Laptop magazine revealed that during a recent test, a Toshiba netbook lost 2.5 hours of battery life when running Windows 7 instead of XP, or about 30 percent (6:53 for Windows 7, versus 9:24 for XP).
Tom's Hardware also reported a similar finding when it tested an Acer Aspire One netbook running Windows 7 release candidate, only to find it lasted 2.5 hours less than when it ran Windows XP Service Pack 3 (5:54 versus 8:28, when both were at a low-power idle state).
Long battery life is one of the key selling points of netbooks, due to their high portability. Many vendors heavily tweak their netbooks to ensure that they can run a full business day on a single charge, or more.
Microsoft previously promised that Windows 7 would improve laptop battery life by about 11 percent over Vista.
That would be due to better use of the graphics chip during tasks such as DVD playback, and improvements in the kernel so that CPU can more quickly switch to an idle state when not in use, and generally run more efficiently, Microsoft said.
A Microsoft spokeswoman declined to comment about the recent reviews and reports, but did point to a white paper, last updated June 23, 2009, describing to driver developers and hardware engineers how to optimise hardware and components for better battery life under Windows 7.
Of course, battery life for Windows Vista was widely perceived to be worse than under XP, due to its bloated codebase, which prevented Vista from running well on netbooks, as well as the poor availability of Vista drivers for many months after its launch.
Hardware drivers and how they interact with an operating system are key for battery drain. For instance, a driver that fails to let Windows turn off a Wi-Fi chip when users aren't surfing the web could accidentally result in poor battery life. Same with a graphics driver that isn't able to shift processing work from an overtaxed CPU to a fresh GPU.
Jack Gold, an independent research analyst, says that it's still too early to condemn Windows 7. "[With release candidates,] Microsoft often has debug code inserted to find and document problems, and the code is not optimised," Gold said.
The same is true of the preliminary drivers available. Drivers are not written by Microsoft, but by the component makers themselves, he said. Rather than simply recycling their Vista drivers, the hardware vendors need the final release of Windows 7, which only arrived last month, and "a little time to perform their magic".
While existing Windows XP netbooks may miss out on some of these optimisations, future models that ship with Windows 7 pre-installed may eventually have the same or longer battery life than XP that Microsoft has promised.
"It does not trouble me that current machines have less than optimum battery life, or performance for that matter. With all the resources Windows 7 will use on a device, optimisation will take a little while to complete," Gold said.