Depends on inherent Linux compatability (Score 2, Informative)

Netbooks that were designed with Linux in mind will most likely experience longer battery life with a recent Linux _and_ modern distribution of your choice.

You need to remember to insure you're using the correct module per hardware device. This really is only a problem with wireless and graphics where an alternative reverse engineered or incomplete open source driver is used instead of the full featured proprietary driver. Here's some examples:

ati, via, nvidia, broadcom (fuck these guys, srsly)

Then just remember to _enable_ powersaving features if they exist such as cpufreq with ondemand, intel wireless power management, sata link control, hard disk power management, pci express, and if possible - dynamic clock control for gpu's (nvidia does this already by default in their proprietary driver).

Just another note, modern Linux distros also use the tickless kernel option by default so you're already saving more power on the cpu than you would in XP would (disregarding cpu clock speed/voltage modulation) when idle.