Quote:
There's some sort of payment system between the companies so that if Duke Power needs more than they can provide, then they pay Progress Energy for it, for example [if their]... customers subscribe to more than they produce, they have to buy power from someone else who is not "green". And if they don't have enough customers, another power company will buy their surplus.


That pretty well sums up my understanding of it from when I briefly worked at Progress Energy (at that time CP&L). In addition, the government regulates how much they can charge (even if that means they're selling at a loss) and requires them to provide service within a designated area even if it isn't profitable.
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~ John