Sun Path Calculator

How It Works

Reading the diagram

The horizontal axis is the compass direction the sun is in and the vertical axis is how high it is above the horizon, so each curve is one day's journey from sunrise on the left to sunset on the right. In Britain every curve peaks close to due south, and the three coloured ones show how far apart the year's extremes are: at London's latitude the midsummer sun reaches about 62° and the midwinter sun only 15°. That is the single most useful fact on this page, because a 15° sun is blocked by things you would never think of as obstacles.

Where the sun rises

Not in the east, except twice a year. At the equinoxes the sun rises very close to due east everywhere on Earth. In London it rises around 49°, well north of east, at the June solstice, and around 128°, well south of east, in December. The June and December sunrise points are about 79° apart on the horizon, which is why a wall that shades a garden all winter can be irrelevant in July.

The obstacle maths

Whenever the sun is lower than that angle and behind the object, the panel is in shade. Something as tall as it is distant blocks 45°, which is most of a British winter's day. A 6 m obstacle 10 m away blocks 31°, which costs nothing in June and more than half the daylight in December. The width setting is how far the object spreads either side of its bearing: a chimney might be 5°, a neighbouring house 30°, a treeline 60°.

This is a geometric answer, not a generation forecast. A shaded panel does not simply lose that fraction of its output: on a string of panels wired in series, shading one can drag down the whole string, which is exactly why microinverters and optimisers exist and why a plug-in kit with one inverter per panel copes with partial shade better than a roof array on a single string.

Which way should the panels face?

Due south, tilted at roughly 35 to 41° in Britain, is the maximum. But the penalty for being off south is smaller than people fear: south-east or south-west costs about 5%, due east or due west about 20%. Due north costs about 45% and is rarely worth doing. An east or west wall that gets sun all day beats a south wall that a tree shades from eleven until three, and this page is here to tell you which one you have.

Worked example

A balcony in Leicester faces south with a 6 m tree 10 m away, dead ahead, spreading about 30° either side. The tree rises 31° into the sky. On 21 June the sun clears it easily and the panel is never shaded. On 21 December the sun never gets above 15°, so the tree blocks the entire middle of the day and takes roughly 57% of what little daylight there is. The panel is still worth having, because June through August is where the year's generation actually sits, but a winter payback estimate from an unshaded calculator will be optimistic. Move the same panel to the east end of the balcony, out from behind the tree, and the winter loss largely disappears.