LED Efficiency and Lumens per Watt
Efficacy compares light output with electrical input.
Lumens per watt
Luminous efficacy measures how much visible light an LED gives per watt of electrical power, in lumens per watt (lm/W). A higher figure means less wasted energy.
Typical ranges
- Incandescent bulb: about 10 to 17 lm/W
- Compact fluorescent: about 50 to 70 lm/W
- LED lamps: often 80 to 150 lm/W or more
Indicator LEDs are rated differently
Small indicator LEDs are usually specified in millicandela (mcd), which describes brightness in one direction, not total light output. You cannot compare mcd to lumens directly without the viewing angle.
Efficiency changes with conditions
Efficacy usually drops as current and temperature rise, so running an LED gently often gives more light per watt. Warm-white LEDs are typically a little less efficient than cool-white ones.
Try it yourself
Sketch the circuit on paper first, calculate the values, then build it on a solderless breadboard using a low-voltage supply. Measure with a multimeter before you make anything permanent.