Choosing LED wattage by guessing from an older bulb can lead to lighting that is too dim, unnecessarily powerful, or inefficient. A better approach is to start with the light output you need and then account for how efficiently the LED converts electricity into visible light. This calculator estimates the required LED wattage from those two values.
How to Calculate LED Wattage (step by step)
Step One: Identify the target light output
Light output is measured in lumens. Enter the brightness you want from the LED lamp, not the wattage of the older lamp you are replacing.
Target light output = 1600 lm
Step Two: Identify the LED efficacy
LED efficacy is measured in lumens per watt. It describes how much light the LED produces for each watt of electrical power. Use the product’s rated efficacy when it is available.
LED efficacy = 100 lm/W
Step Three: Divide lumens by efficacy
Divide the target light output by the LED efficacy. The lumen units cancel with the lumen portion of lumens per watt, leaving watts.
Required LED wattage = target lumens ÷ LED efficacy
Required LED wattage = 1600 ÷ 100 = 16 W
What your results mean
The result is the approximate electrical wattage the LED lamp needs to produce the target lumen output at the stated efficacy. With the default values, an LED rated at one hundred lumens per watt needs sixteen watts to produce one thousand six hundred lumens.
Treat the result as a sizing estimate rather than an exact product specification. Manufacturers may round wattage ratings, and the actual lamp may operate at a slightly different efficacy depending on temperature, driver efficiency, optical design, and dimming.
A higher-efficacy LED produces the same brightness using less power. If two lamps have the same lumen rating but different efficacy values, the lamp with the higher efficacy will have the lower calculated wattage.
Practical selection tips
Select a commercially available lamp whose rated wattage is close to or slightly above the calculated value. Also verify the lamp’s actual lumen rating, color temperature, beam angle, dimmer compatibility, voltage, and physical fit. For an enclosed fixture, confirm that the product is approved for enclosed use because heat can reduce LED performance and service life.
Remember that lumens describe the light produced by the lamp, while lux describes light arriving on a surface. If you are designing lighting for a room, you may need a separate illuminance calculation that considers room area, mounting height, reflectance, and fixture placement.