I LOVE it when a guy like RougeRV2 throws us the REAL MATH on these projects. Bottom line is indeed, its not the best option...... not at first glance in the traditional sense. But there could be an option out there that is not quite so grim if one has the room for it, and it could be done rather inexpensively... certainly under the price of a fancy honda or yamaha gen set.
For many years, I had on a shelf, an old military issue Deco, Cast iron 4hp engine. My goal was to build said DIY generator, but add fully automatic control to the scheme, such that the system could be put into "auto mode", then start and stop whenever it wanted to in order to maintain my electrical needs. Because I have spent my share of time in automation, motion control and simple logic circuits, this really would not be all that hard to accomplish.... it actually has become fairly easy with multi-function electronic relay devices that are sub $100 these days (think simple PLC). Obviously, the 4hp would limit my choices on the alternator side, but I was thinking "compact"......
I never got around to finding the time to build this system, but the dream has never really left my brain, and here is why.
At the time, I was not thinking of the much better engine options available... Since I have a step van, I have enough room to use something much larger as long as it perhaps did a lot more than just provide electricity. because I am a gear head, I at some point found myself with a 22hp Kawasaki liquid cooled V-twin. Thought I would use it on one of my garden tractor resto-mod projects.
When I fired it up on the bench, I was shocked to HEAR how quiet that engine was, and how vibration free it was. After I researched the model number, I found the engine to have a 20 amp charging circuit. A 20 amp charging circuit is slightly less common, but most have by default, a 15 amp system. I found that the engine was actually able to deliver 20 amps of charging already by 1/3 throttle. (I used an Amp/clamp meter on the battery feed, but then loaded the battery incrementally with a battery load tester).
Now personally, 20 amps is 20 amps..... I could live with that real easy. But this dream gets better. As I was researching the engine, I came across a manufacturer that used or uses that engine for all in one, self contained "carpet cleaning" systems that get mounted inside vans. It's primary purpose is to pump/circulate water via tubes dragged into the house and connected to the working end of the machines.
Its SECOND purpose is the one I got real interested in, because they also self create the HOT WATER with that same engines exhaust gas via purpose built heat exchanger. Now I chased the thought of scavenging heat from a small engines exhaust gasses some years ago in another forum, and the guys who had the mind to do the math always told me it could not be done.... and I left it at that for a while.
In later years, I started to wonder why a person couldn't create/use a small catalytic converter on the small engine to create more heat from teh unburned fuel no different than your car..... And years later than that, I read articles where BMW had actually used catalytic converters under their test cars to create pressurized STEAM to actually assist the engine via small turbine. I know, crazy stuff, but hey......
Now, I dont think the carpet machines use platinum in a cat at all. I think their engines are just plain large enough to create the btu they need to make the hot water.... but its not just warm water according to them.... It actually has to be controlled so it does not produce steam.
So, what is the bottom line ? These days, you can find quite a few VERTICAL shaft, V-Twin 14+ HP engines for CHEAP from crapped out zero turn lawn mowers (Horizontals always cost a lot more). A vertical shaft V-Twin is a bit more compact than the horizontal in some cases.
They have electric start, they have 15 to 20 amp charging circuits. They have HOT enough exhaust to create steam via heat exchanger.
To me, this sounds a LOT better than the Honda EU2000 that I DID buy just to save time and have power when I need it. I traded that particular engine away for something else along the way, but I wish now I would have at least hung it on the receiver hitch while running so I could make a decibel level and vibration comparison...... While my on-board underhood mounted honda is certainly quiet and fairly smooth, there is a good chance that the V-twin could have matched it.
The good news is, I'm not dead yet (My wife keeps reminding me of that when I sluff off not taking on another dream), I still trade tractors and engines.... I have room to do it..... and the Honda wouldn't really go to waste just because I pulled it out of the truck.
I'm done now....