Light Efficacy Calculator
A lamp that gives 800 lumens from 10 watts has a luminous efficacy of 80 lumens per watt and uses about 11 kWh a year at 3 hours a day.
Quick answer: Divide the light output in lumens by the power in watts. For example, 800 lm and 10 W is 80 lm/W, 800 lm and 60 W is 13.333 lm/W and 2,000 lm and 25 W is 80 lm/W.
The light efficacy formula
Luminous efficacy equals light output in lumens divided by electric power in watts.
Yearly energy equals watts times hours per year divided by 1,000.
Take 800 lm and 10 W. 800 ÷ 10 gives 80 lm/W.
Take 800 lm and 60 W. 800 ÷ 60 gives 13.333 lm/W.
Light output per unit of power
Luminous efficacy is how well a source turns electrical power into visible light. It equals the luminous flux divided by the power consumed (Lumistrips: What is LED luminous efficacy).
It is a property of the lamp, not of the room.
How to convert light efficacy step by step
- Enter the light output in lumens and the power in watts.
- Divide the lumens by the watts to get lm/W.
- Read the yearly energy at 3 hours a day.
- Compare lamps with the same output.
Example 1: 800 lm and 10 W
- Start with 800 lm and 10 W.
- 800 ÷ 10 = 80.
- The luminous efficacy is 80 lm/W.
800 lm and 10 W = 80 lm/W
Example 2: 800 lm and 60 W
- Start with 800 lm and 60 W.
- 800 ÷ 60 = 13.333.
- The luminous efficacy is 13.333 lm/W.
800 lm and 60 W = 13.333 lm/W
Example 3: 2,000 lm and 25 W
- Start with 2,000 lm and 25 W.
- 2,000 ÷ 25 = 80.
- The luminous efficacy is 80 lm/W.
2,000 lm and 25 W = 80 lm/W
The three examples below use common sizes.
Each one shows the sum in full.
light efficacy conversion table
| Lamp | Efficacy (lm/W) | Energy per year (kWh) |
|---|---|---|
| 400 lm and 5 W | 80 lm/W | 5.475 kWh a year at 3 h a day |
| 450 lm and 7 W | 64.286 lm/W | 7.665 kWh a year at 3 h a day |
| 800 lm and 8 W | 100 lm/W | 8.76 kWh a year at 3 h a day |
| 800 lm and 10 W | 80 lm/W | 10.95 kWh a year at 3 h a day |
| 800 lm and 13 W | 61.538 lm/W | 14.235 kWh a year at 3 h a day |
| 800 lm and 60 W | 13.333 lm/W | 65.7 kWh a year at 3 h a day |
| 1,100 lm and 10 W | 110 lm/W | 10.95 kWh a year at 3 h a day |
| 1,100 lm and 15 W | 73.333 lm/W | 16.425 kWh a year at 3 h a day |
| 1,600 lm and 14 W | 114.29 lm/W | 15.33 kWh a year at 3 h a day |
| 2,000 lm and 25 W | 80 lm/W | 27.375 kWh a year at 3 h a day |
| 2,600 lm and 20 W | 130 lm/W | 21.9 kWh a year at 3 h a day |
| 4,000 lm and 36 W | 111.11 lm/W | 39.42 kWh a year at 3 h a day |
The highlighted row is the default of the calculator. The last column gives kWh a year at 3 h a day.
A quick mental shortcut for light efficacy
Divide the lumens by the watts in your head. 1,200 lumens from 12 W is 100 lm/W.
Round the watts to a friendly number to speed it up.
What is a good efficacy
One guide lists LEDs at 50 to 99 lm/W at the low end, 100 to 149 as good and 150 to 200 as high. Incandescent bulbs fall at about 5 to 17 and halogen at 16 to 24 (Lumistrips: What is LED luminous efficacy).
Compact fluorescent lamps are listed at 46 to 75.
Yearly energy
The last column assumes 3 hours of use a day. Multiply the watts by 1,095 hours and divide by 1,000 to get kWh.
Change the hours to fit your own use.
Source or system
A lamp rating may leave out the driver or fixture losses. A system figure includes them and is lower.
Say which one you quote.
Quick conversion answers
| Conversion | Result |
|---|---|
| 450 lm and 7 W | 64.286 lm/W |
| 800 lm and 10 W | 80 lm/W |
| 1,600 lm and 14 W | 114.29 lm/W |
| 4,000 lm and 36 W | 111.11 lm/W |
Where this conversion is used
Engineers compare lamp data sheets.
Home owners judge the saving from new bulbs.
Common mistakes to avoid
- Using different units. Use lumens and watts.
- Comparing lamps with unlike output. Compare the ratio.
- Ignoring driver loss. System efficacy is lower.
- Using lux. Lux is light on a surface, not lamp output.
Related converters
More lamps
350 lm and 5 W is 70 lm/W. 600 lm and 7 W is 85.71 lm/W. 750 lm and 9 W is 83.33 lm/W.
1,000 lm and 10 W is 100 lm/W. 1,300 lm and 13 W is 100 lm/W. 1,800 lm and 18 W is 100 lm/W.
2,200 lm and 22 W is 100 lm/W. 3,200 lm and 30 W is 106.7 lm/W. 5,000 lm and 45 W is 111.1 lm/W.
Doubling the first number
700 lm and 5 W is 140 lm/W. 1,200 lm and 7 W is 171.4 lm/W. 1,500 lm and 9 W is 166.7 lm/W.
2,000 lm and 10 W is 200 lm/W. 2,600 lm and 13 W is 200 lm/W. 3,600 lm and 18 W is 200 lm/W.
4,400 lm and 22 W is 200 lm/W. 6,400 lm and 30 W is 213.3 lm/W. 10,000 lm and 45 W is 222.2 lm/W.
Lamps of the same output
800 lm and 8 W is 100 lm/W. 800 lm and 10 W is 80 lm/W. 800 lm and 13 W is 61.54 lm/W.
800 lm and 60 W is 13.33 lm/W.
Frequently asked questions
What is light efficacy?
The light a lamp gives per watt of power, in lumens per watt.
How do I calculate it?
Divide the lumens by the watts.
What is a good value for an LED?
100 lm/W or more is good.
How is yearly energy found?
Multiply watts by hours per year and divide by 1,000.
Why is system efficacy lower?
It includes losses in the driver and fixture.
Is this the same as lumens per watt?
Yes. The two names mean the same figure.
Sources
- Lumistrips: What is LED luminous efficacy (lumistrips.com)
