Expanding on (and agreeing with) Charles said: the wall unit (EVSE, what normal people colloquially call a "charger") has a signaling mechanism to advertise how much current it is able to provide. The car then reads that advertisement of capability and the part in the car (that EV designers actually call a "charger") draws an amount of current the battery needs up to the advertised limit of the EVSE.
If I was running new service, I'd run a 50A circuit and terminate with a NEMA 14-50R.
If I already had a 240VAC, 20A EVSE, there's no point in upgrading it, since the car's charger is already the limiting factor.
In slight disagreement with Charles, to find power from voltage and current, multiple current in Amps times voltage in Volts to get power in Watts. 240V * 20A = 4800W or 4.8 kW. (From the context, I'm willing to bet that Charles knows this and just typo'd his calculation from 220V * 40A.)