The ST is a type of generator head. I don't know what it actually stands for, but an ST head is typically 4 pole with a rotating field supplied by brushes and slip rings. ST heads often do not even have voltage regulation. These are the simplest form of generator possible. The only "electronic" parts are the rectifier diodes which provide "DC" to the field. These have the fewest possible things to go wrong, but do not produce a terribly nice AC waveform. Nothing in my house seems to care that the generator makes ugly AC. Even the computer UPS does not mind when I'm on generator.
I don't know about your power needs, but a 20kw generator seems like overkill for a standby home generator. Personally, I have an unregulated ST 7.5 driven by a Changfa (Chinese) 195 engine. I am running my generator at 1800 rpm at 3500 feet elevation, so the unit maxes out at about 6500 watts. With careful load management, I can run anything in my house including my 3 ton air conditioner.
The biggest load that I am likely to see is on laundry day when I'm running the dryer (~5000 watts), washer (~700 watts), well pump (1500 watts) and water heater. My water heater is an oil fired unit, so it only draws an amp or so. That all adds up to 7400 watts which is more than my generator can carry. Sometimes I turn off the well pump and run on the water in the presure tank. If the water presure drops too low, I stop the dryer and turn on the pump. This level of power management is more than my wife is willing to do, so if I wan't to run the generator on laundry day, then it is up to me to do the laundry.
On hot days, I can run the A/C and water the lawn with enough power left over to run the fridge or a few lights. When I'm running the A/C on generator power, my wife as learned to ask before turning on the stove. If I turn off the sprinklers, there is enough power to run one burner on the stove. If more than one burner is required, the house is usually cool enough to handle turning off the A/C while dinner is being cooked.
It would be nice to have a bit more power available, but I can't imagine needing anything more than 10kw. (Unless my wife wants to cook pasta while baking bread on a hot day while doing laundry).