Methodology
How we work out and check our numbers
Every figure on this site is either sourced to a named publication with the date it was read, or is the output of a model whose assumptions are shown on the page and can be changed by the reader.
The rule
Every date, threshold and cap this site asserts is recorded in one register with the primary source it came from, who last read that source, and when. If a fact has not been read from the source, the page asserting it is automatically held out of Google's index and out of the sitemap — regardless of how finished the writing is.
What "AI-assisted primary-source check" means
It is the label on most rows in the register below, and it should mean something precise to you rather than sounding reassuring. It means the named source document was fetched and read, and the value in the register reflects what that document says — usually quoted verbatim on the page that asserts it. It is a genuine primary-source read, not a summary of somebody else's summary.
It is not a substitute for the named, credentialed human reviewer this site still does not have. Those are different things, and on content about money we would rather label them separately than blur them into the word "verified".
The pass on 23 August 2026 found five things we had wrong
The Warm Homes Plan does not open in January 2027 — it is already delivering. GBIS was scheduled to end in April 2026, not 31 March. Plug-in solar is capped at 800 VA rather than 800 W, with panels allowed up to 2,000 W, and compliance runs through the ENA Type Test Register rather than any "whole-kit certification". Warm Homes: Local Grant has three income pathways, not one. And the Future Homes Standard comes into force on 24 March 2027.
All five came from a research pass we had not yet checked. They are logged, dated, in the changelog — which is the point of having one.
That is enforced in code rather than by discipline, because discipline is what fails quietly. The same function that decides whether a page is indexable also decides whether it shows the reader a "not yet verified" notice, so the two cannot drift apart.
What counts as a source
The body that made the rule or published the data: gov.uk, Ofgem, DESNZ, MCS, legislation.gov.uk, the ICO, the FCA, or a supplier's own published tariff page. A competitor's guide, a news article about a scheme, and a comparison site are not sources — they are the thing this site exists to be better than.
The register, in full
Including any row we have not checked. A fact without a date in the last column is one we have not read from its source, and every page depending on it stays unpublished — which is exactly the failure mode we criticise elsewhere, so we would rather show you the queue than hide it.
| Fact | What we say | Primary source | Last read |
|---|---|---|---|
| ECO4 closing date | ECO4 was extended by nine months and ends on 31 December 2026. There is no ECO5, no increase in targets and no carry-over — the extension exists to let suppliers finish existing obligations and installers remediate non-compliant work. | DESNZ — Extending the ECO4 end date: government response | 2026-08-23 · AI-assisted primary-source check |
| GBIS end date | The Great British Insulation Scheme ran from April 2023 and was scheduled to end in April 2026. The government confirmed it would not be extended. It funded insulation measures, never solar. | Ofgem — Great British Insulation Scheme | 2026-08-23 · AI-assisted primary-source check |
| Warm Homes Plan | Published 21 January 2026. £15bn to upgrade up to 5 million homes and lift up to 1 million households out of fuel poverty by 2030. Delivery runs now through the existing Warm Homes Social Housing Fund and Warm Homes: Local Grant, which are to be integrated into a single low-income capital scheme from 2027/28. | DESNZ — Warm Homes Plan | 2026-08-23 · AI-assisted primary-source check |
| Warm Homes Fund consumer loans | Up to £1.7bn of the Warm Homes Fund facility goes to new low- and zero-interest consumer loans, with a further £300m to lower the cost of those loans. Eligible measures include solar panels and batteries. Rolling out in phases, with fuller eligibility detail still to be published. | DESNZ — Warm Homes Plan | 2026-08-23 · AI-assisted primary-source check |
| Warm Homes: Local Grant eligibility | England only, privately owned homes (owner-occupied or private rented), EPC band D to G. Income eligibility is met by any ONE of three pathways: living in an area within IMD income deciles 1–2 (no income check), meeting low-income proxies such as qualifying benefits, or a gross household income of £36,000 or less. | gov.uk — Apply for the Warm Homes: Local Grant | 2026-08-23 · AI-assisted primary-source check |
| 0% VAT on domestic solar | A zero rate applies to the installation of specified energy-saving materials, including solar panels, from 1 May 2023 to 31 March 2027, after which the 5% reduced rate applies. Electrical storage batteries have qualified since 1 February 2024, including standalone batteries charged from the grid. | HMRC — VAT Notice 708/6: energy-saving materials and heating equipment | 2026-08-23 · AI-assisted primary-source check |
| Smart Export Guarantee obligation | Licensed suppliers with 150,000 or more domestic electricity customers as at 31 December of the preceding year are mandatory SEG licensees and must offer at least one export tariff, at a rate above 0p/kWh at all times. Smaller suppliers may opt in voluntarily. The SEG year runs 1 April to 31 March. The rate itself is set by the supplier, not by Ofgem. | Ofgem — Smart Export Guarantee: electricity suppliers | 2026-08-23 · AI-assisted primary-source check |
| Smart Export Guarantee eligibility | Installations up to 5MW (50kW for micro-CHP) located in Great Britain. Solar PV, wind and micro-CHP up to 50kW require an MCS certificate or equivalent, and payments are calculated from export meter readings. | Ofgem — Smart Export Guarantee: generators | 2026-08-23 · AI-assisted primary-source check |
| Plug-in solar legalisation | The Plugs and Sockets etc. (Safety) Regulations 1994 and Electricity Safety, Quality and Continuity Regulations 2002 (Amendment) Regulations 2026 (SI 2026/848) were made on 16 July 2026 and came into force on 27 August 2026. | legislation.gov.uk — SI 2026/848 | 2026-08-23 · AI-assisted primary-source check |
| Plug-in solar product rules | Maximum apparent power 800 VA and maximum current 3.5 A, with PV module DC output capped at 2000 W. Battery systems and solar devices integrated with batteries are out of scope. The specification permits one device per household final ring circuit, but G98 Issue 2 Amendment 1 2026 currently restricts this to one device per household. Manufacturers must type test to G98 and register the device on the ENA Type Test Register before placing it on the market. | DESNZ — Plug-in Solar Device Interim Product Specification v2.0 (July 2026) | 2026-08-23 · AI-assisted primary-source check |
| DNO connection threshold | Generation rated below 3.68kW per phase falls under Engineering Recommendation G98; above that it falls under G99, which requires approval before installation. G98 applies to Great Britain only — Northern Ireland has separate connection requirements. | Energy Networks Association — connecting generation to the electricity networks | 2026-08-23 · AI-assisted primary-source check |
| MCS certification | MCS certification is not a legal requirement to install solar, but an MCS certificate or equivalent is required to claim the Smart Export Guarantee on a solar PV installation up to 50kW, and is required by most grant funding. | MCS — Smart Export Guarantee | 2026-08-23 · AI-assisted primary-source check |
| Planning permission for domestic solar | Installing solar on residential buildings and land may be permitted development with no application to the local authority, subject to limits and conditions. Listed buildings, conservation areas and flats are treated differently and should be checked before committing. | Planning Portal — solar panels | 2026-08-23 · AI-assisted primary-source check |
| Future Homes Standard | The Future Homes and Buildings Standards regulations come into force on 24 March 2027 (24 September 2027 for higher-risk building work), requiring new homes to have low-carbon heating, high energy efficiency and, in most cases, solar panels. Blocks of flats above 15 storeys are exempt from the solar PV requirement. | MHCLG — The Future Homes and Buildings Standards: Building Circular 01/2026 | 2026-08-23 · AI-assisted primary-source check |
| PECR penalties after the DUAA | The Data (Use and Access) Act 2025 received Royal Assent on 19 June 2025 and commences in phases. It aligns PECR enforcement with UK GDPR: the ICO can impose penalties up to £17.5m or 4% of global turnover, and the requirement to show substantial damage or distress has been removed. | gov.uk — Data (Use and Access) Act factsheet: PEC Regulations | 2026-08-23 · AI-assisted primary-source check |
How we model cost and payback
We do not publish a single payback figure, because there is not one. Payback depends on six inputs, the most important of which — how much of your generation you use at home rather than export — varies enormously between households on identical roofs.
So the model exposes all six, defaults them conservatively, and lets you change every one. Two things are deliberately left out: electricity price inflation and a discount rate. Both are defensible, both cut in opposite directions, and both are the levers by which a payback figure gets quietly engineered down to a marketable number.
- Annual yield — default 850 kWh per kWp per year. A south-facing, unshaded roof at a typical UK latitude. East/west split roofs lose roughly 15%, and heavy shading far more. Set your own figure if you have a real MCS estimate.
- Share of generation used at home — default 0.4 proportion. Without a battery, a household that is out on weekdays typically self-consumes 30–40%. A battery, an EV or someone home in the day pushes it to 60–80%. This is the input that moves payback most, which is why it is not hidden.
- Electricity you avoid buying — default 0.27 GBP per kWh. Roughly the price cap unit rate. Every unit you use yourself is worth this; every unit you export is worth the export rate, which is far less. That gap is the whole economics of a battery.
- Export rate — default 0.12 GBP per kWh. The best widely available untied flat rate at the time of writing. Check the current table at /smart-export-guarantee/rates/ — this number moves.
- Annual panel degradation — default 0.005 proportion per year. Typical manufacturer warranty terms imply about 0.5% a year.
- Inverter replacement — default 1200 GBP, once at year 12. Included because leaving it out overstates lifetime return. Set it to zero to see how much difference it makes.
How we handle location pages
A page about solar in a particular town exists only if it carries information no other page on this site has: the network operator that actually governs connections there, a yield figure for that latitude, notes on the local housing stock, and the local schemes or conservation constraints that apply. Without all four it stays unpublished.
The reason is simple. Sixty pages differing only by place name is thin content whether or not the prose is good, it is the fastest available way to get a young domain filtered, and it is no use to the person reading it.
Corrections
When a scheme changes we update the page and add a dated entry to the changelog on the funding tracker. The log is appended to, never rewritten — the archive of what changed and when is the credibility, and quietly editing history would forfeit it. If you think something here is wrong, tell us and cite the source; we would rather be corrected than ranked.