Dear Guy,
I have been shooting a 645 system for the last 2 years as a professional nature photographer. I am often in the field for long periods.
Your film consumption is moderate, and one set of batteries in the 645 should last you a couple days of shooting (220 film gives 32 exposures--expect 10-15 rolls without using AF much). Also, the 645 format probably will reduce your film consumption as you will tend concentrate more on composition as opposed to firing away with the high speed motor drive.
The 645 is a battery hog supreme! There is just no way around this. I gladly make the sacrifice for (1) the optical quality and easy handling and (2) the incredible toughness of the body and lenses (something not often mentioned). One reason for huge battery use is using AF with the heavy all-metal lenses. I rarely use AF in my line of work and so have no real explanation for the substantial battery usage.
There are several ways to address this problem.
First, you can get the vertical grip, which holds a 2CR5 lithium in the handle in the grip and 4AAs in the base. Use lithium AAs and you should see a great improvement.
Second, you can go to NiMh rechargeable batteries. These last a much longer than standard alkalines, but not quite as long as lithiums. They are, though, infinitely cheaper because they can be recharged 1000 times (and also offer excellent cold-weather performance). I simply carry 30 or so of these along with 2 8-battery chargers on trips, and can be assured of enough battery power to reach my next recharging opportunity. I would note that it is truly rare to be away from any electricity for weeks on end (see below)even at "exotic" locations (even safaries have generators nowadays).
Third, you can buy an external Contax battery pack. They make one that holds 4 "C" (or "D"?) cells and should pack quite a punch with lithiums or NiMh batteries.
Finally, if you are really going to be in the field (i.e. wilderness river rafting, backpacking, remote third-world travel, etc.) with no hope of access to electricity for weeks on end--don't take the system. Use instead your 35mm system which, for weight/space concerns would be preferred anyway.
With modern gear, carrying extra weight as batteries is just a fact of life, to me, worth it in exchange for the capabilities of electronic cameras. Given that the 645 system is HEAVY, I think that the extra weight of 30 AA batteries and a charger is, proportionately, a very small increase.
Hope this helps you with your decision. I think that all 645 AF systems are intense in their battery consumption, so I don't think for a second that this should be a deciding factor between 645 systems. It may, though, be a deciding factor in which type of system (35 vs 645) is best for extreme travel.
You can see s&les of my work with the 645 at
www.guyharrisonphoto.com.
All the best,
Guy Harrison