Daylight Autonomy Simple Calculator

Daylight autonomy expresses the fraction of occupied time that a point in a room reaches a target illuminance from daylight alone, letting the electric lighting stay off. The threshold is a design choice rather than a physical constant: 300 lux is the common value for offices and classrooms because it matches the maintained illuminance EN 12464-1 recommends for general office work, while 500 lux applies to drafting and detailed inspection tasks. The ratio form, daylit hours divided by occupied hours, works because both terms count the same schedule, making the result dimensionless and comparable across rooms with different operating hours. The supporting outputs tie that time-based number back to geometry: window-to-floor ratio is the crude first-pass check, typically 15 to 25 percent for side-lit offices, and estimated interior illuminance applies a simple reduction to average exterior illuminance using glazing transmittance, which runs about 0.85 for clear single glazing and 0.50 to 0.70 for low-emissivity double units. The main limitation is that one ratio hides where the daylight actually falls. A room can reach 70 percent autonomy at a desk beside the window and under 20 percent four metres back, which is why formal assessments compute the metric on a measurement grid and report the share of floor area that passes. This tool also takes daylit hours as an input rather than deriving them from weather data, so the result is only as reliable as that estimate. It also says nothing about glare or solar heat gain, and the glazing area that maximises autonomy often worsens both.

The idea of scoring daylight against an annual occupancy schedule rather than a single overcast moment emerged in Switzerland in the late 1980s, when daylight autonomy was defined in Swiss lighting guidance as the percentage of working hours meeting a minimum illuminance without electric light. It remained a niche measure for over a decade because computing it required hourly illuminance across a full year, which was impractical before cheap simulation. That barrier fell in the early 2000s when Christoph Reinhart, then at the National Research Council of Canada and later at Harvard and MIT, developed DAYSIM, which coupled the Radiance raytracer with daylight coefficient methods so a year of hourly values could be produced in minutes from standard climate files. Once the computation was routine, standards bodies formalised the metric. The Illuminating Engineering Society published LM-83 in 2012, defining spatial daylight autonomy, written sDA 300/50 percent, as the share of floor area meeting 300 lux for at least half of occupied hours, paired with annual sunlight exposure to penalise excessive direct sun. LEED version 4 adopted these two measures for its daylight credit, replacing the earlier reliance on static daylight factor and glazing-area prescriptions, and climate-based daylight modelling has since become the expected basis for daylight compliance in green building certification.