Power Reserve refers to the amount of time a mechanical watch can continue to operate without being wound. It is a crucial feature in horology, indicating the duration a watch will keep accurate time after being fully wound. The power reserve is determined by the capacity of the mainspring, which stores energy when wound and gradually releases it to power the watch's movement.

In automatic watches, the power reserve is replenished by the motion of the wearer's wrist, which winds the mainspring through a rotor mechanism. Manual watches require the wearer to wind the crown periodically to maintain the power reserve. The length of the power reserve can vary significantly among watches, typically ranging from 24 to 48 hours, though some high-end models boast reserves of several days or even weeks.

A power reserve indicator is a feature found on some watches, displaying the remaining energy available, allowing the wearer to know when the watch needs winding. This feature enhances the functionality and user experience, ensuring the watch remains accurate and operational. Understanding the power reserve is essential for watch enthusiasts and collectors, as it influences the maintenance and usability of timepieces.