Check out the "Lights, Winter and egg laying" thread on this forum. Note that Roosmom is using a 25 watt red incandescent.
My understanding is that the light does not need to be very bright, only sufficient to read a newspaper. I use an 11 watt compact fluorescent but the lighted and insulated part of the coop is only about 24 square feet, so not much area to light.
Too much light may result in behavior problems . . .
Best of luck,
Steve