How we get the numbers

Live figures

Carbon intensity, national and for the 14 network areas, is the Carbon Intensity API’s own figure in grams of CO₂ per kilowatt hour, with its five bands from very low to very high. We do not recalculate it. Past half hours are the API’s estimate of what happened; later ones are its forecast, drawn hatched so the two are never confused.

Generation in gigawatts is Elexon’s five-minute reading for transmission-connected plant and interconnectors, plus NESO’s estimate of wind and solar on local networks, which Elexon does not see. Shares of the mix are the Carbon Intensity API’s. Agile prices are Octopus Energy’s published half-hourly rates, VAT included.

If a source fails or is late, the page says so and shows nothing in its place. Sources revise their own figures, and we follow them; a half hour can change after it first appears.

How the answers are worked out

The cleanest or cheapest window is the run of consecutive half hours with the lowest average: two hours on the dashboard and Green Signal, one and three hours on Agile prices. The dashboard, Green Signal, Agile prices and the tariff comparison all use the same search, so they give the same answer for the same data.

Bills use Ofgem’s cap rates for your region and payment method: each fuel’s use times its unit rate, plus its standing charge for every day, each rounded to the pound as Ofgem rounds its typical bill. Month-by-month figures spread a year’s use by an estimated seasonal profile and count each month’s real days.

The tariff comparison prices the last 28 days on four Octopus tariffs, for a household shaped like national demand (Elexon) and scaled to your yearly use. It is a replay of real prices, not a forecast.

Records come from the whole half-hourly history since September 2017. A half hour sets a record only if its mix agrees with its own intensity, and the record is the middle of it and its two neighbours, so one bad reading cannot set one. A day counts only if it has at least 46 of its half hours.

Days are London days. When the clocks change a day has 46 or 50 half hours, and every chart and average uses the real number.

Reports and articles

State of the grid is built by a script from public data after each month ends; its sentences are templates filled with computed figures. Articles in Insights and Energy saving are drafted by a language model on our own computer from the past week’s data, and an article with any figure not in that data is rewritten or dropped.

Figures kept by hand, and when we last checked them

Everything not fetched live. The site’s own checks fail when a next-due date passes without an update.