How to Compare Sunlight Between Apartments

Inputs for a useful comparison

The worked example reuses London’s June and December local-noon scenes: south-facing window bearing 180°, clear width 1.5 m, height 1.5 m and sill 0.9 m, with no outside obstacles. These are hypothetical window dimensions at public coordinates, not an actual apartment assessment.

Choose the receiving surface that matters

Pin the exact living-room window, bedroom window, balcony edge, or work area rather than the building entrance. Record the direction the exterior surface faces and the floor level relative to nearby rooflines.

Use one comparison protocol

For each apartment, use identical dates and time intervals. Compare winter, summer, and the months when you expect to occupy the home most. Keep window measurements and obstacle assumptions in the same units.

• Direct-sun start and end times for the important room.
• Approximate floor reach at selected times.
• The obstacle responsible for each transition into shade.
• Confidence level and source of every dimension.

Separate direct sun from room brightness

A room can be bright without a direct sun patch because diffuse sky light and reflections still enter. Conversely, a narrow direct beam does not guarantee comfortable daylight across a deep room. SunCarta estimates direct-beam geometry, not complete daylight performance.

Verify the finalist in person

Use the model to identify critical dates and times, then visit the strongest candidate when practical. A visit can reveal curtains, balcony slabs, tinted glass, temporary structures, or terrain that the simplified model does not include.

Notice the 8 m schematic limit

When the readout reaches 8 m, the simplified room has reached its display/model depth. It is not evidence that a measured 8 m room receives sun all the way across. Check geometry, obstructions and the actual room dimensions before interpreting that result.

Method and a worked example

For an unblocked front-facing solar ray, perpendicular floor reach = (sill + window height) / tan(altitude) × cos(azimuth − facing). The schematic caps this value at 8 m. Width is a scene input, not a multiplier in this one-dimensional reach equation.

The June scene gives about 1.24 m reach using altitude 59.47° and azimuth 150.98°. December’s lower sun would project about 8.9 m on an unlimited flat floor, but the schematic reports its 8 m cap. A larger winter patch does not mean more winter daylight: standard daylight is about 7 h 50 min in December versus 16 h 38 min in June.

How to interpret the comparison

Separate three questions: Is the sun in front of the facade? Does a measured outside obstacle block the ray? How far does the clear beam reach across a simple floor? A room can be bright with zero direct-beam reach because diffuse sky light and reflections are excluded.

What to do next

Open each worked window scene in My space. Change the outward bearing by 60° and inspect how the cosine factor changes reach. For a real comparison, save one named scene per apartment and repeat identical dates and times before arranging a site viewing.

Questions about this method

Sources and further reading

• SunCarta calculation methodology and independent benchmark limits
• Astronomy Engine documentation
• USNO sunrise and sunset definitions