You put her down on Friday night, you take her back on Monday morning: she's stopped. It's not a breakdown. It is the power reserve that has been emptied.
What the Power Reserve measures
An automatic watch stores energy in a spring inside the barrel. When you wear it, the rotor turns with your wrist movements and winds this spring. When you put it down, the spring gradually unwinds to move the hands.
The power reserve is the time a fully wound watch continues to run without being worn. It is measured in hours.
Significant differences between calibres
Many current automatic movements run for around forty hours. Other, more recent ones comfortably exceed three days. The exact figure appears in the manufacturer's documentation or the calibre's specifications.
Two details often change the reading of this figure:
- The figure assumes a fully wound watch. A watch worn for only an hour has not been fully wound and will stop much sooner.
- Accuracy declines at the end of the reserve. When the spring is almost relaxed, many movements become less regular before stopping.
What it changes every day
If you wear the same automatic every day, the power reserve almost does not concern you: your wrist constantly recharges it.
If you alternate between two or three watches, the calculation is done quickly. A forty-hour watch left a day and a half on the table will be stopped when you take it back. We'll have to put it back on time, and the date if it shows one.
This is exactly the situation for which a winder exists: it reproduces the movements of the wrist in short cycles and keeps the reserve full.
Do we have to put everything on a winder?
No. A watch you won't wear for several months has no reason to turn. It can stop without damage and wait in a watch box, sheltered from dust.
The correct distribution is often simple: on the winder, the watches of the current rotation; in the box, those waiting for their turn.
How to know the reserve of your watch
- Look at the manual or calibre card.
- If you no longer have them, your watchmaker will give you the answer in seconds.
- Failing this, do the test: wear the watch for a full day, place it at a specific time and note when it stops.
In summary
Power reserve depends on the calibre and how fully it is wound. A watch worn every day can manage without a winder. A collection worn in rotation benefits from a winder set on the right parameters, which we detail in our guide how many turns per day.