Lux to Watts Calculator
Convert lux to watts for a room: lamp lumens = lux × area / (utilisation × maintenance), then watts = lumens / efficacy. Formula, worked example and a table.
Try your own numbers
Estimate only. Not engineering or safety advice. Check the result against the applicable standard and the equipment data, and have a competent engineer review it before it is relied on. Terms of use.
Open in the calculator with these values
Formula
Worked example
| Input | Value |
|---|---|
| Target illuminance (lux) | 300 |
| Area | 50 m² |
| Luminous efficacy (lm/W) | 100 |
| Utilisation factor | 0.6 |
| Maintenance factor | 0.8 |
- Φ = 300 × 50 / (0.6 × 0.8) = 31,250 lm
- P = 31,250 / 100 = 312.5 W
- Power density = 312.5 / 50 = 6.25 W/m²
Typical values
| Target illuminance (lux) | Electrical power (W) | Lamp lumens needed |
|---|---|---|
| 100 | 104.2 | 10,417 |
| 300 | 312.5 | 31,250 |
| 500 | 520.8 | 52,083 |
| 750 | 781.3 | 78,125 |
| 1,000 | 1,041.7 | 104,167 |
Other inputs are the same as in the worked example above.
Assumptions
- Uniform illuminance across the area
- Factors apply to the whole installation
Use the lumen method calculator for the number of luminaires. Check the power density against your energy code.
Frequently asked questions
How do I convert Lux to Watts?
Φ = E × A / (UF × MF); P = Φ / η; power density = P / A. Worked example: Target illuminance (lux) 300, area 50 m², luminous efficacy (lm/W) 100, utilisation factor 0.6 gives Electrical power: 312.5 W · Lamp lumens needed: 31,250.
What do I need to enter?
Enter target illuminance (lux), area, luminous efficacy (lm/W), utilisation factor and maintenance factor. Change the unit of a value with its selector and the result stays the same.
Why are there two factors, utilisation and maintenance?
Not every lamp lumen reaches the work plane, which the utilisation factor covers, and lamps and luminaires lose output with age and dirt, which the maintenance factor covers. Typical room values are about 0.5 to 0.7 and 0.7 to 0.85, but take them from your luminaire data and design standard.
Is there a fixed lux to watts factor?
No. Lux depends on the area, the lamp efficacy in lm/W and the room factors, so a lux figure cannot be turned into watts without them. For a precise layout use the lumen method or a point-by-point calculation.
How accurate are the results?
Every figure comes from the same tested engine as the Elecmetric calculators and is meant for estimating. Check design values against the standard and the equipment data you work to.
Related
Next steps in a design: Lighting LevelLumens to WattsLED Retrofit Savings and Payback
Opposite direction: Watts to Lux
Also see: Amps to Watts Watts to Amps kVA to Watts Watts to kVA
Formula guides: Lighting Formulas: Lux, Lumens and CandelaPoint Illuminance Formulas
All 78 conversionsLighting Power for a Target Lux in Elecmetric