
The Official Carbon Intensity API for Great Britain developed by National Grid.
UK Carbon Intensity API is a free Environment API that requires no API key, is served over HTTPS and has unconfirmed CORS support.
Details from carbon-intensity.github.io.

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The UK Carbon Intensity API is provided by Carbon Intensity API (carbon-intensity.github.io).
Yes. The UK Carbon Intensity API is listed in the Environment category of the public-apis directory, which only includes APIs that are free or have a free tier. Check its documentation for rate limits and paid plans.
No. The UK Carbon Intensity API requires no authentication, so you can start making requests without signing up.
CORS support for the UK Carbon Intensity API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
Yes. The UK Carbon Intensity API is served over HTTPS, so it's safe to use in production.