The proportion of occupied hours during which a specified indoor illuminance threshold at a defined reference plane is met exclusively by daylight, assuming electric lighting is available but not used; expressed as a percentage over a stated schedule, location, and measurement grid or sensor arrangement.

Principle

Daylighting autonomy quantifies the fraction of time daylight alone can meet an illuminance requirement and thereby serves as a design target that links fenestration, daylight distribution, and lighting control strategy to hours of reduced electric lighting use.

Demonstration

Illustrative scenario → Situation: An open-plan office defines a 300 lx workplane threshold and an occupied schedule 08:00–18:00. Recognition: Daylight simulation or sensor logging shows that at specified grid points daylight exceeds 300 lx for 1,800 of 3,000 occupied hours. Action: Designer reports DA = 60% for the tested grid and uses that result to size shading and daylight controls. Consequence: Electric lighting can be disabled automatically for those hours at the measured locations, reducing operating hours but requiring glare control and occupant override provisions.

Misapplication

Treating DA as a direct measure of lighting energy savings or occupant comfort without accounting for spatial variability, occupant behavior, electric lighting controls, control setpoints, glare, or differing workplane requirements; or confusing DA with daylight factor or useful daylight illuminance (UDI). The error is assuming temporal percentage alone predicts energy or comfort outcomes.

Consequence

When used appropriately, DA informs fenestration placement, daylighting controls, and daylight-responsive lighting strategies; it does not by itself specify uniformity, glare, circadian effects, or actual energy savings, which require complementary metrics and modeling.

Reversal

DA loses relevance or becomes misleading when the occupied schedule is undefined or highly variable, when electric lighting is allowed for reasons other than illuminance (e.g., color rendering, circadian lighting), or when control strategies permit partial rather than binary electric lighting use; in such cases metrics that account for partial lighting use or occupant behavior are required.

Boundary

Clearly within: percentage of occupied hours at a defined workplane and threshold met solely by daylight for a specified schedule and measurement grid. Boundary case: a space where daylight intermittently meets the threshold but sensors average readings across large zones—interpretation depends on sensor placement and control logic. Clearly outside: daylight factor, which is a static ratio under overcast sky, and UDI categories that classify hourly illuminance into ranges rather than a single threshold-based autonomy percentage.

Semantic Tension

Daylighting Autonomy trades off with thermal performance, glare control, privacy, and facade durability: increasing glazed area to raise DA can increase solar heat gain or glare, requiring coordinated mitigation.

Synthesis

DA is a practical, time-based metric for assessing where and when daylight can replace electric light, but its effective use demands coupling with spatial uniformity checks, glare control, occupant behavior assumptions, and energy or comfort analyses.