Pat -

The reading you see on the wattmeter is the current the motor/prop is drawing, minus the small amount of current drawn by the ESC. Think of it this way: When you throttle down all the way, current measures zero. When you throttle up, the motor/prop draws current from the battery. So the current you measure is the current being drawn by the motor/prop.

However, motors and props aren't 100% efficient, so some of the current drawn by the motor/prop ends up being used to create heat. In simplest terms, all of the current you measure is being drawn by the motor/prop, but not all of the power thus produced goes into moving the airplane. Some of the power ends up wasted as heat. The worse the efficiency of the system, the greater the heat produced. Any power wasted as heat is power that does not do anything to propel the airplane.

Think of current as something that the motor/prop draws from the battery, not something that the battery pushes into the motor.

- Jeff