> If you buy a 100W lamp of any type, doesn't that mean that you are expected
> to get 100W of radiant energy out of the thing? And so if you have a 100W
> lamp with 50% efficiency, doesn't that mean you're going to be consuming
> 200W of power?
A "100W" lamp uses 100W of electrical power (about 100/120 = .8 amps).
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).
I think a few percent efficiency is common for incandescent bulbs. That
means for every 100W electrical input, you get ~4% visible power out.
Fluorescents are several factors more efficient. Thats why you can
buy bulbs that are advertised as the "light output of a 100W incandescent,
with only 26W power draw".
-- Rick