Light Pollution Calculator Formula

Understand the math behind the light pollution calculator. Each variable explained with a worked example.

Use the Light Pollution Calculator

Formulas Used

Wasted Lumens

wasted_light = wasted_lumens

Wasted Energy per Year

wasted_energy = wasted_kwh

Annual Wasted Cost

wasted_cost = wasted_kwh * elec_rate

Total Annual Lighting Cost

total_energy_cost = annual_kwh * elec_rate

Variables

VariableDescriptionDefault
num_fixturesNumber of Light Fixtures10
lumens_per_fixtureLumens per Fixture(lumens)4000
uplight_pctUpward Light Percentage(%)30
watts_per_fixtureWatts per Fixture(watts)50
hours_per_nightHours On per Night(hours)10
elec_rateElectricity Rate(USD/kWh)0.15
wasted_lumensDerived value= num_fixtures * lumens_per_fixture * uplight_pct / 100calculated
annual_kwhDerived value= num_fixtures * watts_per_fixture * hours_per_night * 365 / 1000calculated
wasted_kwhDerived value= annual_kwh * uplight_pct / 100calculated

How It Works

How to Calculate Light Pollution

Light pollution is wasted light that shines upward or sideways instead of illuminating the intended area.

Formula

Wasted Lumens = Fixtures * Lumens * Uplight %

Wasted Energy = Total Energy * Uplight %

Types of Light Pollution

  • Skyglow: Brightening of the night sky over populated areas
  • Glare: Excessive brightness causing visual discomfort
  • Light trespass: Light falling where not needed
  • Clutter: Excessive groupings of light sources

    • Fully shielded fixtures eliminate uplight and reduce waste by 30-60%.

    Worked Example

    10 fixtures at 4,000 lumens each, 30% uplight, 50W each, running 10 hours/night at $0.15/kWh.

    num_fixtures = 10lumens_per_fixture = 4000uplight_pct = 30watts_per_fixture = 50hours_per_night = 10elec_rate = 0.15
    1. 01Wasted lumens: 10 * 4,000 * 30% = 12,000 lumens
    2. 02Annual energy: 10 * 50 * 10 * 365 / 1,000 = 1,825 kWh
    3. 03Wasted energy: 1,825 * 30% = 548 kWh
    4. 04Wasted cost: 548 * $0.15 = $82/year

    Frequently Asked Questions

    How much energy is wasted by light pollution?

    The International Dark-Sky Association estimates that at least 30% of outdoor lighting in the US is wasted, costing about $3.3 billion annually and emitting 21 million metric tons of CO2.

    How does light pollution affect wildlife?

    Artificial light disrupts migration patterns of birds and sea turtles, confuses nocturnal insects, alters predator-prey relationships, and disrupts plant flowering cycles.

    What are dark sky friendly fixtures?

    Dark sky friendly fixtures are fully shielded (no uplight), use warm color temperatures (under 3000K), and are properly aimed and dimmed. They provide better visibility while reducing waste.

    Ready to run the numbers?

    Open Light Pollution Calculator