 The Station-to-Shuttle Power Transfer System (SSPTS; pronounced "spits") allowed a docked Space Shuttle to make use of power provided by the International Space Station's solar arrays.Use of this system reduced usage of a shuttle's on-board power-generating fuel cells, allowing it to stay docked to the space station for an additional four days.SSPTS was a shuttle upgrade that replaced the Assembly Power Converter Unit (APCU) with a new device called the Power Transfer Unit (PTU).