Based on the Onan generator specs for the 2800 (gas) and identical 2500 (propane) and the current prices at the Mill Hall PA Flying J, the cost per KW per hr. for propane is higher.

Gas $2.799/gal, 0.3 gal/hr half load, 1.4 kW $0.5998/kw/hr

Propane $2.049/gal, 0.4 gal/hr half load, 1.25 kW $0.6557/kw/hr

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