Methods and sources

Every figure on this site comes from a named primary source, is dated, and appears with a note on how to read it.

Updated 29 October 2026

What we chart

Cooler Indoors covers air conditioning and air-to-air heat pumps in UK homes: how hot summers and homes get, what equipment exists, what it costs to buy and run, and which rules apply. Each chart page carries three to five charts, a table and a list of the sources behind them.

Where the numbers come from

We take figures from the organisations that publish them in the first place: the Met Office (temperature records and the HadUK-Grid summer series), the UK Health Security Agency (heat-mortality monitoring and heat-health advice), the Department for Energy Security and Net Zero (cooling estimates and Boiler Upgrade Scheme statistics), Ofgem (the energy price cap and the grant scheme), HMRC (VAT notices), MCS (installer certification and the MCS 020 noise method), Home Energy Scotland, nidirect, the English Housing Survey, and the legislation itself on legislation.gov.uk. For equipment ratings we use makers' own published specification sheets and the energy-label regulation. We do not use retailers, installers, comparison sites, forums or news reports as sources for numbers.

How we check and date figures

Every figure was read on the source page or in the source file itself, not from a search summary, and each chart says when it was checked. The current check date for the whole site is shown in the footer. When a figure changes (the price cap moves every quarter, for example) we update the chart and the date together.

Provisional data, method changes and conflicts

Some series are provisional or change their method part-way. We say so on the chart itself, in the "How we read this" note, rather than smoothing it over:

The 2026 summer temperature comes from the latest Met Office data file and may be revised.
UKHSA changed how it estimates heat-associated deaths in 2020 and again in 2023, so years either side of a change are not strictly comparable.
Where two official sources disagree (for example on whether a cooling-only unit needs planning permission in England), we show both and tell you who to ask.

Worked examples

A few charts are calculations rather than published figures, mainly running costs. Those charts are labelled "worked example, not a measurement" and list every input: the cooling output, the declared efficiency, the hours of use and the unit price of electricity. Change any input and the answer changes with it.

What we do not do

We do not test equipment, survey households, take measurements or interview anyone, and nothing here should be read as if we had. We do not rank brands or publish reviews. We are not an installer and do no installs, servicing or repairs.

Pictures

Photographs come from Wikimedia Commons under open licences and are credited on the photo credits page. Where no suitable photo existed we used photo-realistic AI-generated pictures, and each one is captioned as an AI-generated picture where it appears. Charts are drawn from the numbers on the page, not as images, so their text can be read by screen readers and search engines.