Carl's NG setup is marvelous- simple, effective. Alas no NG out here in the sticks.
Propane can't be run that way, you're limited to about 25% propane. Carb and regulator for propane run about $220., a bargain compared to an Arrow engine. Other mods are a 1/2" spacer of aluminum under the cylinder to lower compression, hardened valve seats in the head, a black pipe adapter for the spark plug to fit in the injector hole and clamp, and lengthening the shorter pushrod a few inches. Plus hall sensors and circuit to trigger an auto spark module, long reach spark plug, spark module mount, wiring, etc. And an adapter to mount the carb on the intake manifold and a minor mod to the governor arm and new linkage. It is a project- and makes me appreciate Carl's setup.
I'm presently at just 875 watts of PV panels for my 120VDC power system. PV is so cheap now it's shocking. I have to upgrade my PV system to add 5 more 24v panels for my new low EMI inverter design; I'll be adding another 1500 watts so that the well pump (1200 watts) or washing machine (1100 watts) can be used during the day Sunelect.com or Solarblvd.com carry new panels cheap. Locally used panels are a joke- new panels are cheaper. I designed a new seasonal tilt ground mount rack of angle iron that ran about $300 in steel and welding for my new neighbor; I'll use that design instead of my $150 original as it allows one person to do the seasonal tilt. We have wind gusts over 100 mph every spring so I used 1" EMT braces from the top of the 33% cantilevered rack.
Glort, for your electric water heater, I'd suggest going direct DC to the element; use a water heater thermostat to control a solid state relay, or make one with a surplus IGBT module switching the low side. Put your panels in series to get the voltage you need under load. Screw in replacement elements for 240 and 120v at various wattages are common here. You must use dc rated breakers, and properly rated solid state relays- AC rated stuff will melt down the first time you try to turn it off. It might be convenient to isolate one of the elements or add a whole hot water heater (plumbed as pre-heater) to be your solar heater. All that electronic conversion to AC, and batteries are just not needed, and you'll have much better efficiency.
I hurt my left leg last night- one of the flywheels tipped over when going over the concrete lip into the shop at an angle and got me. I was getting too cocky rolling them around. Scrape and monster swelling on my shin bone...no dinner (couldn't stay upright) and not much sleep. Swelling better today, and I can stand for a short while. Living alone is very peaceful but occasionally has it's downsides.