Watt Meter

Watt meters can be digital or analog. It shows the actual power being drawn from power supply. Some digital models can measure and show more information like maximum or average load. Also it can accumulate and store some data and display the amount of power consumed etc.

P=U*I where P is power in (Watt). U voltage (V) and I current (A)

a popular one is the one from HK: https://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=10080

This is very brief and needs to be edited, but still better than nothing.