Sun and solar panels

What we use

PVGIS 5.3, the service of the European Commission's Joint Research Centre (JRC). It calculates the output of a solar power system at a given point from satellite sunlight data for 2005 to 2023 and from the profile of the mountains on the horizon. The data is free to use.

How we use it

The report asks PVGIS for the output of 1 kW of solar panels (about 5 square metres) at the best tilt and orientation, at the address point. It shows the kWh per year, the output month by month and the recommended tilt. To give an idea of scale: in Milan, Rimini and Bolzano PVGIS estimates between 1,340 and 1,370 kWh a year for each kW installed; in southern Italy the figures are higher.

The roof of the building

Then the report looks at the actual roof. From the building's footprint on open maps it works out the roof shape: the ridge runs along the long side, so a long narrow building has two slopes, an almost square one has four, and sheds and very large buildings have a flat roof. Each slope faces a precise direction, and the report asks PVGIS how much a panel produces with exactly that direction and the typical pitch of Italian roofs, 17 degrees. North-facing slopes are left out. From this come the power that fits, the energy in the first year, the cost and the years to pay it back, always as an indicative estimate with the assumptions written alongside.

Limits

• The estimate takes the mountains on the horizon into account, but not the shade from nearby buildings and trees.
• The roof shape is worked out from the footprint, not measured: no open source says what the roof of every Italian building looks like. A site visit may find different slopes, dormers or terraces.
• It does not know any restrictions set by the condominium or by heritage protection rules.
• It is an average over many years: output in a single year may differ.

Official page: PVGIS.