Electronic – If watts is power and amps is current, what is volts

The power of a motor is the product of its speed and torque. The power output is greatest at about half way between the unloaded speed (maximum speed, no torque) and the stalled state (maximum torque, no speed).

The output power in watts is about (torque) x (rpm) / 9.57.

The efficiency of a motor η = Shaft Power Out/Electric Power In

Ref:http://www.me.ua.edu/me416/LECTURE%20MATERIALS/MotorEffic&PF-CM5.pdf

(vector) Input Power = V·I and Loss = I²R

If you can measure real power V*I but cannot measure HP, assume 87~90% for η

Although both values are in watts, they are referring to two different power quantities. The "15 watts" refers to the audio output power, i.e. the power delivered to the speaker. This is the power that generates the sound that you hear. The "180 watts" listed on the back of the amplifier refers to the power drawn from the AC input line. This is the power supplied to the amplifier from the AC wall socket (120 volts at a frequency of 60 Hz). It needs this power to run its internal electronics which produces the 15 watts of audio output power. The AC input power will always exceed the audio output power since the amplifier cannot be 100% efficient. In this case, its efficiency is pretty low (15 watts out for 180 watts in or an efficiency of 8.3%).