I take into use terrarium heaters 2,5 years ago. I have wintered 2-frame hives with electricity and it have succeeded well. Normally to get 4-frame hive over winter is difficult.
Here is a small hive a week ago. Now we have here -5 - -10 C frost.
Box is stryrofoam and medium wall is stryrofoam board. Heater will be in the empty part of hive and closed between thin aluminium plate.
Traffic camera pictures from Finland. My bees are near the point VT 15 ANJALANKOSKI http://www.tiehallinto.fi/alk/english/frames/kelikamerat-frame.html
This is infra radiator warmer 7 W. It does not get warm itself but when radiation meets an object, radiation change to warm energy. Warmer is sold in petty shops.
So I put whole system to my fire wood shelter and I hope that they are satisfied with winter hotel service.