Everything draws power, and when you consider all the elements of a live bands equipment – amps, speakers, lighting, haze, etc… It all adds up.
We are usually fine running on one 13a ring-main (split over a few sockets) in smaller venues as we do not need as much volume or lighting. Ideal if we can split between multiple ring-mains to reduce the load on single sockets. A member of the band is a qualified electrician and always carries out a power recce on arrival. For larger venues, we prefer 16a outlets if possible but we can run again from multiple 13a ring-mains.
Our top-end draw for a room with 500+ people is around 5000w, but if you think of it as 10 watts per person at the venue that will be sufficient, with the minimum being around 1200 watts.