The conversion of watts into lumens for a halogen bulb is a simple calculation once the efficacy of the bulb is known. Lumens are the measurement used to determine the brightness of the light emitted by a bulb. It is necessary to convert watts to lumens to determine how bright a bulb is. Halogen bulbs produce more lumens than their standard incandescent counterparts. This is why they are desirable for a lighting solution.
The job of any bulb is to convert electricity (watts) into light (lumens). Efficacy is the term used for the measurement of lumens produced per watt of power used. The higher the efficacy of a bulb, the more light it will produce at a specified wattage. The formula for calculating the lumens produced by a bulb is a simple one.
watts X efficacy = lumens
The average efficacy rating for a halogen bulb is 18.4. I say average because the rating fluctuates with the amount of heat produced by the bulb and this affects the efficacy.
Let's use a Sylvania 72 watt quartz halogen frosted bulb for an example.
72 watts X 18.4 L/w = 1324.8 lumens
By using this formula, we can compare the different wattages, and determine the appropriate amount of light output for the area being lit.