Application

From the application perspective, a GUI consumes energy through user-GUI interaction sessions, in which a user locates the application, starts it, interacts with it and finally closes it. Such a process usually consists of a series of window operations, such as creation, switching, and manipulation, $etc.$, with intermittent idle intervals, in which the system waits for user input.

Figure 3: Looking for a file and creating an email.

Figure 3 gives the second-by-second energy consumption for two GUI sessions executed on an HP/Compaq iPAQ with Pocket PC 2002. The first session is to look for a file using the file manager. The first peak around four seconds is due to activation of the ``Start" menu and stepping through its items to locate ``Programs". The second peak at $10$ seconds is due to activation of ``Programs". The third peak at $12$ seconds is due to activation of the file manager in the Programs window. The last peak is due to moving of the scroll bar in the file manager to locate the file in the file list and then closing the file manager. The other session, creating an email, is just to open the ``New" menu and create a new email message. The two sessions consume $4.7$ and $1.4$ Joules more energy, respectively, than consumed in the corresponding idle period. Being idle for one second consumes $5.9$ Joules in these measurements. Creating an email consumes much less extra energy because it requires fewer GUI manipulations. Moreover, since it takes much less time, more email sessions are possible compared to file manager sessions in a fixed amount of battery lifetime. From these two examples, it is obvious that window operations are energy-expensive and an energy-efficient GUI should reduce the number of window operations and usage time.