The spec sheet says up to 15 hours, yet by lunchtime the battery is already halfway gone. That is not a fault with your laptop: it is how the number is produced in the first place. Understanding how manufacturers measure battery life helps you read their claims and, more usefully, claw back a couple of real hours with a handful of settings.

Where the 15 hours on the spec sheet actually come from

The headline figures are generated in a lab, under conditions designed to make the battery last as long as possible. Apple, for instance, quotes up to 18 hours of video playback and 15 hours of web browsing on the MacBook Air, but its tests run with the screen at half brightness, the keyboard backlight off and the display at roughly 150 nits.

The industry benchmark MobileMark 25 instead fixes brightness at a minimum of 250 nits and mixes in more varied tasks. The gap shows up plainly on a single model: Lenovo lists up to 22.9 hours of local video on the ThinkPad X1 Carbon, falling to 13.95 hours on MobileMark 25. Neither figure is a lie, but both measure something other than your day of dozens of open tabs, video calls and always-on Wi-Fi.

Independent testing by Laptop Mag found an average gap of just 48 minutes on Apple machines, but more than four hours on some HP models, with a ThinkPad P72 rated for 18 hours managing around four. As a rule of thumb, expect roughly 50 to 70% of the advertised figure in mixed use, and the shortfall swings a lot depending on what you actually do.

Claimed versus real battery life on a laptop rated for 15 hours
Usage Claimed Real (mixed use)
Local video, medium brightness 15 hrs 10-11 hrs
Web browsing over Wi-Fi 15 hrs 7-8 hrs
Video calls and many tabs, high brightness 15 hrs 5-6 hrs

The five settings that genuinely close the gap

The good news is that much of the lab setup is yours to copy. The most powerful lever is the very one that inflates the official numbers, screen brightness: the display draws more power than anything else, so turning it down a few notches gives the biggest return. Here are the five moves, with the exact paths on Windows and macOS.

  1. Turn the screen brightness down before anything else. Windows: Settings > System > Display. macOS: System Settings > Displays.
  2. Switch on the battery saver and set the power mode to "Best power efficiency", the very mode makers use for their records. Windows: Settings > System > Power & battery > Battery saver. macOS: System Settings > Battery.
  3. Drop a 120Hz panel to 60Hz when you do not need the smoothness. Windows: Settings > System > Display > Advanced display. macOS: System Settings > Displays.
  4. Prune your start-up apps, the ones that launch on their own and sit in the background. Windows: Settings > Apps > Startup. macOS: System Settings > General > Login Items.
  5. Find the hungriest app and close it. Windows: Task Manager > Processes > Power consumption. macOS: Activity Monitor > Energy tab.

Taken together these often hand back one to two real hours, without touching the hardware. It is the same logic as comparing UK mobile networks: small adjustments that add up to a different bill by the end.

  • Cap charging at 80% when the laptop stays plugged in all day: many makers offer the option, and macOS runs optimised charging on its own.
  • Ask for the cycle count and battery health before buying second-hand: that single figure separates a bargain from a dud.
  • Do not chase the spec-sheet hours: they are a laboratory ceiling, not a promise for your day.
  • Do not leave the screen at 120Hz and full brightness while on battery: it is the quietest waste of all.

Good to know: on Windows the command powercfg /batteryreport produces a report showing how much capacity remains against the factory figure; on macOS the same detail sits in System Settings > Battery.

Tired battery or hungry software: how to tell

If your runtime has collapsed over the months, the culprit is either the cell or the software, and it pays to start from a hard number rather than a hunch.

The figure to watch is 80% of capacity: Apple designs its batteries to retain 80% after around 1,000 charge cycles. Below that threshold the cell counts as worn and the lost hours are simply age; above it, the problem is almost always software, and the five moves above are usually enough to bring it back into line. And if you are eyeing a second-hand machine, always ask for the cycle count and the battery health report before you pay, much as you would compare networks before you sign up.