Lumens Per Watt — The Efficiency Metric That Actually Matters

Lumens per watt — known in the industry as efficacy — is the single most useful number for comparing light sources on performance. Not wattage. Not the lumen figure alone. The ratio between the two.

A standard incandescent lamp produced around 10 to 15 lumens per watt. Most of its energy was released as heat, with light as the incidental by-product. A compact fluorescent managed 50 to 70 lm/W — a significant improvement, but one that came with colour quality compromises and the complication of mercury disposal. A quality LED today delivers 100 to 160 lm/W. The best laboratory LEDs have crossed 200 lm/W, and the technology is still improving.

For architects working on green-rated buildings, efficacy is a compliance metric. ECBC, LEED, and GRIHA all include lighting power density requirements — the watts per square metre a building is permitted to use for illumination — and higher efficacy products directly enable compliance at lower fixture counts. For contractors comparing products at similar price points, efficacy is the most transparent performance indicator available, harder to manipulate than headline lumen claims and more meaningful than wattage figures. For clients questioning why a specification costs what it does, efficacy is part of the answer — it translates directly into energy bills across the lifetime of the installation.

One important qualification: efficacy is a measure of how well a source converts energy into light. It is not a measure of light quality. A high lm/W product with poor colour rendering, inadequate thermal management, or a failing driver at 18 months is not a good specification. Efficacy earns its place alongside CRI, colour temperature, beam control, and verified lifespan — not instead of them.

Use efficacy to compare. Use the full picture to specify.

Efficiency is how well a source produces light. Quality is how well that light serves the space.