For these calculations, this is the test setup. I bought a tri-fuel conversion kit for my Honda eu2000i. I have two Hondas, a new companion and my original one which is 4 years old now. I bought the tri-fuel kit from central maine diesel for 279 bucks. The kit installed pretty easily using a youtube video from engineer775 to guide me along.
Back to the numbers. I have a backup power setup using 6 x 216Ah 6V Golfcart batteries with a xantrex pro sine SW 2000W inverter and an IOTA DLS-55/IQ4 12 VOLT
55 AMP 4 STAGE AUTOMATIC SMART BATTERY CHARGER / POWER SUPPLY charger. I use #00 wiring for everything. For this test, I pulled 4kw out of my battery setup from fully charged. I measured this using a kill-a-watt meter. I pulled the 4KW using my gas powered furnace and a desktop PC attached to the xantrex inverter. The gas powered furnace draws about 5A to run its fan when the heating system is engaged. The computer draws about 2A continuously and simulates the draw from my hurricane essential items (fridges, some lights, chargers for phones). I pulled the 4kw over a 13.5 hour period. I’d no issues at all with the Xantrex and would recommend it.
The voltage on the batteries after the 4kw was 11.83V under load with the furnace and the desktop running. This was drawing about 5.2A at the time. I removed the load and after allowing the batteries to rest, the voltage was 12.07V. I measured both these voltages at the “battery”.
I used propane to recharge the batteries using my Honda eu2000i.
I attached a full propane tank (36.1lbs gross weight) to my Honda and let it charge the batteries using the IOTA 55A charger. The charger drew 10.1A during the bulk stage. Thats about 860W. I started charging at 12:45pm. The bulk charge finished around 5 hours later. At that point, it had drew 4.4kw from the Honda (measured by the kill-a-watt). The voltage at the battery was 14.06V and in the absorption phase, the draw had dropped to 8 amps.
I stopped it there. I weighed the propane cylinder and it was now 31.7lbs. So, drawing 4.4kw from the Honda took 4.4lbs of propane. Thats 1kw/lb of propane. The math turned out too easy but those are the numbers.
Efficiency wise, this isn't great but I think given we're converting from propane to electricity rather then using it directly then it is what it is. 1lb of propane is about 21600BTUs. 1kwh is 3412BTU. So, our efficiency is 3412/21600 or 15.7%. The eu2000i is regarded as a pretty efficient generator so it's interesting that even with the Honda, the answer seems to be 15.7%.