A year-long study revealed on Friday that laptop manufacturers seriously overstate their claims on a device's battery life, sometimes by hours, not minutes.

Which?, a website that provides reviews and expert advice on a various of topics, including hardware and technology, carried out the tests during the past year as part of their normal review process.

Experts said they tested 8 Acer, 3 Apple, 8 Asus, 10 Dell, 12 HP, 20 Lenovo, and 6 Toshiba laptops. Each device went through two different tests three times.

The first test included watching movies until the battery life died out, while the second test involved surfing the Internet via WiFi, also until the battery gave way.

Results of these tests, portrayed in the infographic below, show that all but one laptop vendor overstated the average battery life for their devices.

Battery life test results
Battery life test results (Source: Which?)

Besides Apple's MacBooks, who sometimes lasted longer than what Apple estimated, all laptops tested poorly. "The most optimistic laptop manufacturers are overstating their battery life by 50% or more, leaving you searching for the power cable twice as often as you’d expect," said Which?'s Jack Turner.

For two decades, users have been whining about laptops not living up to vendor claims. This test shows how inaccurate those claims are to begin with.

The vendors who replied to Which?'s request for comment said the Which? test result differed from their estimations because they used different tests to assess the initial presumed battery life.