DO NOT DO THAT, in my strong opinion... first, it will TORPEDO your hydro bill; and second, I'd be real leery of using something like that anywhere near a dusty environment like a coop (heater is off during day, gets dust or bedding on the element, then turns on, *poof* roast chickens)
Look, if you can afford the hydro bill to run that heater you can FOR SURE afford to buy a buncha insulation for your coop (which you wouldn't need to *buy* anyhow since you can get insulating material free if you look around); and likewise you can afford to buy a heat lamp, which will be cheaper to run (b/c you can reduce wattage as needed) and relatively speaking safer. A heat lamp is like $10-20 for the fixture plus $5-15 for the bulb, depending what exactly you get. That's *all*.
Really.
Good luck,
Pat