How we count

This page is generated from the app's own Measurements screen, so the two cannot drift apart. Where the text says “the switch”, it means Settings › General › Send anonymous statistics inside the app.

In short

Two things here are measured, not guessed: how fast the battery loses energy, and how much energy the chip uses. Everything else — who spent it, and what quitting would give back — is worked out from those two. This page says how the working-out goes, and where it stops.

Where the watts come from

On battery we read the battery's own current and voltage — the same counters macOS uses for the estimate in the menu bar. Separately we read the chip's energy counters: processor, graphics, Neural Engine and a few hardware blocks. The first says how much the machine uses, the second says who caused it. Plugged in, the battery says nothing about power use, so there we read the board's power and the adapter's output instead. Both instruments refresh about once a minute, so a number can sit still for a while and then jump.

One watt on the chip is not one watt from the battery

The chip's counters measure the chip. The battery also pays for converting the power, and for memory, storage, radios and ports — none of which have counters of their own. In our measurements each watt the chip reports costs the battery about two and a half. So the chip's watts are never shown as your power use: they only decide how that use is split.

Why the apps don't add up to the whole

Part of the power is used whether or not you run anything: the screen, the idle chip, memory, storage, radios. We take that part off the top before anything is charged to an app — it is the block you see as "Screen and system". What is left is divided between apps, and only the part that has a per-app counter gets a name. On the Mac this was measured on, apps are typically about a fifth of everything the machine uses.

What the Energy Impact number in Activity Monitor means

It is a dimensionless score, not a physical quantity: Apple calls it "a relative measure, lower is better" and never names a unit. You cannot compare two Macs by it, and you cannot turn it into hours of work. The best public explanation of the formula dates from 2015 and describes Intel processors this machine does not have.

We print watts. A watt is a joule per second: it adds up, it divides into the battery's capacity, and it becomes time. That is why an app here carries "12.4 W" rather than "40 points".

One objection worth answering up front: every menu bar monitor reads the same powermetrics numbers and only renders them differently. For the machine as a whole — true. Per app — not: powermetrics does not split power use across processes, so we build that ourselves from kernel coalitions and IOReport, including the energy of children that have already exited. Hence the gap with any list that merely restates Activity Monitor.

What "Screen and system" is

"Screen and system" is everything that uses power with no app behind it. It holds three rows. "Backlight and services" is the backlight, the idle chip, memory, storage and radios. "Graphics" is measured graphics and Neural Engine work that macOS does not tie to any one app. "Background work" is kernel work and processes that ended between two of our readings — real work, with no owner we are allowed to name. Quitting an app removes none of these three.

Why an app's number moves

Every number is an average over the last few minutes — about six, and the caption on the screen names the span actually covered. One window is not a speed: during a single uninterrupted video call, Chrome's share of the machine moved between 18 % and 65 % from window to window. Averaging is what makes the figure usable, and it also means the figure follows the app with a few minutes' delay.

Where "+N minutes" comes from

Take what the machine uses now, subtract what the app uses, and the charge left lasts proportionally longer. That is the whole arithmetic, and you can do it with the watts printed on the screen. The uncertainty sits in the constant part — it is known to within a few watts — so the answer is a range, and we show its low end. Two minutes after you actually quit something we measure the machine again and print what really changed; if the load moved for other reasons in those two minutes, we say the effect could not be isolated instead of claiming a win.

Why minutes take a while to appear

Minutes need the charge to be actually falling. Right after you unplug — and especially at 100 % — the battery is still settling, and a window that catches that is not a measurement. Half the time the first number arrives within three minutes; sometimes it takes much longer, which is why we never promise a countdown. The app list is there from the first seconds — only the minutes wait. After the Mac wakes from sleep the wait starts over: numbers taken before the sleep are not about now.

Watching video

Most video decoding and encoding happens in a dedicated hardware block, not inside the app. The block has its own counter, so the energy is measured — but macOS does not say who asked for it, so it appears under "Graphics" and not under the player. Two consequences worth knowing: an app playing video looks cheaper in the list than it is, and quitting it can free a little more than its row promises. In absolute terms these blocks are small — during a call with background blur we measured the camera at 64 mW and the codec at 10 mW, while the Neural Engine doing the blur ran at about 0.5 W.

Battery health is macOS's number

Cycles, remaining capacity, lifetime temperature and hours powered on come from the battery controller through macOS. They cover the whole life of this Mac, including the years before WattMate was installed; we only read them and lay them out. One chart on that page is ours: the month-by-month ageing curve, which starts the day you install the app.

What exactly is sent

Only if the switch above is on. Two kinds of message leave this Mac.

Once each — the steps of getting started and buying: the first launch, whether it reached its first numbers, which card met you first, whether you quit an app on our advice, whether you opened the statistics window, whether the price was shown and from which lock, whether you went to pay and on which plan, whether a licence activated or failed, and whether this Mac suits the app at all. Lock taps — every one.

Once a week — one summary: how many of the seven days the Mac ran off the cable, what you opened and how often, how much of the power use went to sleep, how many banners were shown and tapped, whether they are allowed, which card came up most, whether you are on the trial or licensed, and how many minutes the app gave back. Counts leave as ranges, not exact figures; only the days off the cable and the minutes given back are exact.

Never — the names of your apps or processes, window titles, file paths, your watt figures, or anything that could identify the Mac or you.

Everything stays on this Mac

All of it is computed here. App names, process names and your energy history never leave the Mac. Detailed measurements are kept for 48 hours; session summaries stay for good, because they are small. "Erase measurements" in General clears both. Anonymous statistics are on by default and can be switched off in Settings › General. While they are on, what leaves is the app and macOS version, the Mac model, whether it crashed, and once a week a set of rounded numbers: which screens were opened, how many days the Mac ran unplugged, how much of the week went to sleep. Never a name of anything you run, and never an exact count — ranges only.

What we can't measure

• Graphics energy is measured as a total. Which app used it we work out from how long each app kept the GPU busy — macOS has no per-app energy counter for graphics. A second of heavy computation and a second of simple redrawing cost different amounts, so the split between apps is approximate even though the total is not.
• Neural Engine energy is measured, but nothing in macOS says which app asked for it. It is never charged to an app: it stays inside "Graphics" under its own name.
• We can see that the screen costs several watts, but not precisely enough to print a number, and we cannot yet say what your brightness setting is worth. So the screen has no line of its own — it sits inside "Backlight and services". If the estimate ever becomes steady enough, the line will split by itself.
• We ask the system about running processes every ten to thirty seconds — every ten while the Mac is working, every thirty when the work is close to nothing. Anything that starts and finishes in between is invisible to that question — a compiler, a helper, a script. The kernel keeps a per-app tally that gives most of it back, but the kernel's own work and daemons we are not allowed to read stay nameless. They show up as "Background work", and are never blamed on an app.
• When a phone charges from the Mac, we can see how much the port is allowed to give — not how much the phone takes. So the figure is a ceiling and is written as "up to". It is usually the largest single item on the screen, which is why unplugging the cable is worth more than quitting anything.
• History is recorded only while the Mac runs on battery — that is where power use can be measured. Plugged in, the panel still shows what the machine uses right now, read from a different instrument, but no session and no history is written. A Mac without a battery has nothing to measure at all.
• The hours-left figure at the top is Apple's estimate, taken as it is — the same one the menu bar shows. What is ours is the ratio: "without this app the Mac uses 14 W instead of 20". If the two ever disagree, believe Apple about the hours and us about the difference.
• An app's minutes answer one question: what if I quit this one, now. They cannot be added up — the same watt is worth 0.8 minutes on a full battery and 8.3 near the end, and quitting two apps is not the sum of quitting each. Where a total is needed we compute it for the whole group at once, and say so.
• The figures on this page were measured on the Mac WattMate was built on. Another Mac has a different chip and a different screen, so its constant power and its cost per watt differ. The app keeps measuring yours in the background and will switch to your figures once they are steady enough to be worth showing.