If they were going to equate power to a real world unit it shouldn't be wattage which is usually a measure of the amount of electricity consumed by a light.  The light emitted would properly be measured in lumens of light or radiant flux.  Radiant flus is measured in Watts but is the power output not the power consumed by a light source.  Lumens is a measure of the perceived power  and has the advantage of taking into account the human eyes perception of light at different wavelengths.  A good lighting catalogue (the kind used by professional lighting designers) will include this data (you can find it on some lighting manufacturers web sites too).

You'd also want to take into account the distribution from the source of the light generated (usually plotted as one or more polar graphs).  Real world lights don't generate equal lumens in all directions or in the very simplistic way that renderers usually produce spot lights with a simple fall off zone.  I've modelled real world lights from their data using gells in Vue but its hard work converting the graphs into spherical grey scale maps.

There is software that can do all of this and a lot more but it tends to be for specialist lighting designers.  Doing it to a level where the model would really benefit anyone using Vue would be very time consuming.  While we're producing artistic interpretations of our scenes it's probaly not woth e-on's effort and if someone does need to find the luminous intensity on a surface I'd rather they used the real software designed for that work.

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