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Power versus Energy Types
Although the words power and energy are often used interchangeably in casual conversation, it is very important to understand the difference between these two concepts. Power and energy are defined in terms of the work the mobile device performs.
Power = Work/Time (Watts)
Energy = Power * Time (Joules)
The power used by a mobile device is the energy consumed per time unit. Conversely, energy is the time integral of power. Since a battery stores a given quantity of energy, the goal of energy management is to minimize the amount of energy required to perform each task satisfactorily. In some cases, minimizing the power also minimizes energy. However, this is not always the case. Some tasks will require less energy to complete when being performed at high speed and high power for a short duration rather than at low speed and low power for a longer period of time.
Whether one should reduce the energy or power depends on the application. Fixed duration tasks, such as playing audio or video, form an important application class where the energy required is directly proportional to the average power consumed as the duration of the task is constant. This class also includes waiting, whether waiting for user input when the device is on or keeping data in memory and the clock ticking when the device is off. For this class of tasks, focusing on minimizing power will minimize energy.
Therefore when talking about energy conservation, we need to distinguish between power reduction and energy reduction. There is a difference between energy and power. Energy is an integration function of power over time. Just because we reduce power does not mean energy is reduced.
Here is a simple example to demonstrate this fact. When static power is negligible, for a given application that takes the same number of cycles to complete, whether the application runs fast and completes in a short amount of time, or runs at half frequency and takes twice as much time to complete, the same amount of energy is consumed.
However, when there is significant static power in the system, such as bias currents and leakage, running slow (or at a lower power) is less energy efficient due to this constant power being consumed during the entire active period.
The goal is to optimize systems for energy efficiency. Power can be broken into two types: static and active power. Static power comes from any DC current sources such as reference circuits, analog designs, or even unwanted shorts in a product. Active power is from the switching activity of the circuitry where alpha is the switching activity.
Energy, on the other hand, is the integration of power over some period of time:
Figure 4. Energy versus Power 
It is very likely that power reduction does not lead to energy reduction. One such case is if there is high static power and the frequency is reduced in half. The total power will be reduced, but since it takes so many clock edges to complete a function, if the frequency is reduced in half, it will take twice as long to complete which will increase the energyrequired to perform the task (Figure 4 ).