Measuring power consumption / current of an Arduino using a multitester
Select the milliamp range on the meter
Connect the red lead to the meter's milliamp socket
disconnect the power supply from the board
connect the ground of the supply connector to the ground of the board connector
Connect the red probe of the meter to the positive connector on the supply
Connect the black probe of the meter to the positive connector on the board.
observe the meter reading
multiply milliamps/1000 by voltage to get power in watts

voltage measured with second meter
rated voltage of supply
voltage measured earlier or later