Estimate the total wattage needed to light a room.
Wattage is not a measure of light — it is a measure of consumption — so working out the wattage a room needs means going through lumens and then dividing by the efficacy of the lamps you intend to use. Good LED lamps manage 80 to 120 lumens per watt, fluorescent tubes around 60 to 90, halogen about 20, and old incandescent bulbs barely 13, which is why the same well-lit room can need 40 W of LED or 280 W of incandescent. The resulting load density in watts per square foot is the figure electrical designers and energy codes work in, and modern LED lighting typically lands between 0.3 and 0.8 W per square foot for general spaces — a useful sanity check on any lighting scheme.
Lighting wattage from target illuminance and efficacy
Installed lumens = target lux x area in sq m / (1 - loss / 100); total watts = installed lumens / efficacy; load density = total watts / area in sq ft
Because energy codes are written in it and it is comparable between rooms. General LED lighting normally falls between 0.3 and 0.8 W per square foot; well above that suggests inefficient lamps or an over-lit design.
No. Rules like 1.5 W per square foot assumed incandescent lamps at 13 lumens per watt. Applying them to LED over-lights a room by a factor of six or more.
Divide its rated lumens by its rated watts, both printed on the box. A 9 W lamp rated at 806 lumens gives 90 lm/W, which is typical of a decent domestic LED.