Even if your temps get down to -20 you don't need to supplement heat. We humans think "oh gee, I'm cold, the chickens must be cold" .. but that's not true. As the other poster asked, what breeds do you have...hardy breeds? I invite you to go to this new thread here at BYC:Hi guys, I was thinking about buying a 150 watt flat panel heater for my 8x8 insulated coop, I live in mid Vermont and it can get well below zero a lot of nights. Do you think that'll be okay?