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How do I do a sun study of my home?

A useful sun study needs five things: the address's position, the house's north direction, where the windows sit, what shades them from outside — and four dates to calculate.

What a sun study has to answer

A sun study is not a pretty rendering of the house in backlight. It is the answer to one specific question: when does sun reach this window, and what is in the way? Anything that does not contribute to that answer is decoration.

The good thing about the question is that half of it can be calculated exactly. The sun's position over a given address on a given date and time is pure astronomy with no uncertainty. The other half — the house's rotation and what shades it — you have to measure yourself, and that is where a sun study stands or falls.

Step 1: find the address's latitude

Latitude decides how high the sun climbs, and you find it by looking the address up on a map and reading off the coordinates — the method is the same wherever the home is. A couple of degrees matters: a country like Denmark spans roughly 54.6° north in the south to about 57.7° north in the far north, and those three degrees shift the sun height by three degrees all year round. If you do not know the address's exact coordinates, use the latitude of the nearest large town.

Step 2: measure the house's north direction as a number of degrees

North direction is the number the whole sun study hangs on, and it is also the one most often guessed at. There are four ways to find it, and you should use at least two so they can check each other:

1. The north arrow on the floor plan. Remember that the arrow rarely points straight up — it is the arrow that counts, not the edge of the page.
2. A map or aerial photo with north at the top. Read off the angle between the house's long axis and vertical on the map.
3. The phone's compass held against an outer wall. Keep clear of reinforced concrete, radiators and magnetic covers, and read off a number of degrees, not a compass point.
4. The shadow at solar noon. Shadows point due north when the sun is due south — around noon on standard time, 1 pm on daylight saving time, shifted by roughly 4 minutes per degree of longitude west of 15°E.

Write the result down precisely: "the street façade faces 155°" or "the floor plan's up-direction is 23° east of north". Directions are measured from north, clockwise: north 0°, east 90°, south 180°, west 270°.

Step 3: note the windows

For every living room, note for each window: which wall it sits in, how many square metres of glass it is, how high the top sits above the floor, and whether it is vertical or sloped.

The slope matters more than most people think. A vertical south-facing window catches the most sun in winter, because the low sun hits the glass almost head-on — the share follows cos(sun height), so cos(11°) = 0.98 in winter versus cos(57°) = 0.54 in summer. A horizontal rooflight does the exact opposite: the share follows sin(sun height), so sin(57°) = 0.84 in summer versus sin(11°) = 0.19 in winter. Same square metres of glass, opposite seasonal profile.

Step 4: measure the shadow angle to what stands outside

This is the step that separates a sun study from a guess. For each window, measure or estimate how high whatever stands in the direction the window faces rises above the ground.

shadow angle = arctan(height ÷ distance)

Example: the neighbour's house is 8 metres to the ridge and stands 20 metres south of the living-room window. 8 ÷ 20 = 0.4, and arctan(0.4) = 21.8°.

Two corrections make the number more accurate. Subtract the window's height above ground from the object's height — looking out from a window 1.5 metres up, only 6.5 metres of the neighbouring house lies above your eye level. And add any difference in ground level, if the neighbour's plot sits higher than yours.

Rough heights to work with: a storey is typically about 3 metres, a pitched roof adds 2–3 metres, an ordinary hedge is 1.5–2 metres, a full-grown deciduous tree in a garden is 10–15 metres.

Step 5: calculate the sun height for four dates

Four dates cover the whole year, because the sun moves symmetrically around them:

midday sun height = 90° − latitude + declination

• 21 December, declination −23.4°. At 55.7°N: 90 − 55.7 − 23.4 = 10.9°
• 20 March and 22 September, declination 0°: 90 − 55.7 = 34.3°
• 21 June, declination +23.4°: 90 − 55.7 + 23.4 = 57.7°

Now compare these with the shadow angles from step 4. In the example with a shadow angle of 21.8°, the living room gets midday sun from the equinox through summer, but no midday sun in December, January and part of February and November.

If you also need to know when during the day, remember that the sun does not rise due east. The direction at sunrise is found from cos(azimuth) = sin(declination) ÷ cos(latitude). At 55.7°N that works out to roughly 45° (north-east) at the summer solstice and about 135° (south-east) at the winter solstice. In summer the sun sweeps all the way from north-east through south to north-west; in winter it stays within the southern quarter of the compass.

Mistakes that ruin a sun study

• Guessed north direction. The most common and the worst. A house rotated 20–90 degrees wrong produces a sun study that looks correct and is wrong all the way through.
• Summer only. Every home looks good in June. Always calculate December too.
• Forgotten horizon. A sun study without neighbouring buildings and trees shows a house standing on an empty field, not on its actual plot.
• Noon confused with solar noon. On daylight saving time the sun is due south around 1 pm, plus 10–26 minutes depending on how far west in the country you are.
• Deciduous trees judged in February. Bare branches through winter, a dense screen from May to October.

A sun study with a tool instead of calculations

The sums for sun height and shadow angle can be done with a calculator during a viewing, and that is often enough to settle a purchase decision. If it needs doing for the whole home, room by room and hour by hour, a tool that already knows the house's geometry is easier.

In HouseSense the rooms are measured by scanning the home with the iPhone's LiDAR or by drawing it to scale, after which the home is placed on the map so both position and north direction are set. The 3D model then shows genuinely calculated sunlight for a date and time you choose yourself. That does not excuse you from step 4: what stands beyond the plot boundary, you still have to go and look at yourself.