That is true but not just with emulators. Whatever you run that requires CPU processing time will drain a battery for a laptop.
I never run my laptops on batteries unless I am listening to some music and doing something else or sitting on the porcelain throne.
It is amazing what can be done with linux and how you can customize it to run an application like E-UAE and make it look like a real Amiga booting up.
With Windowz you can't change much and you have to wait for the whole os to be loaded before you can execute WinUAE.
Think about it.. Amithlon, KX-Light, all are using minimal linux modified in some way to run e-uae. I personally have Amithlon installed on a Pentium III 550Mhz, 196MB Ram and 13Gigs HDD, Sound Blaster Live, nVidia FX 7000, and Nic card and all runs so smooth and so nice. I'll post some stats soon... If I could only get that wonderful package called Amikit to run on it that'd be a kick arse Amiga that you can't get with classic hardware.