Propane generator for off grid solar system

[hotinhawaii]

Does anyone have a recommendation for a propane generator and installer? I am building a house that will have a 7.2kw PV system with 15 panels and 16 batteries. It has to be propane, so don't even suggest other options, please. I was quoted $22,000 for a 25 kw Onan gen but it just seems huge as a backup! And, it's too expensive. I would like it to automatically go on when batteries are low. The electronics are all Outback.