How do you transfer watt to lumen?

When measuring light, the lumen and the watt both measure power -i.e. the rate at which electromagnetic energy is emitted (for visible light, this 'power' is termed 'luminous flux'). Although they are both SI units, one (the lumen) is used in the photometric system of measurement, whereas the other (the watt) is used in the radiometric system of measurement. Whereas the watt is used to measure the total rate at which electromagnetic energy is emitted from a light source, the lumen is used to measure only the visible part of the spectrum and, then, corrected for the sensitivity of the human eye (the eye is most sensitive to the green part of the visible spectrum. The watt, of course, can also be used to measure the input power to a lamp.

In the radiometric system, the input and output powers of a light are each expressed in watts, but in the photometric system, the input power of a light is expressed in watts, but the output power (luminous flux) is expressed in lumens. The ratio of a light's output (in lumens) to input (in watts) power, is termed efficacy (NOT 'efficiency'), and it various according to the type of light. An incandescent lamp, for example, has an efficacy of up to about 15 lumens per watt, whereas a fluorescent lamp has an efficacy of around 50 or so lumens per watt.