) (verified 4 June 2026)

Industry-standard median module degradation is ~0.5%/year (underlying primary study: NREL, Jordan & Kurtz, "Photovoltaic Degradation Rates – An Analytical Review", 2013, ~2,000 systems, median 0.5%/yr — https://docs.nrel.gov/docs/fy12osti/51664.pdf). EST cited as the UK-facing source for typical lifetime/output retention. Premium modules ~0.3%/yr.

Source: Energy Saving Trust (verified 4 June 2026)

Share of generation used directly in the home for a solar-only system (no battery). ~0.30 is the standard UK baseline; the remaining ~70% is exported. EST uses comparable assumptions in its solar calculator.

Source: Energy Saving Trust (verified 4 June 2026)

Share of generation used in the home when a battery stores midday excess for evening use; ~0.65 typical. Actual figure depends on battery size vs. system size and household load profile.

Source: Energy Saving Trust (verified 4 June 2026)

EST: a typical domestic system is ~3.5 kWp and costs ~£6,100 fully installed (≈£1,740/kWp). Costs fall if scaffolding is already up or on a new build. Battery storage is extra.

Source: Energy Saving Trust (verified 4 June 2026)

SECONDARY / APPROXIMATE. No official published fixed fee exists for a self-installer obtaining MCS certification — MCS is an installer accreditation, and few certified installers will sign off a self-install. Real-world routes: pay an MCS installer a fee to commission/certify (hundreds of pounds and up), plus a DNO G98 notification (usually free) and Part P building-control notification (~£100–£300, England/Wales). ~£500 is an order-of-magnitude placeholder for the certification path; quote it as a rough range, not a fixed price. Some suppliers (e.g. Octopus) accept non-MCS installs for SEG for a ~£250 fee with electrical/building certificates.

Source: MCS (Microgeneration Certification Scheme) (verified 4 June 2026)