I'm going to make a waterbath for my fermenting vessels.
I calculate it'll have a maximum of about 45litres of water in it, less whenever there's more than 1 demijohn in there. Much less if I fill it with 4 of the things.
It'll be powered by an aquarium heater. The specs for them suggest that for >40 litres you need at least 50W.
BUT my waterbath will be insulated all over. Losses /should/ be minimal.
Tell me 25W will be OK.