Activity Monitor, Battery Status Menu and top

This article describes the Activity Monitor, Battery Status Menu, and top — three related tools available on Mac OS X.

Note: The power profiling overview is worth reading at this point if you haven’t already. It may make parts of this document easier to understand.

Activity Monitor

This is a built-in OS X tool that shows real-time process measurements. It is well-known and its “Energy Impact” measure is likely to be consulted by users to compare the power consumption of different programs. (Apple support documentation specifically recommends it for troubleshooting battery life problems.) Unfortunately “Energy Impact” is not a good measure for either users or software developers and it should be avoided. Activity Monitor can still be useful, however.

What does “Energy Impact” measure?

“Energy Impact” is a hybrid proxy measure of power consumption. Careful investigation indicates that on Mac OS 10.10 and 10.11 it is computed with a formula that is machine model-specific, and includes the following factors: CPU usage, wakeup frequency, quality of service class usage, and disk, GPU, and network activity. The weightings of each factor can be found in one of the the files in /usr/share/pmenergy/Mac-<id>.plist, where <id> can be determined with the following command.

ioreg -l | grep board-id

In contrast, on Mac OS 10.9 it is computed via a simpler machine model-independent formula that only factors in CPU usage and wakeup frequency.

In both cases “Energy Impact” often correlates poorly with actual power consumption and should be avoided in favour of direct measurements that have clear physical meanings.

What does “Average Energy Impact” measure?

When the Energy tab of Activity Monitor is first opened, the “Average Energy Impact” column is empty and the title bar says “Activity Monitor (Processing…)”. After 5–10 seconds, the “Average Energy Impact” column is populated with values and the title bar changes to “Activity Monitor (Applications in last 8 hours)”. If you have top open during those 5–10 seconds you’ll see that systemstats is running and using a lot of CPU, and so presumably the measurements are obtained from it.

systemstats is a program that runs continuously and periodically measures, among other things, CPU usage and idle wakeups for each running process. Tests indicate that it is almost certainly using the same “Energy Impact” formula to compute the “Average Energy Impact”, using measurements from the past 8 hours of wake time (i.e. if a laptop is closed for several hours and then reopened, those hours are not included in the calculation.)

Battery status menu

When you click on the battery icon in the OS X menu bar you get a drop-down menu that includes a list of “Apps Using Significant Energy”. This is crude but prominent, and therefore worth understanding — even though it’s not much use for profiling applications.

Screenshot of the OS X battery statusmenu

When you open this menu for the first time in a while it says “Collecting Power Usage Information” for a few seconds, and if you have top open during that time you’ll see that, once again, systemstats is running and using a lot of CPU. Furthermore, if you click on an application name in the menu Activity Monitor will be opened and that application’s entry will be highlighted. Based on these facts it seems reasonable to assume that “Energy Impact” is again being used to determine which applications are “using significant energy”.

Testing shows that once an energy-intensive application is started it takes less than a minute for it to show up in the battery status menu. And once the application stops using high amounts of energy it takes a few minutes to disappear. The exception is when the application is closed, in which case it disappears immediately. And it appears that a program with an “Energy Impact” of roughly 20 or more will eventually show up as significant, and programs that have much higher “Energy Impact” values tend to show up more quickly.

All of these battery status menu observations are difficult to make reliably and so should be treated with caution. It is clear, however, that the window used by the battery status menu is measured in seconds or minutes, which is much less than the 8 hour window used for “Average Energy Impact” in Activity Monitor.


top is similar to Activity Monitor, but is a command-line utility. To see power-related measurements, invoke it as follows.

top -stats pid,command,cpu,idlew,power -o power -d

Note: -a and -e can be used instead of -d to get different counting modes. See the man page for details.

It will show periodically-updating data like the following.

PID            COMMAND                  %CPU         IDLEW        POWER
50300          firefox                  12.9         278          26.6
76256          plugin-container         3.4          159          11.3
151            coreaudiod               0.9          68           4.3
76505          top                      1.5          1            1.6
76354          Activity Monitor         1.0          0            1.0
  • The PID, COMMAND and %CPU columns are self-explanatory.

  • The IDLEW column is the number of “package idle exit” wakeups.

  • The POWER column’s value is computed by the same formula as the one used for “Energy Impact” by Activity Monitor in Mac OS 10.9, and should also be avoided.

top is unlikely to be much use for power profiling.