Power output neccesary to power multiple USB devices at once?
You will be connecting all devices in parallel, so the voltage requirement stays constant at 5V. To get the needed current just sum up all device input currents.
If you have 4 devices with 2A each you need 8A. The power is V*I, in this case 40W.



If you want to power the devices from battery for a longer time, you obviously also have to keep in mind the capacity of the battery. 8A for 2h requires the battery to have at least 8A*2h = 16Ah = 16,000 mAh.
When choosing the equipment for this make sure all devices can handle the current you need. Check this question for more detail on power handling of USB devices.

Is this answering you question completely? Otherwise feel free to specify more precisley what you want to know.