What is Luminous Efficacy?

Why Efficacy Matters

Higher efficacy means more light output for less energy consumption — directly reducing electricity costs and carbon emissions. In commercial buildings where lighting typically accounts for 25–40% of total energy use, specifying high-efficacy luminaires is one of the most impactful decisions a designer can make.

For context, here's how different light sources compare:

• Incandescent lamp — 10–15 lm/W • Halogen lamp — 15–25 lm/W • Compact fluorescent (CFL) — 50–70 lm/W • T5 fluorescent — 80–100 lm/W • Standard LED luminaire — 100–130 lm/W • High-efficiency LED luminaire — 130–170 lm/W

Modern LED luminaires deliver 5–10x the efficacy of the incandescent sources they replace.

Efficacy and Part L Building Regulations

Part L of the Building Regulations (Conservation of Fuel and Power) sets maximum energy consumption targets for lighting in new and refurbished buildings. Luminaire efficacy directly determines whether a lighting scheme can meet Part L compliance.

The current Part L (2021) requires lighting power density targets that effectively mandate LED luminaires with efficacy above 100 lm/W for most commercial applications. Projects targeting BREEAM credits for energy performance need even higher efficacy to score maximum points in the Ene01 category.

Luminaire Efficacy vs LED Chip Efficacy

Be cautious of efficacy claims that quote raw LED chip performance rather than complete luminaire efficacy. The light output you actually receive is reduced by:

• Driver losses — typically 5–10% • Optical losses — diffusers, lenses and reflectors absorb or redirect 10–25% of light • Thermal derating — LED output decreases as junction temperature rises

Always compare 'luminaire lumens per circuit watt' — the total light output from the fitting divided by the total power consumed including the driver. EOS quotes luminaire efficacy on all product datasheets.