The wattage of a heater depends on the temperature difference between the tank and the room. I have a heater box in front of me that states if there is a 10 degree difference between the room and the temperature you want to achieve the heater size should be a minimum of 80 watts. That is for a 20 gallon tank. For a 20 degree difference the minimum requirement rises to 187 watts. That is in degrees Fahrenheit of course.
