Mycelium
Corporate carbon emissions disclosures (scope 1, 2 and 3), company-reported and estimated
Open data from Great Britain’s Electricity System Operator.
National Grid ESO API is a free Environment API that requires no API key, is served over HTTPS and has unconfirmed CORS support.

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Corporate carbon emissions disclosures (scope 1, 2 and 3), company-reported and estimated
Yes. The National Grid ESO 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 National Grid ESO API requires no authentication, so you can start making requests without signing up.
CORS support for the National Grid ESO API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
Yes. The National Grid ESO API is served over HTTPS, so it's safe to use in production.