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Giacomo Ravetta · PoliticsItaliano

Tool · Energy

The sun on the roofs, hour by hour

For 19 municipal buildings Google’s Solar API provides, besides the modules, the survey layers: how much energy reaches each square metre in a year and in each month, where the sun shines at a given hour, which surfaces Google recognises as roof and the heights the shadows come from. Here they can be browsed on the map, building by building.

Google Solar is the service with which Google, starting from its own high-resolution aerial imagery, reconstructs roofs in three dimensions and computes for every point how much sun it receives over the year, taking into account tilt, orientation and the shadows of trees and nearby buildings. It is the same engine behind Project Sunroof, the tool Google uses to estimate the photovoltaic potential of homes in the United States; since 2023 it has been available as a developer service and covers Italy too. The data on this page come from there and are not surveys on site: they are a way to reason about roofs before climbing onto them.

The menu lists only the buildings whose roof was traced by hand: it is that area, not the footprint Google recognises on its own, that drives the numbers and the layout of the modules. Where Google splits a building with a courtyard or a cemetery made of blocks into many small “buildings”, the layers cover the whole area and show the real potential. The other buildings remain on the map of the proposal.

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Photovoltaic modules

Over the year

Shape from PVGIS at the optimal tilt for Saronno, rescaled to the estimated annual output of the roof: reliable on seasonality, indicative for single months.

Source: Includes solar data from Google

Google aerial imagery of 17 April 2023, resolution 25 to 50 cm per pixel depending on the extent of the area (50 cm for the monthly layer, 1 m for shadows). Layers downloaded on 10 October 2026.

Method

How to read the layers

One layer at a time: open the entry you are interested in.

Sun over a year

The solar flux reaching each point, in kilowatt-hours per kilowatt peak installed, already net of the shadows of trees, plant rooms and nearby buildings. The lighter zones are where a module would yield the most.

Sun month by month

The same measure, computed for a single month: it shows how it changes with the season and reveals the parts of the roof that stay in shadow in winter. The month slider chooses which one to show.

Shadows hour by hour

For the 15th of the chosen month, which points see the sun at that hour. It is the layer that best explains the differences between one roof pitch and another. Hours without sun are not shown; the month and hour sliders pick the moment.

Recognised roofs

The surfaces Google classifies as roof: outside them the Solar API places no modules. If a block of burial niches or a low wing does not appear, it is not in the count of Google modules even if the sun shows it well exposed: a limit of the survey, not of the roof.

Heights

The surface model: roof pitches, plant rooms, trees and nearby buildings with their elevation. This is where Google computes the shadows from, and where the site derives the slope and orientation of the pitches to lay out the modules.

Survey photo

The aerial imagery Google used for this analysis, at the date shown below the map. A roof rebuilt after that date does not appear as it is today.

Hand-traced areas

The polygons drawn on the aerial photo with the site’s internal tool: the surface we consider genuinely available, for instance the blocks of burial niches in a cemetery even where Google does not recognise them as roof. Selecting one shows the layer values computed pixel by pixel over that surface: share recognised as roof, average annual and monthly sun, share in the sun at a given hour and the modules Google places there.

Modules over the whole area

The module slider can lay them over the whole traced area, not just where Google recognises a roof: a grid filling the polygon with the orientation and tilt of the pitch on which Google places the most modules; the orientation can be changed with the slider. Rows run along the contour lines of the pitch. On a sloping pitch the modules lie flat on it; on a flat roof they sit on tilted racks, and the spacing between rows grows with the tilt so that one row does not shade the next at noon on the winter solstice, when in Saronno the sun is 21 degrees above the horizon. Each module’s yield is read from Google’s annual flux at the point where it sits, shadows included. The best modules come first, as in the Google survey.

The layers derive from aerial imagery with a resolution of 25 centimetres per pixel, are regenerated at every rebuild of the site and are not kept for more than thirty days, as the Google Maps Platform terms require.

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