Not to hijack the thread... I am planning to charge from the 110v GFCI outlet at home, at least initially, so I am definitely interested in the topic and have marked it for watching.
But on the quoted subject. Is this
https://www.amazon.com/AC-Connector...coding=UTF8&psc=1&refRID=AZ3MX87DYFDY7MK74MA5 the kind of adapter you recommend? How would I know if my circuit is 20a? Is simply buying and plugging this adapter, and UMC into it, a safe enough test, as in, either it will work or it won't? I started learning about electricity as it applies to Tesla charging, but it was giving me headaches, so I stepped back into blissful ignorance.
The adapter I am talking about is the NEMA 5-20 one that Tesla sells here:
Model S/X/3 Gen 2 NEMA Adapters
The car comes with a NEMA 5-15 from the factory with the UMC (and a NEMA 14-50). (note the "5" in 5-15 means 120v, 6-xx means 208/240v)
These Telsa adapters are special in that they send a signal to the UMC which sends a signal to the car telling it how much current it is allowed to draw.
If you bought that adapter and plugged your 5-15 UMC adapter that came with the car into it, then it would be totally safe to plug into any outlet since it still would not draw more than 12 amps (it is only allowed to draw 80% of the rating of the circuit due to the face that EVSE's are considered "continuous loads - this is a safety factor). Doing this however would tell you absolutely nothing and be kind of silly. ;-)
Now if you bought the Tesla 5-20 adapter AND that adapter from Amazon you could break one of the safety features of the NEC. Basically that 5-20 adapter tells the UMC which tells the car that it can draw 16 amps. If you plugged it into a 15 amp circuit you would (eventually) blow the breaker assuming the breaker functioned (this is the second line of defense).
Now comes the somewhat complex thing: The NEC allows you to put either 15 amp (straight vertical blades) or 20 amp receptacles (left blade is horizontal) on a 20 amp circuit. 20 amp receptacles will accept either 15 or 20 amp plugs. 15 amp circuits can only have 20 amp receptacles.
It is *super* common to have all 15 amp receptacles on 20 amp circuits in the US. Kitchens are required to have two 20 amp circuits but they nearly always just use 15 amp receptacles. So looking at outlets may not tell you much. What you really want to do is go look at the circuit breakers. If it is a 20 amp breaker it is a 20 amp circuit (unless someone heaven forbid used 14 gauge wiring instead of the required 12 gauge - this is super dangerous and would need fixed).
My garage had a 20 amp circuit and 12 gauge wire, but all the receptacles were 15 amp ones. Many were cracked anyway so I replaced them all with the 20 amp receptacles just for fun in case I wanted to run something that needed the 20 amp receptacle (like my UMC at 120v 16a...). If you have a 20a circuit and want to charge your Tesla using 120v for some reason, I would recommend swapping a receptacle (or all of them on the circuit if you wish - not required) as I did. I would try to charge from the outlet that is "first in the daisy chain" closest to the panel generally so you are going through the smallest number of connector hops.
Now to your question on that adapter:
So I did not know those existed, I think it is cool and I ordered one! Though I am not 100% sure on the safety factor. The use case would be that you know there is a 20a circuit but it only has a 15a receptacle and you want to be able to charge at the fully 16 amps (which is what you get on a 20a circuit for continuous draw). From my understanding, what I have seen is that the 15a outlets have the exact same guts in them as the 20a ones, just they don't have the sideways notch. So the theory is that you can draw 20a no problem from the 15a outlet (though UL may disagree with this assessment - I presume they only test 15a outlets to 15a?)
So in general, I would not recommend the use of this adapter (even though I bought one for fun). If it is your own house and it is a 20a circuit, then just replace the receptacle for a safer solution. I might consider using this adapter if I have no better option and I am darned sure it is a 20a circuit and I am there to actively monitor it at all times for heat and have a fire extinguisher handy. It could come in handy for a 33% boost in charge rate if you are stuck with just a 120v circuit.
P.S. If the right side notch is horizontal or both are horizontal then it is a 240v circuit of 15 or 20 amp ampacity.