A wise old friend of my husbands told us this when asking him about getting a "heater" for our chicken coop.
"When I was kid on the farm, we had 1000 layers in a 30 X 50 2 story building thats 3 sq ft per chicken. If your brooder light has a 100 watt bulb it will give 312 BTU's
adjust the height to give a thermometer laying on the floor to read 50 deg directly under the light should be fine.
In mild weather your bulb doesn't need to be a 100 watt.
You don't need to heat all the room. The chickens will move in or away from heat as they feel, too hot they'll move out, cold they move under it more. Now realize as it gets colder you need to lower the hood.
100 watts = a penny an hour or a quarter a day. 2 or 3 eggs worth."