A watch’s official power reserve is the amount of time a mechanical watch will run after it’s fully wound In other words, if you wind you watch all the way, how long would it be before it stops That’s the réserve de marche (power reserve or pr). When buying an automatic watch, one of the important specifications indicated is the power reserve feature The power reserve is at the 6 position on this automatic watch Here it is indicating that 25 out of 42 hours remain
A power reserve indicator (originally called réserve de marche) is a complication of the watch, which is designed to show the amount of remaining stored energy The power reserve indicator indicates the tension on the mainspring at any particular moment. This is determined by the amount of energy that can be stored in the mainspring. The majority of the automatic watches from longines offer an average 64 hours of power reserve In manual wind watches, the power reserve supply is a bit longer than in an automatic watch, but it still lessens and is spent when the watch spring is fully uncoiled unless the owner winds the watch crown by hand before that point in time. For automatic watches, it's how long the watch will keep running off stored energy when it’s not being worn
Just as a phone battery tells you how long your device will last before needing a charge, a power reserve shows how long your watch can operate without external input.
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