Work out the hours of sun on a site

Sun hours counts the hours of direct sun that reach each wall of each building mass and the ground round them, on a day you pick or averaged over a season. The masses and the ground itself cast shade; nothing else does (no trees, no neighbours you have not drawn). Sky light that is not direct sun is not counted.

Before you start

The sun's place comes from the site's latitude and longitude, which come from the project's coordinate system. A project with none is refused with a message: set it first (Coordinates in the GIS group). A latitude and longitude typed by hand would be a guess, so there is no box for them.

Work it out

In Properties, under Building masses, Sun hours:

1. Pick the Day. It starts at the day the sun is lowest where you are (21 December, or 21 June in the southern hemisphere). For a season, tick Over a season, pick the Last day and how many days apart to sample.
2. Type the Time zone: a name such as America/Mexico_City, or an offset from UTC such as -6. A name follows that zone's daylight saving rules for the day; an offset you type is used exactly as typed, so use the one in force on that day. The line under the box tells you which offset it found.
3. Choose how often to place the sun (every 5, 10, 15 or 30 minutes), then Work out the sun hours. A season runs in background workers; Stop cancels it.

What you get

• A table by mass: the worst wall, the average and the best wall, in hours a day, and a table of every wall with the way it faces and its hours floor by floor.
• The ground round the masses, drawn on the plan from dark (no sun) to yellow (the most any cell gets), with a key. Cells under a mass are left blank. Show the ground on the plan turns it off.
• Walls as CSV and Ground as CSV save the numbers.

For a season the figures are the average a day over the days sampled. Nothing is saved in the project: the result is for looking at, and running it again makes it again.

Good to know

• A wall counts a moment as sunny only when it faces the sun and nothing is in the way. A wall facing away gets nothing.
• The ground is sampled on a grid that depends on the size of the site, and the sun's place at the middle of each time step, so hours are right to about the step you chose.
• The sun's place is a published astronomical formula, good to about a minute of arc.