Lighting Power Density

Watts per square metre is the planning figure for a growing space, and its usefulness has nothing to do with photosynthesis. It tells you two things the photon units cannot: what the electrical service must carry, and how much heat the room has to reject.

Typical densities run from about 150 W/m² for propagation and leafy greens up to 400–600 W/m² for high-light fruiting crops and cannabis. The important physical fact is that essentially all of it becomes heat. A fixture converting 45% of its input into photons is exceptionally good, and even those photons largely end up as heat once absorbed. For sizing cooling, assume the entire lighting load appears as sensible heat in the space — 600 W of lighting is 600 W of cooling duty, or about 2,050 BTU/h.

This is why watts per square metre is a poor way to compare fixtures and a good way to size a room. Two lamps drawing identical power can differ by 40% in photon output, because efficacy — micromoles per joule — varies enormously between technologies. Modern LED fixtures reach 2.5–3.0 μmol/J, high-pressure sodium about 1.7, and older fluorescent and metal halide well below 1.0. Compare fixtures on efficacy and PPFD; size the room and its cooling on watts.