Light Efficacy Calculator
Turn lumens and watts into efficacy plus yearly cost.
What it does
Efficacy tells how much light each watt buys. An 800 lumen bulb at 9 watts gives 88.9 lumens per watt, far above a 60 watt incandescent near 13. Higher means more light per dollar of energy.
The tool above takes lumens, watts, daily hours, and electric rate and returns efficacy, yearly kWh, and yearly cost. The defaults 800, 9, 3, and 0.15 match the worked example, so the page opens showing 88.9 lm/W.
Efficacy divides lumens by watts. Yearly kWh multiplies watts times hours times 365 over 1000. Cost multiplies by rate. Here 9 times 3 times 365 over 1000 is 9.855 kWh for $1.48.
Use this to compare bulbs at equal brightness, not equal watts. A 1600 lumen 14 watt tube at 114.3 lm/W beats two dim bulbs on both light and bill. Dimming and controls cut further.
This page shows the formulas, one full hand checked example at 800 lm, a reference table, and practice checks that run through the tool.
How to use
- Type lumens from the bulb box.
- Type watts from the same box.
- Type hours per day of use.
- Type electric rate per kWh.
- Read Efficacy in lm/W.
- Read Yearly cost to compare bulbs.
Formula
- lm
- Total light lumens.
- W
- Electric watts drawn.
- kWh
- Kilowatt hours per year.
Rated lumens and watts assumed. Dimming scales both roughly together for LEDs.
Worked example
Example: 800 lm, 9 W, 3 h/day at 0.15
A19 LED rated 800 lumens at 9 watts, three hours daily.
- Efficacy: 800 / 9 = 88.8889 lm/W.
- Round: 88.9 lm/W.
- Year hours: 3 x 365 = 1095 h.
- Energy: 9 x 1095 / 1000 = 9.855 kWh.
- Round: 9.9 kWh.
- Cost: 9.855 x 0.15 = $1.48.
Reference table
| Bulb | lm/W | Year cost at 0.15 |
|---|---|---|
| 800 lm, 9 W | 88.9 | $1.48 |
| 1600 lm, 14 W | 114.3 | $2.30 |
| 800 lm, 60 W incandescent | 13.3 | $9.86 |
Limits
- Rated values only. Dimming and heat shift real draw.
- No purchase price here. Payback needs bulb cost too.
- One bulb math. Multiply by fixture count.
- Light quality (CRI, color) sits outside.
Common mistakes
- Comparing watts instead of lumens for brightness.
- Forgetting hours when comparing yearly cost.
- Mixing kW with kWh.
- Using peak rate for average cost.
- Ignoring controls and daylight savings.
Try it yourself
Quick checks. Answers sit tucked under each question.
1600 lm, 14 W, 5 h/day at 0.12?
114.3 lm/W, 25.6 kWh, $3.07.
800 lm, 60 W incandescent, 3 h/day at 0.15?
13.3 lm/W, 65.7 kWh, $9.86.
Questions
800 at 9 W?
88.9 lm/W.
Year cost here?
$1.48 at 3 h/day and 0.15.
Higher better?
Yes for energy. More light per watt.
LED vs incandescent?
LED near 90 plus, incandescent near 13 lm/W.
Dimming?
Scales both roughly. Re measure for exact.
Many bulbs?
Multiply one bulb cost by count.