Well, you have just re invented the wheel!
This has been done many times with highly variable results over the years. Remember that it will take about 1.4hp to get 1kw of power
out the wires.....This is in a perfect world, with no friction......Your results will vary.....
I built a rig like this to power up my "Bach" in NZ about 30 years ago. I had a 2 cyl Kawasaki motor from a "Mule 500" (500cc) that
I converted to electric for the customer. Brand new, and I converted it to LPG as well!
Using an 150 amp marine cruising alternator with precision 3 stage regulator, I was able to get about 75 amps consistently. into
400ah worth of flooded batteries. 150 was a real reach for it!
The main issue with automotive alternators and small engines is that, being designed to be small and light, they get their high output
only at high rpm. Not a problem under the bonnet. As they reach their rated HP at about 3600 Rpm or "Flat screech"
as it is called by those around them.......Also, being air cooled, lawn mower engines are difficult to sound proof. Muffler is only 1/2 of the issue.
Lead Acid batteries (Flooded, Gel, and AGM) all share the same basic characteristics in that they will all accept a high amperage charge initially, but their ability to accept the current will fall off rapidly. As the voltage rises, the current will fall.
If you try to shove in more current you will need to raise the voltage to do so. If you exceed about 14.6 (Avg) on a sealed
battery (AGM/Gel) you will damage the battery by causing gassing in excess of what the cells can re absorb. Battery will vent, and the
lost moisture cannot be replaced. Do this enough, and the battery will dry out, and die. Use a quality regulator with sealed batteries!
Mechanically, some of the best ones I have seen are direct drive. If you have a deck type mower, just remove blade and couple up alternator.
Belts are tricky unless you go with flat or cogged types they will usually slip. Also, the main bearings on a lawnmower engine are
not designed for side loading, and there is a LOT of side loading if you want to transfer 3-5hp. so straight shaft will last longer as well.
Running 2 in parallel is not really a problem, aside from the mechanical complexity. In your case here, Richard is correct. There will need to be a battery in parallel with the inverter. If you tried to run directly, the noise alone (Electrical) would probably do in a cheepo high frequency
transformer-less switching inverter. Further, as the alternator will need B+ to make power, if there is no battery it will not excite the field.
The "Inverter" generators you see on the market today are high voltage DC generators that use a trick inverter to hold frequency and
voltage stable over a surprisingly wide RPM range. Honda does this best..........If I wanted a small, efficient, quiet, stable genset,
I would buy one of those.....
Having said that....I would be happy to share any tips on a home brew! It's fun, if not really practical......
Hmmmmm There are those who say that about P-38s!
OH! as for the French mill?? Let me know, I will fly over to help! Hydro is the BEST form of renewable energy, and a typical 1800's grain mill will give
you about 10kw/hr of energy, with a LOT of torque, so it will not stall! If the power happens during the winter, do the mill
wheel generator, and run electric heat. For FREE!