What does power reserve mean?
A mechanical watch stores energy in a mainspring. Power reserve describes how long the movement is designed to run from a full wind before stopping. An indicator, when present, shows the approximate remaining energy; otherwise the owner may infer it from winding and elapsed time.
Automatic movements replenish the mainspring through a rotor as the watch moves, while hand-wound movements are powered by turning the crown. Both have a finite reserve, and the published duration depends on the calibre and test conditions.
Why running time may vary
A watch that was not fully wound may stop sooner than its stated reserve. Automatic winding depends on wear patterns and the movement's rotor system. Age, lubrication, condition, service history and how the watch is positioned can also affect performance.
If a watch repeatedly runs for much less than expected after correct winding, have it assessed by a watchmaker. Do not use a watch winder to hide a service issue or assume that a longer reserve means a more accurate movement.
How to wind and store a watch
For a hand-wound watch, use the crown as directed and stop when the mechanism reaches its normal resistance; never force it. Automatic watches can usually be manually wound, but the number of turns and crown procedure depend on the model. Follow the manual.
If the watch will not be worn, a clean, dry and secure storage place is usually sufficient. A winder is optional and may keep a watch running, but it does not make servicing unnecessary. For watches with calendars, follow safe-setting instructions before restarting.
Power reserve and buying
Compare reserve as one part of movement design alongside accuracy, winding feel, frequency, serviceability and complication. A watch with a long reserve can be convenient across weekends, but personal preferences differ.
On a pre-owned watch, ask when the movement was last serviced and test the reserve only after proper winding in accordance with instructions. An accurate reserve test is not a substitute for a movement inspection.