The measurement of the power of sunlight emitted from the sun is called Radiant Flux and is measured in watts. The measurement of the perceived power of sunlight is called Luminous Flux and is measured in lumens. Lumans are directly related to candelas (or candlepower), which is the luminous intensity emitted in any one direction. Measurement Methods: Surprisingly, luminosity is still often measured by eye. Another popular method is by measuring the current emitted by a photovoltaic cell.
http://en.wikipedia.org/wiki/Radiant_flux http://en.wikipedia.org/wiki/Candlepower http://en.wikipedia.org/wiki/Lumen_(unit) http://en.wikipedia.org/wiki/Luminous_flux