I have known heating 40 years but 4 years ago I was obliged to try what ever to keep my coffee cup size colonies alive. I lost 60% of my hives. But that catastrophe was plessing to me.
Electrict has high price. It is cheaper to winter with sugar. Too much heating disturbs winter ball.
I have wintered 2-3 frame colonies and they go splended over winter with 3W heater. Such colony is value queen.
Spring heating is very productive. With pollen patty and heating spring build up is 3 times faster than with natural way.
When outer temperature is +17C, heating is not needed. But nights are cold in spring. Heating makes hive corners warm and bees may occupy larger area from frame for brood.
Big hives get biggest advantage from heating. Build up is enormous. Then you may give emerging brood frames to weak colonies and they will be quickly normal foragers.
15 w cabel heaters are best for big hives. They are cheap and easy to use. http://www.reptilica.de/product_info.php/product/Lucky_Reptile_Thermo_Cable
I have now 25 heated hives. I heat them 3 months from March to beginning of June.The basic of warm economy is insulated brood boxes.