One difference between Linux and Windows is that Linux will allocate pretty much as much RAM as the machine makes available and use it to avoid swapping pages to the swap disk versus Windows which performs similarly when a bunch of RAM is reported as free. Or to put it another way, Linux using all the RAM is a good thing for the same reasons that Windows not using all the RAM is...it means the system is running more or less optimally.

None of which directly relates to battery life. That's mostly a measure of Microsoft being the target platform of system designers and Linux not being the target. Linux power management will probably never be as good.