Solar energy is one of the environmentally sustainable resources for producing electricity using photovoltaic (PV) systems. The main input data used in the planning process is solar radiation. The Europe JRC has an app online which you can use to calculate PV values, bus sadly they have no (good) API. That's where this website/API comes into play.
The PVGIS API provides a simple API endpoint which you can get the same values via JSON. The API and source code is freely available to use under the MIT license.
See https://pvgisjson.com for details on how to use the API.