Apple introduced new MacBooks and MacBook Pros yesterday, and eagle-eyed Macworld readers quickly spotted that battery life appears to have declined in comparison to previous models.

MacBooks are now marketed as offering 4.5 hours battery life, the 15.4-inch MacBook Pro offers five hours and the 17-inch topk-of-the-range laptop promises just 4.5 hours, on Apple's website.

This doesn't mean the Macs don't run as long between charges, what has changed is the manner in which Apple tests for battery life - it now promotes this with reference to the number of hours use you'll get when using WiFi on your system.

The reason for the change of heart? When Apple launched MacBook Air, it introduced a machine which by its nature required that WiFi be active at all times. WiFi use drains battery power, so Apple announced the MacBook Air as a computer with five hours of "wireless productivity".

In layman's term, that's five hours of normal use with the WiFi connection switched on and the screen at normal working brightness.

Ars Technica spoke with Apple spokesperson, Anuj Nayar, who said: "Battery life has actually improved across the board." What has changed is how Apple represents battery life in laptops.

Nayar explains Apple has always conducted three battery life tests on its laptops: DVD playback, wirelesss productivity and a test in which the computer is set up to achieve the best battery life - and that's the figure Apple used to promote.

"The wireless productivity test is the closest to normal usage, right in the middle of the road with WiFi, text editing, sending email, and so on," Nayar told Ars Technica.