There is power at almost every stadium, the biggest reason not to use it is it's not consistent in voltage. Too many times we've gone somewhere and the same settings have been louder or softer due to the amount of power available.
I would definitely go the generator route instead of batteries. Supposedly we had enough power with the one we purchased, but we can't run the amps up to full power like we've needed and have had shutdowns before. One season we changed to just one amp to avoid that. A big enough generator will keep that from being an issue!