Residential Lighting (W/m²)

Sizing residential lighting in watts per m²

Modern LEDs put out somewhere around 80–100 lm/W, so the usual targets people work with are 3–5 W/m² for the living room (150–300 lux), 5–7 W/m² for the kitchen (300–500 lux), 4–6 W/m² for the bathroom, and 2–4 W/m² for the bedroom. Take a 15 m² living room at 4 W/m². You need about 60 W of LED, which works out to roughly seven 9 W bulbs. Hit the same light level with halogen and you're looking at ~200 W; with old incandescent, ~600 W. That gap is exactly why the W/m² number only makes sense once you know what kind of lamp you're using.

Applications

This comes up in residential lighting design, when you're trying to trim the energy bill, for Procel labelling, and on projects that have to meet NBR ISO 8995. It tells you how many LED bulbs each room actually needs, and it's handy when swapping halogen or incandescent out for LED, where consumption usually falls 70–85 % at the same lux level.

FAQ

Is W/m² the same as lux? No. W/m² is power per area; lux is illuminance. The two only line up once you pin down the lamp efficacy in lm/W.

Why so little wattage with LED? LED efficacy (~80–100 lm/W) runs 8–10× higher than incandescent, so you get the same light for a fraction of the power.

Does ceiling height affect it? It does. Once you're above 3 m, bump the figure up by 20–30 % to make up for the flux you lose.