I am new to all of this. I am more of a 'computer person' than a 'bee person' right now. With this in mind I hope that you will forgive any lack of bee knowledge in this post.
I have noticed several posts, and even an article on this site, relating to hive heating throughout the winter months. I also know of wonderful little items called Thermoelectric Heaters (Sometimes called Thermoelectric Coolers or TECs) or Peltier units. These have fairly recently been used to allow 'supercooling' of home computers by individuals who want to increase the overall speed of their main processors (Overclockers). These units are better suited for heating than cooling CPUs though.
Peltiers come in many sizes. Those best suited for CPU cooling (40mm X 40mm X 7mm) would probably be a bit much for use in hives. Typical units provide a temperature rise of up to 120 Degrees Centigrade - This is well above the boiling point of water. They would also prove quite costly to maintain and potentially dangerous if used on a standard electrical outlet. Smaller Peltier units would prove much more practical for beekeeping. I would speculate that a good deep-cycle 12V marine battery ($70US ea) and a quality 800ma solar battery charger ($90US) could keep up to six Peltier units running for several months at a time. (The battery cycling would eventually drain the battery completely and require a 'swap' with a fresh one.)
If an industrious person were to incorporate a small thermostat in each box then the Peltier unit in that box would only be activated when needed. In this case a single battery would likely last the entire cold season without need of refreshing.
10mm X 10mm Peltiers could be effectively used to heat six or eight bee boxes to 40 or 60 degrees Farenheit. They could be placed under the outside of the boxes to create a radiant heat. The thinner the wood in the bottom, the less energy would be required to pass the heat up through the insulating wooden box bottoms.
I have obviously typed more than enough here. I have found that most Beekeepers are 'DIY' type people, so I will leave it up to your own research to decide whether or not this is a viable idea. If it is then I am glad to place it in the Public Domain for any and all to freely use.
Under normal conditions I don't think heating is wise at all. Under extreme conditions where a hive is really not strong enough to make it throug the winter, it might be useful to heat it to say 40 degrees Farenheit. Any warmer and the bees might try flying. I have considered it for hives that were having difficulties. Also, maybe it would be helpful for overwintering nucs in the far north to sell in the spring.
I assume these are simply someting that gets hot, not something with a fan involved? I don't think you want a fan. you just want some warmth on the bottom that will heat the air some.
I think this would be good to use in very early spring to help raise queens and drones. It could be used on a drone mother colony and the queen rearing colony. I would like some more info on this. I have both a PV unit 30 watt and a 12 v deep cycle battery.