OK, thanks for the replies, rurwin for the warnings and pointing me to the FAQ's, which I read completely, and realworld for the link which I read with interest. Formerly I powered pics with 2 or 3 AA batteries, and that worked out really nice.
But I can see from your replies that RaspberryPi is in a completely different category - more of a computer than a pic, and uses a lot more power. Battery power will probably not be practical for me.
I'm only about 3 weeks into my RPi, so I'm new to it. I appreciate that you took the time to help me with this question that was important to me.