Power from wall socket with USB connected?

Will this work? Connect a 9V/1,1A power adapter AND usb cable from my pc to read the serial monitor. Will the Arduino Mega 2560 select the power adapter and can the USB still be used to read the serial monitor?
It's just for testing purposes, I'm having some strange issues with my Mega 2560 connected to a nRF24L01+. When I just have the USB cable connected I get very bad reception and lots of time outs but with the power adapter connected it seems to work fine. I want to have a look with the serial monitor to see what happens.

Yes. You can plug in both the USB and power adapter.

Ok, thank you.

Perhaps you are maxing out the usb allotted 100ma , I had a serial bluetooth that every other second when it searched the leds would dim as it draws too much,
I've used the nrf without any problem before, to the range of about 65 feet off usb

I power the nrf24l01 from the 3,3V pin, according to the official Arduino website the 3,3V bus is limited to 50mA. The datasheet of the nrf24l01 says that the chip only draws 12mA max. If I connect the module to my Uno I works without problems (powered by USB only). Weird :~