Calculate bulb luminous efficacy in lm/W from lumens and total electrical watts, or estimate LED replacement wattage using editable incandescent, halogen, CFL or custom efficacy assumptions. The replacement estimate compares equal assumed lumens, not product compatibility.

Use rated lumens and total electrical watts. Replacement presets are illustrative assumptions; compare actual product labels for equal light output.

Luminous Efficacy Formula

LE = LF / P

Variables:

  • LE is the Luminous Efficacy (lumens/watt)
  • LF is the luminous flux (lumens)
  • P is the power (watts)

To calculate Luminous Efficacy, divide the luminous flux by the power.

How to Calculate Luminous Efficacy?

The following steps outline how to calculate the Luminous Efficacy.

  1. First, determine the luminous flux (lumens). 
  2. Next, determine the power (watts). 
  3. Next, gather the formula from above = LE = LF / P.
  4. Finally, calculate the Luminous Efficacy.
  5. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

luminous flux (lumens) = 34

power (watts) = 10; efficacy = 34 / 10 = 3.4 lm/W.

FAQ

What is luminous flux?

Luminous flux is a measure of the perceived power of light. It is quantified in lumens and represents the total amount of light emitted by a source or received by a surface.

Why is luminous efficacy important?

Luminous efficacy measures how effectively a light source converts electrical power (watts) into visible light (lumens). It's important for evaluating the energy efficiency of lighting systems, helping to reduce energy consumption and costs.

Can luminous efficacy vary between different types of light bulbs?

Yes, luminous efficacy can significantly vary between different types of light bulbs. For example, LED bulbs are generally more efficient than incandescent bulbs, producing more lumens per watt.

How can I improve the luminous efficacy of my home or office?

To improve luminous efficacy, consider using energy-efficient light bulbs such as LEDs, optimizing natural light usage, and ensuring that lighting is appropriately sized and placed for the intended area.