It takes up to 470mA at 5V according to here so if your current scheme is powering the Pi for a couple of hours, then expect this time to halve (this also assumes your current battery power pack can supply enough current to run the Pi and display simultaneously, some won't).
Thanks for the quick reply Shabaz!
So assuming 2 usb ports being used and the screen, I would expect to draw about 2A. So a 10,000mAH pack should be able to provide 3-5 hrs of power. Can you suggest a good battery pack that can provide this output and charge at the same time without eventually drying up (ie charge rate is faster than discharge rate)?
I think most battery packs are not capable to charge at the same time as discharge, however you could disconnect and just power the Pi+display from a power supply while that power supply is also charging the battery pack. It would need some switching and circuit design though if you didn't want to do it manually. Also some battery packs have digital on/off circuitry push-buttons so that would complicate things further - I think there is no easy solution to automatic switching unless you were to design a totally custom battery pack.
I don't use such high capacity battery packs, so I couldn't recommend one, but I did notice one that contains a 5000mAH cell for £14.97 in the latest cpc.co.uk catalogue.
I am not able to get clear info on the power requirements of the new 7" touchscreen. Since it can be run through usb, I can imagine its not a lot.
Basically I am trying to find a simple solution to run the pi and the screen using a typical portable battery charger (like the ones used to charge phones and give 1 or 2A output). The pi works fine with this for a couple of hrs (depending on its AH rating). But I don't know what rating and power would be required to run both together?
Please help! Thanks very much!!