The first couple of years, they supplied 12A EVSEs in the US, then they dropped it to 10A units. The US electrical code says a device like an EVSE must derate the supply to NGT 80%, so the 12A would be okay on the nominal 15A circuits in the US. The hassle is, there could be lights, a garage door opener, and who knows what plugged into that circuit, so that's why they dropped their standard issue unit to 10A, so there'd be less likelihood of tripping breakers when people didn't have a dedicated one for their EVSE to plug into.
Since power = volts * amps, whatever amps you can supply off of that 120vac circuit, would be half what it could be if you had 240vac available. And, the AC-DC conversion done in the vehicle is less efficient, so it's actually less than half, and because the charging takes longer, that may mean the vehicle will have to supply cooling for longer to keep everything working correctly as the batteries warm as they're being charged.
2014 i3 BEV, 2021 X5 45e
(The i3 will be sold soon, <17K-miles, interested?)