Do monitors and other peripherals - with their own power source - drain power from MacBook when plugged in via USB-C?
This is from both experience and speculation.

My MacBook Pro needs to be connected to power while the lid is shut in order to project the screen onto an external monitor. I imagine this is due to power limitations from the battery, as well as the need for greater power draw from the monitors. 

If the lid is open, however, you can keep the power unplugged while still using monitors. This would certainly drain the battery faster, as the device now needs to project the display to one or more external monitors.