will the tank have a hard time staying @ 79 degrees if your room temp is 68 degrees? (with no lid of course)
Should I invest in a higher wattage heater?
It is possible you may need a bigger heater. Although yours isn't as extreme of a case as mine.
I have a 40 gallon in my store (sans the lid and with a sump) and the store is unheated at night. During the winter, it easily gets into the 50's in the store. If it is real cold outside, it will get into the high 40's. I keep a 300w heater in the sump and it easily copes with the outside temperature change. In fact, there are times when it is too warm in the tank.
As you can see, my heater is way oversized for my tank. However, I felt that because of the vast temperature difference I would need something large. I don't think you will have a problem since you building isn't as cold as mine.