The EVSE that comes with a US spec car is a 120vac device. The car can be charged on a 240vac device (or via DC). This is so that it is compatible with the common receptacles used in the USA. As a result, it's fairly slow. Depending on the model year, it's either a 12A or maybe 10A device, so not all that many watts. The stock one sold in the rest of the world is mostly a 240vac device. Note, the USA and a few other countries use a J1772 plug which differs from the ones sold in many places including Europe. The actual charging protocol is identical in the car, though, so it's primarily just the receptacle in the car.
2011 535i x-drive GT, 2014 i3 BEV