I have a 40qt stockpot that I sit on a 1300 watt hotplate (but for making broth, not beer, at least not yet ;-) To take water from 50F to 190F takes it about 4-5 hours. The temp rises quickly at first then really tapers off as it starts getting near boiling. I don't know if it would actually be able to boil it or not. It might if the water was already boiling going in to keep it boiling. I have four of the cordless like tea kettles that I run when I need to boil water, they're each around 1500 watts I think.
I think the equipment that actually has the turned 20A blade is more likely to be commercial or industrial appliances. I've never seen a home appliance that has the 20A blade. I do know for short term loads, some appliances far exceed the permitted 'continuous' draw. I was having trouble with a certain brand of laser printer some years back, and when I asked them about it, they told me it draws 40A for a few seconds at startup to get the first page out faster.