Methodology

Our rules

Seven rules govern everything on this site. They are not aspirations, and most of them exist because we got something wrong first.

1. Every figure is real, sourced and dated. Nothing is estimated, illustrative or filled in from memory and presented as fact. If a number cannot be sourced, it does not go on the page. Where sources conflict, we publish the conflict rather than picking one and presenting it as settled — that disagreement is usually the most useful thing on the page.

2. Every page says when it was last checked. Not when it was published, and not when a word was changed: when a person last read the sources and confirmed the page still matches them. Pages that go unchecked for more than 90 days are flagged automatically, and the badge on the page changes colour.

3. We name no private individuals. Not installers, not company directors, not people named in rulings or judgments. Public officials acting in an official capacity, sourced to an official publication, are the single exception.

4. Answer first. Every page opens by answering its own title directly, in about fifty words, before any context or qualification. If the honest answer is “probably not, for you”, that goes first too.

5. Corrections are public. We do not quietly edit. Every correction is dated and logged.

6. Where a page tells someone to write a letter, we write the letter. If our guidance says “put the request in writing”, leaving the reader to work out the wording is an unfinished job. The templates are free, ungated, traceable clause by clause to the law they rest on, and honest about what they cannot do.

7. Cite the issue number, never just the URL. Standards get revised and the publisher’s own website is not a reliable guide to which version is current. We record the issue number and the document’s own internal date, and we trust the number printed inside the document over the filename, the folder or the upload date.

Rules 6 and 7 were both added in August 2026. Rule 7 exists because we cited a superseded version of a standard across nine pages and only caught it by reading the document rather than trusting the link.

How a page gets made

Read the primary source, in full. The regulator’s decision, the statutory instrument, the engineering recommendation, the standard. Not a summary of it, and never another commercial solar site’s account of it.

Quote the load-bearing sentences exactly. Where a page turns on what a document says, we quote it and attribute it, so you can judge our reading rather than take it on trust.

Model the numbers ourselves where we can. Our generation figures are not copied from anyone. We run the European Commission’s PVGIS model with the loss factor the UK industry standard specifies, and publish both the output and the scripts.

Check every number against the data before publishing. Claims in prose are verified programmatically against the datasets they came from. This is not a formality: it has caught a wrong dataset minimum, an arithmetic slip repeated across two pages, and a statistical range quoted two decimal points too narrow.

Say what we could not establish. Most pages carry a section listing what we failed to answer and why. A visible gap is more useful than a confident guess, and it tells you where to be sceptical.

Where our data comes from

We work from primary sources: the regulator, the government department, the standards body, the statutory register. We do not cite other commercial solar sites as sources for facts, because most of them are citing each other.

Full detail, including licensing, is on our sources page.

Sources we are not allowed to republish

Some of the most important documents in UK solar are not open, and it shapes what we can publish. We take three different approaches depending on the licence:

Open Government Licence material — Ofgem reports, government guidance, legislation. We republish with the required attribution. This is why our SEG tariff table can exist as a table rather than a summary.

Restricted material we can model around. MCS publishes the UK method for estimating generation and states that its own figures come from a public European model. The method is public; the compiled tables are not. So we run the model ourselves and publish our own output, which is why our generation dataset is free to download while MCS’s cannot be mirrored.

Restricted material with no public model behind it. MCS’s self-consumption lookup tables and the Energy Networks Association’s engineering recommendations fall here. There is nothing to reproduce independently, so we read them, quote sparingly for the purpose of explanation, and link to the source.

Where getting at a source is itself difficult — a paywall, a registration wall — we say so in the citation, because you deserve to know that before you click.

How the calculators work

Every calculator on this site runs entirely in your browser against pre-computed data shipped with the page. Nothing you type is transmitted to us.

Each calculator publishes:

• the data behind it and where that data comes from
• every assumption it makes, stated as a number you can disagree with
• the arithmetic, in enough detail to be reproduced by hand
• the conditions under which the answer stops being reliable

We would rather give you a range with the assumptions visible than a single confident figure that hides them. A payback figure quoted without the electricity price, the self-consumption share and the export rate it assumes is not a result — it is a marketing device.

We do not publish payback periods, and this is deliberate rather than an omission. Payback requires an installation cost, we have no maintained source for installation costs, and inventing one would breach rule 1. Our calculators tell you what a system produces and what that output is worth. The rest is a number you can only get from a real quote.

Where an assumption turned out to be wrong, we changed it and said so. Both calculators originally used self-consumption ranges we had chosen ourselves. When we read MCS’s guidance note properly we found its figures were far lower, so we replaced ours with the sourced ones and wrote the page explaining the difference. That correction reduced the headline saving our own calculator reports. It is logged.

What we check before anything ships

Every change runs through the same gates:

• the site builds with warnings treated as errors
• every internal link resolves
• every template has the partials it references
• no email addresses appear anywhere in the source or the output
• British spelling, enforced against a dictionary that rejects American variants
• numeric claims cross-checked against the datasets behind them

None of this makes a page right. It makes a page consistent, which is the floor rather than the ceiling.

When we get it wrong

We correct the page, add a dated note at the top of it, and record it in the corrections log. Where the error affected a figure someone might have acted on, the note says what the figure was, not only what it should have been.

We log the corrections that reached you. Errors caught in review before publication are recorded in the changelog instead, because calling those “corrections” would flatter us — nobody was misled.

Spotted something? Tell us.

What we do not do

We do not accept payment to cover, omit or soften anything. We do not run affiliate links. We do not generate or sell leads. We do not publish “best of” lists assembled from commission rates. We do not gate a download behind an email address.

We are not a government service, we are not endorsed by one, and we never present ourselves as either.