How many hours of sunlight will the flat you want to buy get?

What does each orientation mean?

In Spain the sun passes due south at midday: a south façade gets sun all year, a north one almost none in winter, an east one the morning sun and a west one the afternoon sun. In Madrid the day lasts from 9 h 17 min to 15 h 3 min, according to Spain's National Astronomical Observatory.

In Spain the sun passes due south at midday. A south-facing façade gets sun in the middle of the day all year round; in winter the sun is low and reaches deep into the rooms, while in summer it is high and comes in less.

A north-facing façade gets almost no direct sun in winter; in summer, only early in the morning and late in the afternoon. An east-facing façade gets the morning sun and a west-facing one the afternoon sun, which is the hottest in summer. The in-between orientations (south-east, south-west, north-east and north-west) share those traits.

A real example: in the sample report, a building in Valencia, the main façade faces south and gets 8 h of direct sun on 21 June and 8.8 h on 21 December. Its west façade gets 4.5 h in summer and 3 h in winter.

Why can a south façade get less sun in summer?

Because in summer the sun rises in the north-east and sets in the north-west, and a south façade only gets it while the sun is in front of it. In Madrid on 21 June the sun rises at 6:45 and sets at 21:48, according to the National Astronomical Observatory, but much of that time it lights other façades.

In summer the day is longer, but the sun rises in the north-east and sets in the north-west. A south façade only sees it while it is in front of it, so it can get fewer hours of direct sun in June than in December, as in the Valencia example: in June the sun reaches it from 11:24 to 19:24 and in December from 08:24 to 17:12.

In winter the opposite happens: the day is short and the sun is low, so the shadows of the buildings opposite grow longer. That is the season when the floor and the width of the street matter most. If winter sun matters to you, always look at the figure for 21 December.

The floor changes the result

The higher the flat, the less shade the neighbouring buildings cast on it, especially in winter. In the sample report, the south façade gets 8 h of sun on 21 December on the ground floor and 9.3 h on the eighth; on 21 June, by contrast, every floor gets 8 h because the sun is high and the shadows are short.

On a narrow street the gap between floors can be much larger than in that example, and a south-facing first-floor flat can get less sun than an east-facing penthouse. That is why the report gives the hours per floor rather than one figure for the whole building.

How does the report work out the hours of sunlight?

From the Catastro's geometry: it takes the outline of the building and its neighbours, separates exterior façades from party walls and assesses each façade at the height of every floor. The sun's position is calculated every few minutes with the NOAA solar algorithm for 21 June and 21 December.

Check 18 answers “How many hours of sunlight does it get?”. It is our own calculation on the geometry held by the Catastro (Spain's land registry of buildings): it takes the building's outline, separates the exterior façades from the party walls (walls shared with another building) and assesses each stretch at the height of each floor, at 3 m per floor. Neighbouring buildings cast their shadows according to their outline and number of floors in the Catastro.

The sun's position is computed every few minutes with the solar algorithm of NOAA, the US atmospheric agency, for 21 June and 21 December. The result includes the orientation of each façade, the hours of sun in summer and winter, when the sun arrives and leaves, the hours per floor, the daily average for each month, the annual total and, if there is an inner courtyard, its hours per floor.

Start with the free report: it shows the check with your building's real data and the key information blurred. The essential report costs €11 and the complete report, which adds the recorded building condition, €19. That is 22 checks on official sources, as a PDF in a few minutes, with a single payment and no account.

What the calculation cannot see

It is the direct sun that can reach each façade under a clear sky, not an on-site measurement. It does not account for terrain, such as a nearby hillside or mountain, nor for trees, awnings, balconies or how far each window is set back. Heights are estimated at 3 m per floor, and a new building not yet in the Catastro casts no shadow in the calculation.

Nor does it know which rooms face each façade: you will see that on the floor plan or at the viewing. The report does not replace a technical inspection of the building.

How to check it at the viewing

Take a compass (the one on your phone will do) and note which way the living room and the bedrooms face. If you can, visit at two different times, say in the morning and mid-afternoon, and look at the buildings opposite and their height.

Ask the neighbours what the flat is like in winter. Then compare what you see with the report's hours per floor: it will tell you whether the difference between a first and a fifth floor in the same block is worth what it costs.