I only use my ‘heated’ fridge as a fermentation control for cider/wines during the warmer months, so it is energised as a fridge at times. The correct version of the STC1000 controllers works perfectly to maintain 18 degrees, or close to, for the fermentation period. It could be used for lagering as well (but I don’t make many beers) or liquefying honey if I so wished.
As I see it, the post has been useful for the OP as perhaps now he realises that it matters not whether the heater was a few hundred Watts or 10kW, the time to liquify the honey would be unchanged as long as the thermostatted temperature is maintained. An over-sized heater would only cycle the controller more, in fact, and wear out the switching contacts that much more quickly.
A water bath is so much faster, to warm honey. Just look up the heat capacity of water versus air to perhaps understand the benefits of using a water bath.