How We Measure Battery Life, and Why Manufacturer Figures Differ (MacBook Air M4)
How We Measure Battery Life, and Why Manufacturer Figures Differ explained, with the MacBook Air M4's own measured figures as the worked example: quoted battery figures are produced under conditions a...
Daniel Reyes
In brief
MacBook Air M4, weight 1.24kg — How We Measure Battery Life, and Why Manufacturer Figures Differ. Measured on hardware rather than argued from theory, on a device we rate for exceptional battery life.
Quoted battery figures are produced under conditions almost nobody replicates. Ours use a fixed brightness and a scripted workload so numbers stay comparable.
Why this matters: Battery is consistently the specification buyers care about most and the one most loosely reported.
The mechanism
We set the display to a measured 200 nits rather than a percentage, disable adaptive features that vary between devices, and run the same mix of browsing, video and standby. The result is not the best number a device can produce — it is the number you can expect.
In daily use
Take the MacBook Air M4 as a worked example. A ten-minute 4K export finished in 4m 12s with no fan to spin up. That is the difference this makes once you stop reading specifications and start using the thing.
| Specification | MacBook Air M4 |
|---|---|
| Display | 13.6in, 2560 x 1664, 60Hz |
| Chip | Apple M4 |
| RAM | 16GB |
| Storage | 512GB |
| Battery | 53.8Wh |
| Weight | 1.24kg |
| Runtime | 16h 20m mixed use |
The spread across devices
One worked example is an anecdote. Here is how the figure that drives this behaviour actually varies across the hardware we have measured in this category — same test, same conditions, so the spread is real rather than an artefact of who reviewed what.
| Model | Display | Price | Our score |
|---|---|---|---|
| MacBook Air M4 | 13.6in, 2560 x 1664, 60Hz | £1199 | 5 / 5 |
| HP Spectre x360 14 | 14in OLED touch, 2880 x 1800, 120Hz | £1499 | 4 / 5 |
| Acer Aspire 14 AI | 14in IPS, 1920 x 1200, 60Hz | £649 | 3 / 5 |
| Dell XPS 14 | 14.5in OLED, 3200 x 2000, 120Hz | £1749 | 3 / 5 |
What counts as good here
It helps to have a number in mind before you start comparing. Across the laptops we have measured, the useful thresholds are the ones where a difference stops being visible: below them you notice, above them you are paying for a specification rather than an experience. The MacBook Air M4 reports 13.6in, 2560 x 1664, 60Hz for display, which sits comfortably on the right side of that line, and Apple M4 for chip. Treat anything materially below those as a real compromise and anything materially above as a number you are unlikely to feel.
Myths worth dropping
- A larger battery always means longer life, when efficiency varies enormously
- Fast charging damages a battery meaningfully in normal use
- Closing background apps extends runtime
Checking it yourself
- Reproduce the conditions rather than trusting a one-off reading — run the same task three times and take the average.
- Note the figure your device reports and the figure it actually achieves; on the MacBook Air M4 those diverge under sustained load.
- Change one variable at a time. Brightness, background sync and ambient temperature all move this number independently.
- Compare against the manufacturer’s stated figure and treat any gap as the cost of real-world conditions, not as a fault.
- Repeat after a firmware update — this is one of the numbers that moves without an announcement.
Where it shows up on a spec sheet
On a laptop listing this usually appears as the display line — the MacBook Air M4 quotes 13.6in, 2560 x 1664, 60Hz. That figure is necessary but not sufficient: the chip entry (Apple M4) constrains it in practice, and the two are almost never presented together. When you see one quoted without the other, assume the omitted one is the weaker of the pair.
Putting it to work
Compare figures from one source measured the same way, and ignore cross-site comparisons of quoted numbers.
A battery figure is only meaningful alongside the method that produced it.
Behind this piece
Every product covered here was bought at retail and lived with for at least a fortnight before a word was written. Battery figures come from a scripted day rather than a manufacturer estimate, and sustained performance is measured warm — after half an hour of load — because that is the state a device spends most of its life in.
Where a firmware update changes a result, the piece is re-tested and the change noted rather than quietly corrected. No review unit arrives with conditions attached, and no score is agreed in advance with anybody.
Where to go next
If you are weighing this against the rest of the laptops we have covered, the category archive lists every review in date order. The closest comparisons we have published are How We Measure Battery Life, and Why Manufacturer Figures Differ (Razer Blade 16) and How We Measure Battery Life, and Why Manufacturer Figures Differ (ThinkPad X1 Carbon Gen 13).
For the reference material behind these figures, the specification pages carry the measured numbers, and the common questions collect what people ask once something is in daily use.
