Use only the power adapter that comes with the e-mark for charging. Your e-mark also charges when turned off, but you will not see any light indication. To check how much it has charged already, switch the e-mark on briefly and check the lights at the back (see What do the lights and beeps of my e-mark (create) mean?) or connect it to the app and check the exact battery level.
Note: the USB interface is only for data transfer, the e-mark cannot be charged via USB.