Lumens to Watts Calculator
Luminous efficacy of a lamp, the incandescent equivalent and the yearly running cost.
Results
Luminous efficacy
117.00
lm/W
Power
117.0
W
Cost
3.42
Savings
27.33
What this tool does
Lamps are bought by wattage out of habit, but what you actually want is light, and that is measured in lumens. Modern LEDs reach well over 100 lumens per watt while an old filament bulb managed about 13 — which is why the second result shows the wattage the same light would have cost you then.
Formula
efficacy = lumens ÷ watts
Variables
| Symbol | Meaning | Unit |
|---|---|---|
lm | Luminous flux | lm |
w | Power | W |
h | Hours per day | h |
pr | Price per kWh | — |
EF | Luminous efficacy | lm/W |
IN | Power | W |
CY | Cost | — |
SV | Savings | — |
Worked example
- Luminous flux1521 lm
- Power13 W
- Hours per day4 h
- Price per kWh0.18
- Luminous efficacy117.00 lm/W
- Power117.0 W
- Cost3.42
- Savings27.33
Limitations
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
- The tool works with whatever currency you use for the inputs; it does not convert between currencies.