PVGIS: the details
It is developed by the Joint Research Centre (JRC). From the location, installed power, tilt and orientation of the panels, it calculates expected output month by month and, if needed, hour by hour, using radiation data series derived from satellites. It also offers an API, which lets another program run those calculations.
It’s the usual reference for sizing an installation and for preparing a self-consumption quote: how much it will produce, what share will be consumed and how long it takes to pay off. It’s useful afterward too: comparing actual output with what PVGIS estimates for that site is the simplest way to detect that a plant is performing below what it should.