Hello Ueli

Yes, this is not properly explained in the user manual (which does not really deserve its name).
But there are basically two ways to charge the Elite:
Possibility 1: You take 2 normal USB chargers (they should deliver 2A at 5V) and charge via the two USB-C plugs. You don't have to do anything special, because the PowerCore automatically recognises what is connected to it. I can't say how long the charging process takes, except that it was done in the morning.
Option 2: You can buy the "Anker PowerPort+ Atom III (60W, PowerIQ)". This is intended for the PowerCore. I tried option 1 and the Atom III is on its way to me right now.
I hope this helps.
LG,. Ed