From English Spielbox (see also file English Spielbox page with rules entry):
Storage power plants are used by pumping water into a reservoir at times of low electricity consumption (at night, for example). This allows you to generate additional water power at times of peak demand. The following expansion simulates this functionality.
- There are two storage power plants available per player; nobody is allowed to own more than two.
- For 10 "Elektro" per piece, you can buy a storage power plant at any time during the game. Upon their purchase, the storage power plants are uncharged.
- If charged, a storage power plant can supply exactly one city. After that, the storage power plant is exhausted, but can be recharged.
- Storage power plants are in play in addition to the regular power plants. They are not counted among the power plant limit and are not subject to other restrictions regarding power plants.
- You can charge your storage power plant(s) only if you generate more energy than you need to supply your cities.
- To indicate that a storage power plant has been charged, put a coin on top of it, and remove it when it is discharged.
- If you have one or two charged storage power plants and generate less energy than is required for supplying your cities, you have to discharge these storage power plants so that you supply as many cities as you can. So, in this case, you may not keep the energy you have stored in order to use it, for instance, to your advantage in the end game.
It has turned out during development that storage power plants may not always take effect, but that, if they are bought, they integrate smoothly into Power Grid and provide an additional depth of play. If they are used, they can be of great importance in the end game.