There are tons of small digital meters available on e bay. I bought one when I was building a pid controller for my beer brew kettle. It has a inductive pick up coil for the amps so no current goes through the meter.
I bought these for my cars so I also have a usb charge port for my cell phone. It displays volts normally and displays charging amps when the phone is plugged in.
Dual Ports 3.1A USB Car Cigarette Charger Lighter 12V/24V Digital LED Voltmeter | eBayView attachment 1715303234
