With this online calculator you can convert lumens to watts and vice versa.
Formula:
P = Lf / Lc
P - power
Lf - luminous flux
Lc - lamp characteristic
Lamp characteristic:
Incandescent lamp = 10 ... 15 lumen/watt
Fluorescent lamp = 30 ... 40 lumen/watt
LED lamp = 80 ... 100 lumen/watt
Luminous flux is a physical quantity that characterizes the amount of light power in the radiation flux.