"Every bit of energy you put in comes out as "light".
However, only a very small fraction of that "light" is
visible. Most of it is at such a long wavelength that
it is usually called "heat". (eg: blackbody radiation
at 2500-3500K)."
That's _almost_ true. A good deal of it also comes out as blackbody
radiation at a mere 350K or so, directly from the ballast & other
parts of the box. This is particularly true of things like halide
lamps, HPS lamps, etc., where the ballast may be only 80-95%
efficient.
In general, though, Rick has it right: the wattage rating on a lamp is
how much it gets, not how much if gives off in light. As Rick points
out, regular lightbulbs are about 4% efficient in the visible range,
which means that a 100-W lightbulb would have to draw many thousands
of watts from the wall in order to actually produce 100 watts of
light.
Another way you can verify this for yourself is that a 400-watt halide
lamp is MUCH brighter than 4 100-watt incandescent lamps. It is
therefore impossible for the "4 x 100 watts" or "400 watts" to refer to
the amount of light that comes out, because they'd have to be the
same. Right?
Someone else, I forget who, said something about "...edwardian...,"
referring to the display case that got us into this discussion in the
first place. I think that's actually "Wardian," named after a person.
Cheers!
jon