Assuming you have:
1) 300 gallon wide
2) no air movement in the room
3) no evaporation at all
4) 100 watts of heat in the water from pumps and lights
You will need a minimum of 182 watts to reach 80 F in a 72 F room.
However, I highly encourage you to get 2 heaters (roughly 200-250 watts each) because:
a) it's safer for the stock;
b) it will be impossible to heat the tank for ich (up to 88 F) if needed unless the room temperature is raised to 80 F;
c) you likely are really losing a lot of heat from air movement and evaporation (especially if you have a sump); and,
d) extra heaters for higher wattage doesn't "cost" more. It costs a bit more for an extra heater but the difference in cost from electric usage is insignificant.
Many people will recommend a controller for the heaters as well.