If you want to estimate how much power the bands may need, call a few typical bands directly and ask what they expect to see when they arrive at the venue for their keyboards, amplifiers, electronic drumsets, specialized lighting and the rest.
Then consider rounding that requirement upwards and treating it as a continuous load since you never know how long the band will have this stuff switched on for.
Be ready to duplicate this setup wherever the band may be setting up for the event.
