I went further and started searching for a good candidate for the motor.  There is a bunch of electric scooter motors on Ebay selling for less that $10 each.  For example:

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=75210&item=7157958719&rd=1 These are 24V and rated for 2500 RPM.  The voltage is wrong and the speed rating is a little low.  There are some rated at 2750RPM, but I'd like to see 3000.  16A is plenty for a backup generator.

Bill, what do you think?  Could you suggest a better candidate motor.

        I would stay away from the inexpensive brush motors. They really don't last unless they are high-quality. You want the back-up to work when you need it.

        Small brushless DC motor/generators are out there, typically in scooters. They are typically set up for 24 volt input power, but I suspect they would happily put out 12 volts with modification to the controller (or complete substitution of the controller for a simple regulator.)