Some light sources such as flashlights are rated in various different units, such as candlepower or watts. To be able to compare different types of light sources, it would be necessary to convert between these types of units.
The short answer is, you cannot. Unless the manufacturer already has calculated both candlepower and watts, you're pretty much out of luck. You could try searching review sites in addition to the manufactures site, in hopes of finding the information already calculated.
Different light sources could have the same power requirements, but vastly different light output. They are just not directly related enough.
For example, searching Amazon.com I found descriptions of two different 12,000 candlepower flashlights. One was listed as 3 watt and another listed as 1 watt.
Here are the definitions of some units:
One candlepower is the radiating power of a light with the intensity of one candle. This unit is considered obsolete as it was replaced by the candela in 1948, though it is still in common use. 1 candlepower is equal to about 0.981 candela. *
The standard unit for measuring the intensity of light. The candela is defined to be the luminous intensity of a light source producing single-frequency light at a frequency of 540 terahertz (THz) with a power of 1/683 watt per steradian, or 18.3988 milliwatts over a complete sphere centered at the light source. *
The standard unit for measuring the flux of a light being produced by a light source. One lumen represents the total flux of light emitted, equal to the intensity in candelas multiplied by the solid angle in steradians (1/(4Ěpi) of a sphere) into which the light is emitted. *