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It's an all electronic camera, moving a large assembly to obtain autofocus, so it will use more power than the RTS II which only powers the shutter and metering systems. And they use different types and sizes of battery, so a meaningful comparison isn't easy.
Using re-chargeables means you can carry several batteries and top them up ready for the next session.
Unless you use a manual camera with a separate selenium meter, then batteries will be required.
I'm a bit late on this discussion but I get about 46-48 rolls of film on average when using AF, though for some reason it drops to about 40 if I switch a lot between macro and standard modes. I always used to carry a P-8 Power pack with me but now, since I am always shooting with my Contax 645, I have also bought the P-8D and get up to 60 rolls when using it. The combination of the AX, D-8 back and a power pack is superb, though not exactly something you can put inside your pocket!