There is nothing better than Solar Energy. This heater works so well, you may end up putting one or two in your home.  This is how it works: 

Forced air solar window heater- unit sits in a south window sill of your coop .  When the sun shines, it collect on the panel  and when the built in thermostat detects heat, if pulls air from the coop through the warm panel.  Keeps a 650 sq ft coop 70 degrees!

Ideal for sunny locations. You should have a few hours of sunlight a day, for this to work well.