IS lenses versus battery consumption

Dear folks -
I'm looking to purchase a 100-400 L lens, and am wondering if the IS function will cause extreme battery depletion on my 400D (XTi).
I know that the answer is to obtain the 'battery-grip', but I like the size of the camera as it is now.
Obviously, I can carry an extra battery with me, but how often will I be exchanging for a fresh battery with an IS lens, versus a non-IS.
I'm also comtemplating the purchase of a 400 f/5.6 prime, if the battery drain for an IS lens is extreme. (sorry, I know 'extreme' is a subjective term, but it's the best I could come up with)

Thanks for any replies.

I dont think its really noticeable on any camera body... I would however suggest you at least try the grip on the 400D though. Its IMO the best grip/body combo canon has ever made in the DSLR line.

I can only speak about my 20D with the 70-200mm F4L IS but I did not notice a huge difference. I have never counted the shots on a single charge but it was hundreds.

Not a concern. Don't let that stop you from buying an IS lens.

I for one have noticed an increase in battery usage when using the IS (on both the 17-55 and 70-200). However it's nothing an extra Sterlingtek battery can't take care of. As Nicksan said, don't let it stop you from buying the lens.

Thanks for your input, friends.
It sounds like a non-issue. I just wanted to make sure it didn't drain batteries VERY rapidly. I've noticed that using the onboard flash can deplete a battery rapidly, and I was curious if an IS lens would do the same.

Maybe I'll ask for the 'grip' for Father's Day this year. I should have known that a gearhead like me would keep spending the bucks ! ......oh well, you only live once.