Sun Windows

What it is for

Before you rent a flat, place a raised bed or choose a balcony plant, you want to know: when does the sun actually get here? This tool calculates the sun's position for your location and date, then removes the times when the sun is below the horizon, behind a building you measured, or behind the wall of a window facing the wrong way. You get the direct-sun windows of the day, a daily solar path chart and the same calculation for the 21st of every month.

It is pure geometry. Clouds, weather, trees in leaf, terrain beyond your obstacles, and atmospheric refraction are not included, and it is not a solar-panel yield calculator.

Worked example

The default is midsummer, 2026-06-21, at latitude 48.89 and longitude 8.7 with UTC offset on selected date 2, an open sky (Window facing -1) and one obstacle: bearing 180°, 40° wide, 5 m high at 10 m distance. Result: 16 hours of direct sun, from 05:25 to 21:25, a Maximum altitude of 64.562° and a Modeled solar noon of 13:27. The obstacle never blocks the sun because the sun stands far above it in June.

Now a Berlin flat in midwinter: 2026-12-21, latitude 52.52, longitude 13.4, UTC offset 1, a south-facing window (Window facing 180) and a building across the courtyard at bearing 170°, 30° wide, 12 m higher than the window and 20 m away. The result: "5.25 approximate hours of direct sun", in two windows, 08:20–10:15 and 12:25–15:45. The sun is above the horizon for 7.417 hours, peaks at only 14.059° at 12:02, and the building, which rises about 31° above the window, hides it around midday. The Monthly comparison shows 6.25 hours on 21 January, 12 in March and September and 9.25 in June, when the sun rises and sets behind the window wall.

How to read the result

• Direct sun (hours) is the total time the sun is up, inside the window's view and not behind an obstacle.
• Sun above horizon (hours) is the day length; the gap between the two figures is what obstacles and the window cost you.
• Maximum altitude (°) and Modeled solar noon describe the sun's highest point in local time.
• Sun windows lists each sunny period with From and Until times.
• The chart switches between Daily solar path and Monthly direct sun. Inspect time shows the modelled sun position for any interval of the day.
• Monthly comparison repeats the calculation for the 21st of each month at 15-minute steps with the same UTC offset.

Tips and common mistakes

• Enter the offset valid on that date, including daylight saving: Central Europe is +2 in summer and +1 in winter.
• Bearings are compass degrees clockwise from north: 90 east, 180 south, 270 west. Use a phone compass and correct for magnetic declination if you need precision.
• Measure obstacle height from the window or balcony, not from the street. A 15 m building seen from a 3 m high balcony counts as 12 m.
• Split wide or stepped buildings into several obstacles with their own bearings and heights.
• Set Window facing for a window or balcony; leave -1 for a garden spot with open sky all round.
• The monthly table keeps your offset all year, so its times are not adjusted for daylight saving.

FAQ

How accurate are the times?

The solar position follows NOAA equations, so the sun's track is accurate. Errors come from your measurements of bearings, heights and distances, and from the 1, 5 or 15 minute sampling.

What does Window facing change?

A window only receives direct sun from the half of the sky in front of it. With facing 180, sun from the north-east in the early morning does not count, even if nothing else blocks it.

Does it account for trees?

Only if you enter them as obstacles. Remember that deciduous trees block more in summer than in winter.

Can I export the chart?

Yes: vector SVG, result JSON, CSV tables and a print or PDF view.