at what average daytime temperature would you turn the heater on and approximately when would you turn it off?
Actually I live in capital city and I have 150 km trip to my hives.
Heaters are all the time on at spring. This spring I put thermostat which turn off circuit when day temperature rise to 15C. When day tempereture is 20C I may see that bees have too hot.
you said to help small colonies survive winter you have placed these heaters on the side of the hive,but I do not recall how this plan worked for you.
During 2 winters I have had 2 frame colony over winter. They succeeded splended. Just now I have again 2 small colonies and 3W heaters inside hives. Out temperature has been -25C during a week. Hives are in the firewood shelter. Another colony is two twist size. Temperature rised tomorrow to -9C and netx day
Long period of cold is dangerous, 1-1,5 month. Bees go to some corner and use food to end.
It is impossible to get honey 2-frame colony with the help of itself. I give emerging brood fames to hive . When it has whole box bees since that it will develope in normal way. So I have got 150 lbs average yields from those dwarfs. I even foraging powers of hives or put hives together.
I know that USA is a vast country and you have many kind of weathers there.