Depends on location, if it is in your house then yes, however I have 20-40 gallon tanks in my garage (while it is insulated and heated) I require a 150Watt heater for the tanks to maintain 80 deg....the 100 watt could do it but it was on 24/7.Yup, people tend to go overkill on the wattage. Then if the heater gets stuck in the on position....cooked fish anyone?
100 watts is good for 20-40 gallons, so I wouldn't go more than that.