How much propane will I use

I will be building an IG pool and IG spa (7ft spa). I will be heating the spa with a 400K BTU propane heater. I would like to heat the pool a couple of time per year for special events. Local code limits the size of a propane tank to 125 Gallons; anything bigger will require a permit from the fire dept. While I can certainly get the permits required for a larger tank my question is will 125 gallons of propane be enough to run the spa without needing to fill it too often? I live in northern CA (Sacramento area).

One gallon of propane contains about 91,500 BTU, so you will consume roughly 4.5 gallons per hour when the heater is on.

What is the worst case scenario that you can think of that you will need to heat up? In other words, how cold will the water be and what temperature will you want to get it up to?
You can calculate how many btu's needed for this temperature rise. Once you determine how many btus needed, you can calculate the costs.

Do you know what the cost of LP is?

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?

It's going to depend on how you use it, how much water it holds, how hot you want it and what the water temp is when you start. 8 BTU raises a gallon of water one degree. If your spa is 500 gallons, in a perfect world, to raise the water from 80 degrees F to 104 degrees would take (104-80) X 8 BTU X 500 gallons = 96,000 BTU. That would be about a little more than a gallon of propane. Heating a spa is pretty wasteful, but I would guess you could heat it with 2 gallons.

My guess is you will be fine, we are with the same size tank. There's a loose rule about how low you can go on the tank, something a little less than half so all 125 gallons are not really available. Talk to the propane company.

Time how long it takes to raise the spa 10 degrees and figure out how much propane you used with the 4.5 gal/hour rate. You can apply that math based on the water temp since it will obviously take longer starting at 55 than 80. I doubt you will actually heat the spa 8 hours per week since it will start cycling on/off once temp is reached. It will run non-stop until you hit the desired temp but then only about 15-20min/hour to maintain it while you enjoy!

You might consider installing solar panels and plumbing them to your spa as well as to your pool.

I have a friend who uses his spa almost every evening. He routes the solar heating to it during the day for just a couple of hours and heats it up to 110 degrees. He says once in a while he forgets about it and it gets as high as 135 degrees. I know you have plenty of solar heat up there in Sacramento!