Keep in mind that the input voltage first has to be converted into DC by the charger in the car, and then that charger also down-steps some of that energy to recharge the 12vdc battery. Then, the active heating/cooling circuits for the batteries needs to be managed - cooling if they get hot during the charge (or too hot to start) and warming before they actually start to charge as a cold battery is not a great place to try to push electrons. So, while the i3 is pretty efficient for its type, by far, not all of the power going in ends up in the batteries, at least the main traction motor high voltage set. SOme of it just ends up in wasted heat - the ac-dc conversion is never perfectly efficient - some ends up in heat losses.
Out of the 22Kw battery capacity, the computer maintains a buffer both at the top and the bottom that you as a user can't touch. This is to prolong the life of the battery.