Understanding Lumens to Watts Conversion
Lumens measure how much visible light a source produces. Watts measure how much electrical power it consumes to produce that light. The two are related — but not fixed — by a bulb's luminous efficacy, expressed in lumens per watt (lm/W). Older incandescent bulbs waste most of their energy as heat and only convert a small share to visible light, so they need many more watts to hit a given lumen target than an LED does.
The formula
The standard conversion is:
- Watts = Lumens ÷ Luminous efficacy (lm/W)
For example, an LED rated at 100 lm/W producing 800 lumens draws about 800 ÷ 100 = 8 watts. An incandescent bulb at roughly 14 lm/W needs about 800 ÷ 14 ≈ 57 watts to produce the same 800 lumens.
Typical luminous efficacy by technology
- Incandescent: roughly 10-17 lm/W (most of the input energy is lost as heat).
- Halogen: roughly 16-25 lm/W (a modest improvement on incandescent).
- CFL (compact fluorescent): roughly 45-75 lm/W.
- LED: roughly 80-120 lm/W for common household bulbs, with some high-efficiency models exceeding that.
- Linear fluorescent tubes: roughly 60-100 lm/W.
These ranges are typical manufacturer specifications, not physical constants — always check the lumens and wattage printed on a specific bulb's packaging for an exact figure, since efficacy varies with bulb quality, wattage tier, and color temperature.
Why an "incandescent equivalent" is useful
Packaging for LED and CFL bulbs often lists an "incandescent equivalent" wattage (for example, "60W equivalent") because shoppers are used to thinking in old incandescent watt ratings. That equivalent is simply the bulb's lumen output divided by roughly 14.5 lm/W, the typical efficacy of an incandescent bulb — it tells you how bright the new bulb will look, not how much power it actually draws.