Lumen or watt: why the watt no longer tells you anything
For decades the watt meant light. With LED it is merely what you pay on the bill: the light you get is a different matter.
- Reference standard
- lm/W · EN 13032 (flujo de luminaria)
- Scale
- LED técnico 90–173 lm/W
- Where it applies
- Comparar productos y calcular W/m²
- How it is measured
- Esfera integradora sobre luminaria acabada
The watt (W) measures consumption, the lumen (lm) measures emitted light, and their ratio, efficacy (lm/W), measures how much you actually gain. With incandescent lamps every manufacturer had the same efficacy (about 12 lm/W), so comparing by watts worked. With LED, efficacy ranges from 80 to 200 lm/W: two 30 W spotlights may deliver 2,400 lm or 5,000 lm. Comparing by watts today compares bills, not light.
The industry’s most frequent trap is module flux versus luminaire flux. One manufacturer may publish the LED chip lumens (the supplier’s figure) while another publishes the lumens that actually leave the product after the reflector, the optic and thermal losses. The gap can exceed 25 %. The valid figure is luminaire flux measured in an integrating sphere on the finished product, per EN 13032.
Even more important than the lumens is where they land. A 4,000 lm spotlight with a 60° optic spreads light over a huge area; the same flux with a 20° optic concentrates four times more illuminance on the object. Comparing two products by lumens alone is almost as bad as by watts: without photometry you know nothing.
The indicator that really matters in a project is power density (W/m²) for the required lux level. It is what building codes measure, what appears in the energy certificate and what the client pays every month. A more efficient product means fewer luminaires, fewer drivers and less installation.
- Always ask for luminaire lumens, not module lumens, and at which CCT and CRI they were measured.
- Efficacy drops as CRI rises: CRI >96 has fewer lm/W than CRI >90 of the same product. That is physics, not marketing.
- Compare in W/m² for the same lux level, not in W per luminaire.
- Be wary of a very high lm/W with no stated CCT: 5000 K always looks better than 2700 K.
