Board Thread:Questions and Answers/@comment-25155237-20150808024341/@comment-26109599-20150808173835

The upgrade button that appears when you hover over the windwill/watermill does not actually upgrade it. It bring up a screen which informs the player that the maximum level for that power source has been reached, and the upgrade button will not be able to be pressed. The player will also have the option of demolishing the building.