Power reserve is how long a mechanical watch keeps going after a full wind before it has to be wound again. It tells the owner how often the watch needs winding. Most watches run for roughly 40 to 48 hours. Some high-end models run for a week or longer.
Why does a mechanical watch stop?
A mechanical watch stores its running energy in the mainspring. This is a long metal strip, wound up inside the barrel. The watch stops once the spring’s force is too weak to do its work. At that point it can no longer keep the balance swinging widely enough, or it can no longer drive the escapement properly. So the mainspring sets the limit on power reserve. A longer spring might seem the obvious answer, but it brings a problem of its own. It can make a watch run fast at first and much slower later.
How long does a power reserve last?
Both hand-wound and automatic mechanical watches usually have a reserve of anywhere from a day to several days. Some run far longer than that. Watches with a long reserve often use two barrels, or silicon parts, to keep running longer. They often have a power reserve indicator as well. This shows the wearer how much stored energy is left, so the watch can be wound before it stops.
Not to be confused with
Power reserve is the running time itself. A power reserve indicator is something different. It is a separate complication that shows how much stored energy is left in the watch. Every mechanical watch has a power reserve, but only some have an indicator to display it. The indicator was first called the ‘Réserve de Marche’. It is a common feature on watches built to run for a long time between windings.
Where it comes from
Power reserve indicators appeared early on marine chronometers. They were later used on railroad-grade pocket watches. Breguet made the first wristwatch with a power reserve indicator in 1933. It was a prototype, and only one was built. In 1948, Jaeger-LeCoultre introduced a power reserve indicator built for series production. The LeCoultre 481 calibre used a differential gear to show the reserve. It worked with both self-winding and hand-wound mainsprings.
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