I owned a Chinese gen set I bought at a Cummins Tool sale. 6500W peak, 5500 nominal. Came with 2 duplex 120V outlets, & one 3-pronged 120V twistloc. Had a clone of a Honda GX190 engine (13HP, overhead valve, electric start) I only paid $325 for it, as it fell out off a loading dock & was all bashed up -- tank mashed in, gen vibration isolators snapped of the gen housing. I blew the tank back into shape with compressed air, & threw a rubber exhaust hanger under the gen housing to support the unit.
First thing I did was ditch that 120V twistlock for a 30A 4-prong 220V plug ... and wired it in after fumbling around with strange Chinese wiring practices.
Now I was able to plug it into an outdoor plug which is connected to my transfer panel, & power my well pump, fridge, boiler, certain lights & everything in the TV room including the computer. It ran everything without a problem ........ UNTIL I ran an extension cord out the window, plugged into one of the duplex outlets, & ran my washing machine. It ran that fine too, but I forgot to turn off the WATER HEATING feature it has ... which I would imagine draws some big amperage! Well once that kicked in, some lights in the house got REAL BRIGHT ... & I thought oh sh**!! Figured my spankin new fridge was toast. Then I heard a loud POP, & the kitchen light was out ..... then heard another smaller more muffled POP .... and my motion light on my porch was toast. I checked on the fridge, & the light inside was flickering super bright, then back to normal. This also happened later when I ran the dishwasher with the hot water feature on also.
I later realized that if I drew too much current off one leg of the generator (two 120V legs together are 220V), it would cause a voltage spike as high as 140V ... or higher if I drew more of a load on the other leg!!
This gen powered everything fine -- including the blower fan motor on my Mark III .... as long as both legs were reasonably balanced.
I tested out my new inverter generator that I recently bought on the same circuits, & it too ran all of them fine ... although I did not attempt to run the washing machine or dishwasher. The gauge on the gen said 3 bars out of 10, with every possible circuit running everything at once. (Yamaha EF6300iSDE)