That reflects the production capacities available, ranked by their production cost. So this is, in a very real sense one hal of your supply-demand graph.
For a given level of demand, you can determine very precisely the price that will be needed to fulfill that demand (that of the producer that allows you to reach that same number of MW at that moment, adding all the cheaper producers in the list before that producer's capacity is needed).
And all producers will get that price - and all consumers will pay that price (noting, of course, that the market is for wholesale electricity, and is likely to apply only a 1-hout slot of demand or other similarly short period).
With capacity essentially fixed in the short term, it is demand that varies and sets price as a first approximation. Of course, power plants can decide to bid for power at prices that are different from their actual cost, for various technical or other reasons, and modify the curve - but you can still rebuild a curve based on their bids.
If you look at the demand side, it may look flat (ie demand is not sensitive to price) but within ranges. Industrial users will have cutoff prices (and may thus take out a big chunk of demand is prices go above a certain level). Some may have storage capacity and similarly drop out of the market at some prices (and become sellers). etc...
And of course, long term decisions will be affected by how the short term S-D curve looks like, and will in turn help shape it differently. In the long run, we're all dead. John Maynard Keynes
So if the supply curve changes for some reason, then the demand should be able to change immediately in response. On the other hand, there is some hysteresis in the demand curve also, because the consumer is not likely to make a quick change in usage habits.
For example, when the price of gasoline jumped from $2.90 to $3.10 per gallon last week, I did not suddenly reduce my driving to compensate. That is, there was a change in the supply curve (downwards), but I continued to demand the same amount as I did previously.
If it takes energy consumers a while, say perhaps two months, to change their consumption habits, then the operating point will be offset from the theoretical operating point for that period. I suppose that my demand would be expected to gradually move down my demand curve until it intersected with the newly changed supply curve. So this would be an example of where my small part of the economy is operating outside of the simplistic "intersection of the supply and demand curves" model.
I suspect that this is dicussed in one of the later chapters of my old copy of Samuelson's Economics...
there was a change in the supply curve (downwards)
Actually the supply curve moves to the left, which makes it move "upwards" on the S-D curve...
EDF has and had some interesting tariff arrangements with some of them - like interruptible supply pricing: such users get a low price, close to wholesale, but may be cut off (or have to pay punitive prices) at EDF's discretion, when it needs to deal with demand spikes. That reflects that they have processes that are not sensitive to cutoffs, or that they have alternative (and flexible) ways to get power at hand, and they act as 'peak supply' providers for EDF. In the long run, we're all dead. John Maynard Keynes
