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

    Ready to run the numbers?

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