Which of the following is not a unit of power?

Power is defined as the rate at which work is done or energy is transferred over time. The standard unit of power in the International System of Units (SI) is the watt (W), which is equivalent to one joule per second (J/s). This means that both horsepower, a common unit in engineering and automotive contexts, and joule per second also qualify as units of power.

Watt per second, however, is not recognized as a unit of power. Instead, it conveys a quantity that implies energy change with respect to time. Using watt per second suggests a notion of power being described in terms of change over time rather than the rate of doing work or energy transfer at a given instant. Therefore, it does not accurately represent a measure of power, making it the right choice for the question.

In summary, while watt, horsepower, and joule per second are all units that measure the concept of power effectively, watt per second fails to do so, thus marking it as the exception.