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There could be a simple and cheap option.

Maybe just a lorry battery with an inverter (in a box to look pretty). ( 
About $100 ? ) You could use a standard PA and not sacrifice sound quality.
A PA system shouldn't use more than about couple of hundred Watts. ( 10 
Amps from a 24volt battery )
The battery will be about 60AmpHours so should run the PA for several 
hours without need for recharging.)

It will of course be heavy, but then on a trolley that shouldn't pose a 
problem.

Kevin

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