As two previous answers already give some references for the power consumption of different models of the Pi and various load cases, I would like to add a note on the amount of energy a portable "power-bank" will have to provide over an 8 hour period.
Assuming the bank can provide its power at a constant 5V level. It's really as simple as this:
E = I * t * SF
with I being the current drawn by the Pi and the additional peripherals (measure this for the real thing), t the duration (8 hours), and SF a safety factor to ease your mind and accommodate for aging of the battery. So as an example:
E = 800 mA * 8 h * 1.2 = 7680 mAh.
On a nit-picking side note: technically it should read not power but energy in "how much power this Pi would be using over an 8 hour period". The integral of power over time defines the work performed - which is an amount of energy.