Great question, polar99! We need to decide this as well. Hopefully others with experience will chime in.

I chose a 500 gal underground for the on demand hot water, cooktop, fireplace, bbq grill, and 20kw emergency power generator. My heat is an electric heat pump with electric back up.

We have a 500 gal underground tank used for cooking, dryer, and gas fireplace. A fill-up typically lasts us 2-3 years. It depends upon fireplace use more than anything else for us. In the colder months we'll use it for 3-5 hours a day every day. That amount of use for about 4 months typically uses about 1/3 of the tank. When we had it connected to the pool heater, we used a tank per year. Our main heat source is geothermal with electric backup. I don't know how the water heater would impact use of propane. My guess is that you might use 1/2-3/4 of a 500 gallon tank/year. I think the 1000 gallon tank is probably overkill. You can do a partial fill in the summer if you want to take advantage of lower prices and the tank isn't empty.

Thanks folks.. I decided on 1000g tank because this is my main backup (no electric backup).. You cannot beat the 1$ a gallon ..