Joshua Putnam writes:
We use a compact fluorescent specifically designed for this application.

It's a compact twist design, 150-watt equivalent, designed for cold
weather exterior use, with a built-in photocell in the base. Just screw
it in to replace a standard bulb and leave the switch on. No problems
with it so far.

Sounds like just what I need (though maybe not as much as
150-watts). Where can I buy?