I am happy to share that I was able to get my model 3 to charge from my new inverter generator today. I charged at a steady 10 miles per hour, 2.83 kW for my P3- on 19", 24A @ 118A. It costs ~$0.63 per kWh for $3 per gallon gas.
Why did I buy a generator? The main reason was backup power for my fridge and electronics since my house power in LA goes out way more often than it should, about 3-5 times per year. The longest it's been off was 10 hours. The secondary reason was to charge my Tesla if the power went out. I did some research, this site was helpful:
https://generatorgrid.com/blog/tesla/
I learned that you have to use an inverter generator that produces a clean sine wave, this means the Total Harmonic Distortion (THD) is less than 5%. Most normal generators have THD of 20-30%, so called "dirty power". I choose the Generac GP3500IO inverter generator, it has an open frame and can put out up to 3 kW max running power, 25A @ 120V.
Generac Power Systems - 3500iO watt GP Series Portable Generator - 5982
I bought a "surge guard generator neutral" plug on Amazon, without it the Tesla won't charge from a Generac. I think Hondas may not need this plug.
https://www.amazon.com/Southwire-Co...or+Neutral&qid=1588226816&s=automotive&sr=1-1
Finally, to get the 24A in a single plug I bought a L5-30 120V adapter, it plugs into my mobile charging cable.
L5-30 120V Adapter for Tesla Model S/X/3 Gen 2 – EVSE Adapters
I followed the instructions on the website listed above as well as the manual. The generator started right up (add oil first), it's quieter than the other open frame generators, but is not whisper quiet. I measured 72-76 dB from 20-30 feet away. Procedure: start the generator with nothing plugged in, once running plug in the ground plug into one of the normal 12A outlets, plug in the L5 adapter connected to your Tesla, and start charging.
Summary:
10 miles per hour charging (4 miles per kWh for my P3-)
24A @ 118V = 2.83 kW charging power
Can charge 40 miles on one tank, which will take 4 hours.
$0.63 per kWh assuming $3 per gallon gas (much more expensive than the $0.13 per kWh that I pay overnight)
19 miles per gallon (I was shocked how low this is!)