The most common and logical measure of lighting efficiency is calculated by dividing light output (measured in lumens) by the electric input (in watts). The resulting value is lumens per watt, sometimes referred to as LPW. Since the value for lumens per watt is always greater than one, the lighting industry correctly refers to the "efficiency" of a lamp as the lamp's "efficacy."
Be careful about your assumptions of LPW since some light sources require the use of electric ballasts to provide required circuit conditions. Input wattage values from most manufacturers' catalogs and tables refer only to the wattage of the lamp alone.
The figure shown here illustrates the efficacy of various light sources (including ballast load) as well as the average rated life of different lamp types.
