Also known as Power ReserveRéserve de marche Also known as Power Reserve. A watch's power reserve refers to the amount of time a mechanical watch can run without being wound. The power reserve of a mechanical watch can vary depending on the size of the mainspring, the efficiency of the gear train, and the rate at which the mainspring releases energy. If a watch has a Power Reserve 'complication' it simply means that the status of the power reserve can be seen on the watch itself (either on the dial or movement side of the watch). [Learn More]. A watch’s power reserve refers to the amount of time a mechanical watch can run without being wound. The power reserve of a mechanical watch can vary depending on the size of the mainspringMainspring The mainspring is a spiral-shaped metal spring that stores energy in a mechanical watch movement. It is located inside the barrel. The mainspring is wound, either manually or automatically. The mainspring unwinds over time, and the watch will stop running when the mainspring is fully unwound. [Learn More], the efficiency of the gear train, and the rate at which the mainspring releases energy. If a watch has a Power Reserve ‘complication’ it simply means that the status of the power reserve can be seen on the watch itself (either on the dial or movement side of the watch).