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WattFigure API is a free Environment API that requires no API key, is served over HTTPS and supports CORS, so it can be called from the browser.
Details from api.wattfigure.com.

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The WattFigure API is provided by WattFigure API (api.wattfigure.com).
Yes. The WattFigure 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 WattFigure API requires no authentication, so you can start making requests without signing up.
Yes. The WattFigure API supports CORS, so front-end JavaScript can call it directly.
Yes. The WattFigure API is served over HTTPS, so it's safe to use in production.