A lot of factors go into sizing the heater:
1) evaporation rate
2) insulation of the tank
3) added wattage from other equipment
4) air flow around the tank
5) actual tank dimensions
6) wall / stand thickness of tank
7) ambient temperature of the room
Assuming no evaporation, no insulation, no air flow, no added wattage, a standard 180 gallon, normal wall thickness and ambient temps of 40F, you are looking at more than 1,000 watts. Insulation will help some, but evaporation will hurt. I'd insulate as much of the tank as possible, cover any areas where evaporation will occur, and start at 750 watts. I think that will be low however and you face a serious problem if you lose power for even 6-12 hours.
I'm skeptical that the room gets to 40F. The upper surface of the earth is 56F, so unless you have an unheated partial basement with poorly set windows/doorways leaking heat, the temp is probably higher. (I lived near Chicago with an unheated partial basement that had a sump, a dozen windows and where outside temps went to -20F, and my basement was never that low.)
In that case, fix your windows and doorways. If your basement is a full basement, it's not 40F.