There are a bunch of variables when it comes to this. How much insultation is there between the room and the tank, how much between the tank and the floor, how much evaporation is there?

When the losses to the environment equal the heat input, the temperature stops going up. If the temperature surrounding the tank drops, the heat input must go up. During the summer when the basement is at 75F, I get by with 1KW cycling and keep 85F. In the winter with basement temps at 60F I have 2KW and have trouble keeping above 75F.

Hope this helps, but you can't really figure this out exactly because there is so much going on.