No, in short unless you live somewhere warm.
I have a 60x15x18 with a 300w heater and its set at 30deg (just below the point that has it on permanantly) and it holds 25. I have a spare 100w that will be going in if i see the temp dropping or the heater on too long.
1w/litre seems to be the reccomended minimum, my experience would say more than that. More like 1.5/litre even 2 but it may just be my circumstances.
When i heated the tank up after taking the goldfish out ready for the tropicals the tank increaced roughly a degree every 3 hours slowing as it got up to 25. I'm not sure how much hotter it would actually heat the tank.