<a href="https://en.wikipedia.org/wiki/Bookkeeping" rel="nofollow">https://en.wikipedia.org/wiki/Bookkeeping</a><p>This is the equivalent of saying the $999 you saved for your Macbook didn't actually buy you that Macbook, because once your money entered the bank, it became indistinguishable and inseparable from all the other money in the bank, and who knows which pennies went where.<p>Well, that's true, and if you insist that <i>your</i> $999 bought <i>your</i> Macbook, you can take precautions to ensure that (at enormous temporal sacrifice). As a practical matter, this doesn't matter to anybody, so we come together as a society and agree that transactions don't need a perfectly isolated series of exchanges, so long as the credits and debits match up.<p>The same is true for energy.
The fossil plants are either needed for reliability purposes, i.e. spinning reserve, voltage support, or they are online because it is more economical for them to stay online. It is often better for the plants to stay online at minimum generation than shut down and endure a restart time of several hours.