Incandescent lights give off heat as well
as light energy. The higher the wattage
of the light bulb the higher the temperature.
A compact fluorescent bulb gives off very
little heat energy because they do not use
resistance and cause a light to glow hot.
In a home or office, lots of incandescent
lights means that the air conditioner would
have to use more energy during the summer
to remove the extra heat given off by lights.
Also, some lights such as torchieres, can
be very dangerous as the bulbs are rated
at 300 watts or more and get VERY hot. They
can catch drapes or other materials on fire
if you're not careful.