20 m² bathroom — 4000 lumens

Watts are not brightness

Equal lumens mean equal brightness; what changes is the electricity. An LED makes 100 lumens per watt, a filament 13 — nearly eight times apart. The monthly figure below assumes five hours a day for thirty days.

• LED100 lm/W40 W
• fluorescent60 lm/W66.7 W
• halogen20 lm/W200 W
• incandescent13 lm/W307.7 W

Several lamps beat one

The same lumens crammed into one fitting light the spot beneath and leave the corners dark. Splitting them across several 800-lumen lamps softens shadows, and one failure no longer means a dark room.

Nearby cells

Same room, other uses

Same use, other sizes

How to read this

• The light you need is lux times floor area; the use decides the lux.
• Watts are only that light divided by the lamp’s efficacy — not a measure of brightness.
• Bulb counts round up from one 800-lumen lamp.
• Ceiling height, wall colour and shades are not in this calculation.