What kind of fish do you keep and where is the temp now. Could running the tank at 72 be an answer?
In my case I figured during the winter my room temp will be about 20 degrees colder than the tank needs to be. This made it easier for me to calculate what size heater I'd need.As for heating requirements, I recommend 25 watts for every 10 degrees of ambient temperature per 10 gallons you need to raise your aquarium temperature. EX: If your home is 68 degrees and you have a 40 gallon aquarium, to reach a temperature of 78 degrees you would need a 100 watt heater