We installed a light in our coop last fall after egg laying totally stopped. It's just a 60-watt bulb (in a protective cage kind of cover) suspended in the corner, and it's set on a timer. Within a week most our girls were laying again. We also put plexiglass over the windows for the winter, and between that and the bit of heat the light let off, they seemed comfortable.
My vet has a backyard flock and he advised against a heater beyond the lightbulb and blocking drafts. He told me people often insulate/heat their coops too much and the hens are susceptible to lung infections, so going from too warm to the cold in the run/outside, along with decreasing ventilation, is not a good idea. If you have breeds that are winter hearty, you shouldn't need a heater at all. I was really worried they'd be cold...but they do have all those feathers
Our light is just on for 4 or so hours in the early morning.
And they LOVE warm food in the winter! On really cold days I'd make them oatmeal, and every day I'd fill their waterers with warm water, 2x a day if it was freezing out (since we don't have a heated waterer).
Oh, I don't know how much heat it emits, but we also do the deep litter method thing -- basically never cleaning out the coop, haha -- letting it compost in there and adding fresh pine shavings and DE. That's supposed to generate heat, as well.
