Yes, Finski used reptile heaters (15W).
I use them in spring build up. Never in wintering.
And not heating on bottom in wintering.
A cluster must have winter rest. Added heat by electric is like a bigger cluster in the hive. Heat comes from side or it is up.
But other sides are cold and bees do not wander around during winter.
I have wintered 2 frame nucs with electric, but problem is that that nuc is not able to start brooding during winter. It is only curiosity and not real wintering. I know the case now and do not mind work with those trouble makers any more. It is easier when you give couple on brood frames to the nuc and the colony is big enough for winter.
If you have for example 3 frames bees, you must first reduce the wintering room to 3 frames.
Then use 3 W heater and put it on the top of frames.
If heating is under the cluster, it breaks the cluster.
During winter I can see that on +C weather bees are spread
quite largely inside the hive, but during -20C they are around the heater like a heater is part of cluster.
In Robo's picture a mesh floor and bottom heating is a strange combination.
When I have solid ply bottom, 15 W heater keeps the whole bottom warm and the ply is very warm on opposite side too.