Lumens per Watt Calculator Formula

Understand the math behind the lumens per watt calculator. Each variable explained with a worked example.

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Formulas Used

Luminous Efficacy

efficacy = watts > 0 ? lumens / watts : 0

Annual Energy Cost (8hr/day)

annual_cost = watts * 8 * 365 / 1000 * 0.15

Variables

VariableDescriptionDefault
lumensLight Output(lumens)800
wattsPower Consumption(watts)10

How It Works

Luminous Efficacy

Efficacy = Lumens / Watts

Higher efficacy means more light per unit of energy.

Efficacy Benchmarks

  • Below 20 lm/W: Poor (incandescent)
  • 50-70 lm/W: Good (CFL)
  • 80-120 lm/W: Very good (modern LED)
  • Above 120 lm/W: Excellent (high-efficiency LED)

    • Worked Example

    800 lumens from a 10 watt LED.

    lumens = 800watts = 10
    1. 01Efficacy = 800 / 10 = 80 lm/W
    2. 02Annual cost at 8hr/day: 10W x 8 x 365 / 1000 x $0.15 = $4.38

    Frequently Asked Questions

    What is a good lumens-per-watt ratio?

    For modern LED bulbs, 80+ lm/W is good and 100+ lm/W is excellent. The theoretical maximum for white light is about 370 lm/W.

    How does efficacy save money?

    A more efficient bulb uses less electricity for the same brightness. Replacing a 60W incandescent (14 lm/W) with a 10W LED (80 lm/W) saves 50W and about $22/year running 8 hours daily.

    Ready to run the numbers?

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