Calculating Energy Consumption
It's not going to consume that. That's what the fuse is rated at. It will depend on what you're running. Maybe 200-300 for Ethernet would be the largest in a standard application, so typically I'd say under 500mA. The 5v supply tends to droop as well, so that isn't exactly a fixed number either.

If you go worst case then you can just use 1.2A x hours for an amp-hour rating if that's how the batteries are specified. There is more to it than that though. You don't really want to run the batteries too low etc. Google some pages. There's plenty of info.