It’s not easy to establish the true battery life of a laptop—at least not without using it.
Manufacturers aren’t exactly honest in their advertising, and the facts are obfuscated behind all sorts of numbers and technical jargon. For someone who spends a lot of time on the road, that lack of transparency can quickly become problematic.
For example, if poor battery life is a deal-break when you’re buying a new machine, how are you supposed to make an informed decision? You don’t want to be forced to constantly take battery saving measures just because a manufacturer conned you.
Luckily, you can take matters into your own hands—you just need a special math formula (and possibly a calculator).
The Formula for Estimating Laptop Battery Life
Just divide your computer battery’s watt-hours by your estimated wattage. The result is the number of hours you can expect your battery to last.
Your expected wattage varies depending on the task you’re performing. Gaming will use between 40 and 60 watts, watching video will consume 15 to 30 watts, and web browsing will eat 10 to 15 watts.
The watt-hours of your battery can be found on the battery itself, on the manufacturer’s website, or on the accompanying spec sheets.
You can also find the information using the operating system. On Windows, open Command Prompt, type powercfg /batteryreport, head to the path it spits out, and open battery-report.html in your browser. On Mac, hold Option, then navigate to Apple > System information > Hardware > Power.
Remember, all batteries degrade over time, so this calculation is only going to be accurate on relatively new devices. If you’re not getting the amount of usage out of your battery as you think you should be, check out our guide to troubleshooting battery issues on Windows and Mac.