I figure we will use the spa on average 8 hours per week throughout the year. Heating the pool every once in a while would be nice but I am really only concerned with heating the spa.

Will I need to run the heater the entire time we are in the spa? 4.5 gallons per hour looks expensive. We are able to get propane at $1.76 per gallon. I want to have a tank that is big enough that I will not need to fill it all of the time, will 125 gallons work or should I go bigger?