Efficacy: lumens per watt explained
Led Lighting · Lesson 2 · Intermediate
Purpose
Calculate efficacy at a declared product boundary and distinguish it from efficiency, output and maintained performance.
Learning objectives
- Trace electrical-to-light conversion
- Use declared operating boundaries
- Separate component and luminaire metrics
- Check control and load compatibility
Luminous efficacy is lumens per electrical watt. Package, module, lamp and luminaire efficacy differ because driver, thermal and optical losses occur at different boundaries.
Core theory
Use measured lumens and input power for the same state, temperature and boundary. Do not combine initial chip lumens with complete-luminaire mains watts.
Luminous efficacy weights radiation by human visual sensitivity; radiant efficiency is optical watts/electrical watts and is not numerically interchangeable.
Compare maintained output, driver efficiency, temperature, dimming, tolerance, spectral quality and lifetime. A high lm/W source can still create poor illumination through distribution or glare.
| Term | Meaning | Symbol | Unit |
|---|---|---|---|
| Luminous flux | Visible-light output weighted by human sensitivity | Not applicable | lumen |
| Luminous efficacy | Luminous flux per electrical input power | Not applicable | lm/W |
| Radiant efficiency | Optical radiant power divided by electrical power | Not applicable | Not applicable |
ηv = Φv / PinUse lumens and watts at the same declared boundary.
lm/WAssumptions: Supplied photometric/electrical values share the stated measurement boundary; Manufacturer limits remain governing.
- Boundary: Both values describe the complete luminaire.
- Divide: 4200/32=131.25 lm/W.
- Report: Luminaire efficacy is about 131 lm/W at the stated condition.
131.25 lm/W (about 131 lm/W).
Reasonableness check: The result identifies whether it describes LED package, module, driver or complete luminaire and avoids claiming field performance from one laboratory number.
- Dividing lumens by LED-chip watts while calling it luminaire efficacy
- Connecting constant-current LEDs to a constant-voltage source without current control
- Assuming every dimmable driver works with every dimmer
Where this appears in practice
A reliable LED installation coordinates light source, optics, thermal path, driver, supply/protection, controls, environment and maintainability.
Knowledge check
Is LED-package efficacy automatically the installed luminaire efficacy?
No. Driver, thermal and optical losses change the boundary.
Answer: No. Driver, thermal and optical losses change the boundary.
Practical exercise
Compare two products using complete-luminaire efficacy, maintained lumens, distribution and color quality.
Summary
- Metric boundary changes the answer
- LED current and junction temperature govern life/output
- Driver and dimmer must match electrically and functionally
Sources and review
- IEC 62031:2026: LED modules safety requirements: IEC; 2026; International
- IEC 61347-2-13:2024: LED controlgear safety: IEC; 2024; International
- DALI-2 certification overview: DALI Alliance; Current certification database/status must be checked; International
Editorial review date: 2026-08-22. Professional electrical review is pending.