[Baz]

Google is our friend, once again.

Just found this info:

With a fuel capacity of 4 gallons, the R4400DF can operate for up to 18 hours using gasoline (fuel consumption around 0.22 GPH), or for up to 11 hours (on a 20 lbs tank - propane consumption around 0.43 GPH) using propane.

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This means if I start off with a full tank of gas and full tank of propane I will have approx. 29 hours of run time before any re-fueling is needed. That's probably an optimistic number so will take with grain of salt, but at least it's a data point mark for now.