I visited the station a few years ago and it is the reason for my original comment, at the time this was explained in some detail by one of the people on the tour. Very impressive by the way and highly recommended if you're in the region.
As for the bookkeeping:
A certain amount of electricity is produced using renewables which is always < the current baseload. It is then (much) more profitable to sell this energy to consumers directly and then to use the reduced base load requirements to top off the lakes before reducing base load generation capacity. At night the process reverses and now the 'newly minted green electrons' get added to the green energy already being sold. This makes more money than selling the nuclear/coal/NG generated electricity directly even though there is some loss from the whole pumping operation.
Note that the scheme already made money based on the demand pricing of electricity, the 'green' aspect simply made it more profitable.
You are describing some blatantly fraudulent bookkeeping right there.
Let's say you charge $0.15/kWh for "green" electricity and $0.10/kWh for "black" electricity. You can't change the color of the electrons once they are in the grid, just to make more money.
To say otherwise is to end up with stupid schemes like using a sulfur plasma lamp, powered by coal-burning, to shine artificial light on a solar cell. Or run air fans with your nuclear steam turbine to blow air across your wind turbines. Or endlessly pump the same water over a hydro turbine.
Power from pumped storage generates the same "color" of electricity as the electricity used to run the pumps. If the grid was 15% green when it went up, it will be 15% green when it comes down.
Also this chap has a fairly good bunch of images and maps of various scheme throughout Scotland, apparently blagging his way into some facilities to take photographs:
As for the bookkeeping:
A certain amount of electricity is produced using renewables which is always < the current baseload. It is then (much) more profitable to sell this energy to consumers directly and then to use the reduced base load requirements to top off the lakes before reducing base load generation capacity. At night the process reverses and now the 'newly minted green electrons' get added to the green energy already being sold. This makes more money than selling the nuclear/coal/NG generated electricity directly even though there is some loss from the whole pumping operation.
Note that the scheme already made money based on the demand pricing of electricity, the 'green' aspect simply made it more profitable.