POCO's are generally secretive about this. The link Dennis posted alluded to this but commercial customers are charged based on peak demand. I'm not sure of the exact formula in which this is billed but in laymens terms if your demand at peak intervals is say 50 KW you are billed at the 50KW rate (say $0.50/KW hour). If, at any point during peak usage times, you require 60 KW then you are billed at the 60 KW rate (say $0.60/KW hour) for the entire month or whatever interval the POCO decides. From my experience you can place as many phone calls as you wish to your local POCO but no one at the POCO will tell you what you want to know. If you end up being more successful at getting this info than I was would you please post your findings here ? Thanks.
Phil