150 watts more than enough. need to remember how warm you keep the place where it's located if it's already always 75 degrees then that's not much, if it always 50 degrees in the house or garage then it will need to work more. All heaters are not equal also some are better and don't take as much wattage as other. I am running a 125 gallon tank and about 30 gallon sump on a 250 Watt heater. But I know with house heat and such it's not being over worked. Look at the aqueon pro heaters. Lifetime warranty and have a light always on letting you know it's working and different light color when the heater is heating the tank