The size of the tank may be determined by either how many days of potential outage you want to cover or the minimum size needed for the generator capacity. The generator, operating at maximum output, needs a certain propane flow rate at some minimum tank pressure to support that output. Liquid propane in the tank must vaporize at the corresponding rate, and the heat absorbed by this vaporization must come from ambient air. The design condition is at some appropriate low winter temperature (perhaps zero F). The rate of heat transfer is proportional to temperature difference between air and boiling propane temperatures and to tank surface area. The generator vendor or propane provider can look up the minimum tank size needed easily.
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