We're playing Wembley, can we use a portable PA system?

Depending on the size of the room and the amount of background noise, smaller portable PA systems will easily cover an audience of between 40-70 people. Larger single systems will cover around 150-200 people. If you need to perform to a larger audience, adding an additional system will increase your volume. For Wembley, however, you're going to need a bigger system...

Do any Portable PA systems run on batteries?

There are a select few systems which operate on batteries, for example; the Bose S1. However, due to the lower wattage of these systems, it is possible to power them via a small generator.