It's best to have a minimum of 2 heaters. It really depends on what temperature your room is kept at. If you keep room at 68 or above in the winter and it's not super drafty two 250 watts or two 300 watt heaters would be ok. I use a 300 as my primary set at 78 and a 250 that kicks in if temp drops below 78. On really cold nights in the winter the 300 will stay on and the 250 will cycle. Thats on a 240 with a 75 sump all controlled by apex