What does energy impact mean on a Mac, and which number should you trust?
Energy impact is a relative score, not watts. On our Mac WindowServer scored up to 65 while kernel_task scored 0 at 67% CPU. How to read it and act on it.
What does energy impact mean?
It is Apple's estimate of how much power a process is using right now, combining CPU, GPU, wakeups from sleep, disk and network into one score. It has no unit. A higher number means more power relative to the others, and the scale is not watts or percent.
| Process on our Mac | Energy impact | CPU in the same sample |
|---|---|---|
| WindowServer | 32.0 | 31.9% |
| A terminal tool | 5.8 | 5.7% |
| An editor's helper | 5.7 | 5.7% |
| NotificationCenter | 4.9 | 4.9% |
| Google Chrome | 1.8 | 1.6% |
| trustd | 1.4 | 1.4% |
| kernel_task | 0.0 | 66.8% |
For ordinary apps, energy impact followed CPU closely. kernel_task is the exception: its time is not charged, partly because some of it is the system holding the CPU back to cool down.
How do I see it in Terminal?
top -l 2 -s 3 -o power -n 15 -stats pid,command,cpu,powerThe second sample is the one to read; the first has nothing to compare against.
What is 12 hr Power?
Activity Monitor's 12 hr Power column averages each app over the last twelve hours, or since it opened. It is the better column for finding what drained a battery overnight, because the live number only shows this moment.
See it live
Free Mac's System tab shows energy impact per process, the same measure Activity Monitor shows, beside what is draining the battery, and marks system processes as protected. See WindowServer high CPU, kernel_task and what drains a MacBook battery.
Related: MacBook battery drains overnight while asleep and Mac battery time remaining is wrong.
Common questions
What does energy impact mean on a Mac?
A relative estimate of a process's power use, combining CPU, GPU, wakeups, disk and network. It has no unit.
What is a high energy impact?
There is no fixed threshold. Compare processes against each other in the same moment; the top few are where battery goes.
Why does kernel_task show 0 energy impact?
Its CPU time is not charged to it. On our Mac it read 66.8% CPU with energy impact 0.