LED efficiency: what lumens per watt really tell you

Luminous efficacy in lm/W relates light output to electrical input. Check the system boundary when comparing products. An LED-chip figure is not directly comparable with a complete-luminaire figure: the driver, cover and optics form part of the operating system.

Luminaire efficacy divides emitted luminaire flux by total input power. Compare suitable variants under similar conditions, including colour temperature, colour rendering and temperature. An impressive headline value may describe a different configuration from the one your project needs.

Installation efficiency also depends on how much light reaches useful surfaces and when it is required. Appropriate distribution and control may matter more than a small difference between datasheet figures. The goal is to achieve suitable lighting quality with an appropriate energy input, rather than optimise one number in isolation.

What to clarify for your project

• Do not mix LED-chip and complete-luminaire figures.
• Include driver power in the system input.
• Assess energy only after checking lighting suitability.

Remember: Compare complete systems, then compare their performance in the space.