I've tried to find the heat cable at the few pet shops around here, but haven't had any luck. Any suggestions?
Well, I was too cheap to drop $20 per hive for a heat cable, so I tried a different method. I bought some night/Christmas light sockets, the ones that snap into the ceramic Christmas village, at the dollar store. I mounted 2 in each hive between the bottom board screen and the slide in cover, so that the bees can't get to it.
I used 2 because Finman suggested 15W and each night light is 7W. I find 2 lights to be too much and causes the bees to fly when it is to cold out, so I have been only using 1 light now. I an trying it with 2 single deep splits that I did late in the fall. They are both still alive and as far as I can tell (too cold to open them up) doing OK.
Take it for what it is worth, I'm not trying to endorse iit at this point, but since you asked..........
Next year I'm going to try it with a few more hives and control 1 or 2 lights based upon the outside temperature. This way when the sun comes out and heats up the hive, I can turn 1 or both off to prevent it from getting too warm and them flying to their death.
Finman, have you any problem with it getting too warm for them and causing them to fly when it is too cold? I would think the insulated hives would experience this more.