Luminous efficacy (lm/W) is the ratio of luminous flux output (lumens) to electrical power input (watts). It is the single most important metric for energy efficiency comparisons. LED efficacy has improved dramatically: 2010: ~70 lm/W → 2024: 200+ lm/W. Theoretical maximum for white LED: ~300 lm/W. Standards: EU Ecodesign requires ≥120 lm/W for new commercial installations. China GB standard: ≥100 lm/W for LED luminaires. Always compare efficacy when selecting fixtures—a 150 lm/W fixture uses 33% less energy than a 100 lm/W equivalent for the same light output.