How many lumens to equal a watt?

Answer 1

Incandescent bulbs: 10 lumens per watt

Halogens: 13 lumens per watt

CFLs: 50 lumens per watt

A useful bulb to light a small room is 600 lumens, so that would need a 60-watt incandescent, or a 45-watt halogen, or a 12-watt CFL energy-saving bulb.

The best CFL bulbs are the spiral ones.

Answer 2

Lumens and watts measure different things, so they cannot be "equal" or converted unless you make some big assumptions. Lumens refers to the amount of light in the visible spectrum within a viewing angle; watts refers to the amount of power.

For instance, an incandescent bulb of 100 Watts might emit about 1500 lumens (360 degrees), but a compact fluorescent (CFL) bulb might only need 22 watts to generate the same amount of light and an LED bulb might give 1500 lumens with only 15 watts or so (typically in a much narrower viewing angle).

Another Answer

A lumen measures the output power (i.e. its luminous flux) of a lamp in the photometric system. The watt measures the output power of a lamp in the radiometric system (it is also used to measure a lamp's power input).

In the radiometric system, the watt measures the rate at which emits electromagnetic energy across all frequencies, whereas in the the photometric system, the lumen measures the rate at which electromagnetic energy is emitted as visible light, corrected for the sensitivity of the human eye.

The relationship between a lamp's input power (in watts) and its output luminous flux is termed the 'efficacy' (not 'efficiency'!) 's the light source, and is expressed in lumens per watt. Efficacy depends upon the type of light source -e.g. an incandescent lamp has an efficacy of between 5-15 lumens per watt.