Enter the total light load (watts) and the total light area (ft^2) into the Lighting Power Density (LPD) Calculator. The calculator will evaluate the Lighting Power Density (LPD). 

Lighting Power Density (LPD) Calculator

Enter any 2 values to calculate the missing variable

Related Calculators

Lighting Power Density (LPD) Formula

Lighting power density measures how much installed lighting power is assigned to a given area. It is a simple but useful metric for comparing lighting layouts, estimating electrical demand, and evaluating whether a space has relatively low or high connected lighting power for its size.

LPD = \frac{LL}{LA}

Where:

LPD
Lighting power density, usually expressed in watts per square foot or watts per square meter.
LL
Total light load, or total connected lighting wattage for the space.
LA
Total light area used in the calculation.

If you know any two values, you can rearrange the equation to solve for the third:

LL = LPD \times LA
LA = \frac{LL}{LPD}

How to Calculate Lighting Power Density

  1. Determine the total connected lighting load for the space.
  2. Measure or confirm the floor area being served by that lighting load.
  3. Use matching units for the area and the output you want to interpret.
  4. Divide the lighting load by the area to get the LPD.

The calculator above can solve for LPD, total light load, or total light area when the other two values are known.

Units and Conversion Notes

LPD is only meaningful when the wattage and area units are interpreted consistently. A result in W/ft2 is not numerically equal to the same result in W/m2.

Light Load Input Area Input Resulting LPD Unit
W ft2 W/ft2
W m2 W/m2
kW ft2 kW/ft2
kW m2 kW/m2

Common area and density conversions:

1 \ \text{ft}^2 = 0.092903 \ \text{m}^2
1 \ \text{m}^2 = 10.7639 \ \text{ft}^2
1 \ \text{W/ft}^2 = 10.7639 \ \text{W/m}^2
1 \ \text{W/m}^2 = 0.092903 \ \text{W/ft}^2

Example

If the total connected lighting load is 500 W and the total light area is 300 ft2, the lighting power density is:

LPD = \frac{500}{300} = 1.67 \ \text{W/ft}^2

This means the space has 1.67 watts of installed lighting power for each square foot of area.

How to Interpret the Result

  • Lower LPD generally means less installed lighting wattage per unit area.
  • Higher LPD means more connected lighting power is concentrated in the same footprint.
  • LPD is a power metric, not a direct measure of brightness, visual comfort, or lighting quality.
  • LPD is not the same as energy consumption; schedules, dimming, sensors, and operating hours also affect actual energy use.

What Affects Lighting Power Density

  • Fixture quantity and wattage
  • Lamp or LED module efficiency
  • Driver or ballast losses
  • Ceiling height and spacing strategy
  • Task lighting versus ambient lighting approach
  • Controls such as dimming, occupancy sensing, and daylight response

Common Input Mistakes

  • Mixing area units such as entering square meters but interpreting the result as W/ft2.
  • Using an incomplete wattage total instead of the full connected lighting load for the space.
  • Using the wrong area by including spaces not actually served by the lighting being analyzed.
  • Using zero or near-zero area, which makes the result invalid or unrealistically large.
  • Confusing LPD with illuminance; power density does not tell you the actual foot-candles or lux on a surface.

Quick Reference

Known Values Use This Relationship
Total light load and total light area 
LPD = \frac{LL}{LA}
LPD and total light area 
LL = LPD \times LA
LPD and total light load 
LA = \frac{LL}{LPD}

Frequently Asked Questions

Is lighting power density the same as illuminance?
No. LPD measures installed power per unit area, while illuminance measures how much light reaches a surface.

Should the area be the entire building?
Use the area that corresponds to the lighting load being evaluated. If only one room or zone is being analyzed, use that room or zone area.

Do controls reduce LPD?
Controls can reduce actual energy use, but they do not change the connected lighting power unless the installed wattage itself changes.

Why is my result very high?
The most common reasons are an overstated wattage input, an understated area input, or a mismatch between square feet and square meters.