Power reserve and Power-reserve indicator

Every mechanical watch has a power reserve but not all display a power reserve indicator. Let’s find out how power reserve works in our mechanical watches and how useful they are!

The Power reserve:
When you wind up a watch, the spring accumulates power that it will eventually release into the watch. This accumulated power is the power reserve of the watch. When we say a watch has 48-hour power reserve, it means the calibre mainspring can store a maximum of 48 hours of power for the timepiece. After 48 hours, the spring will be totally unwound and won’t deliver any force to the mechanism anymore. In watchmaking, units commonly used when talking power reserve are hours or days.

As a reminder, the power is released to the watch calibre through the main wheels to the balance spring. It is then the balance spring that regulates the power with its movement (the famous “tic tac”). To go even further, the power delivered by the mainspring is not totally constant. Usually, when fully wounded, the power delivered is higher than in the last hours of the spring. This can have an incident on the spiral as the balance spring we be under different pressures from the main spring. This is why you can observe strange measures while checking your watch accuracy in the end of the power reserve of your watch. Watchmakers have invented several constant force mechanisms to avoid this problem but it’s not today’s topic! (this will be discussed in our next article !).

For convenient purposes, power reserves need to be of at least 24 hours (hello Apple watch!). By fully winding up, you’ll know how long the watch will live before it stops. It’s important to know this period especially regarding manual-winding timepieces (for self-winding ones it’s clearly not an issue as long as you wear it). This is why watchmakers have decided to display the power reserve either on the dial or the bridges side of the calibre.

The power-reserve indicator:
Power reserve indications helps monitoring the “battery” of mechanical watches. You’ll know exactly when the timepiece will run out of energy. The indication can be materialized in four different ways:

  • A pointer
  • A disc
  • A linear device
  • A 3D structure

Some power-reserve display examples:
Power reserve indicators

How does it work?
All those displays are based on the same mechanism: a specific wheel (or a quarter wheel as on the picture below) is usually linked to the barrel thanks to a differential system which rotates according to the mainspring’s power reserve. Then, for the traditional hand indication (as shown below), watchmakers attach the hand to a pivot linked to this mechanism and then display the graduation wanted.

Power reserve mechanism

The power reserve indication is not the more complex complication in watchmaking but it could add some thickness to a calibre. Nevertheless it is important to monitor the power reserve. We also recommend you to wind your manual watch every day at the same time as it is better for keeping a good accuracy. You can also check our article about winding watches and do not forget to check and monitor your watch accuracy with Toolwatch.io

One last thing! Do you know which manual-winding watch has the longest power reserve?
The Hublot MP-05 LaFerrari with a 50-day power reserve! Watchmakers used 11 mainspring barrels to reach this power reserve! It is roughly 1’200 hours and the watch also features a tourbillon!

Hublot LaFerrari

Hublot LaFerrari

See you soon for a next article to share our love for mechanical watches ! Do not hesitate to tell us in the comments which article you would like to see here !