Power reserve is the length of time a movement keeps running without being wound. It is measured in hours, from a fully wound barrel.
The thresholds that matter
Three thresholds shape current catalogues.
- 38 to 48 hours. A watch worn daily winds itself. Set down on a Friday evening, it has stopped by Sunday morning.
- 60 to 72 hours. The watch clears the weekend. Set down on Friday evening, it still gives the time on Monday morning.
- Beyond 100 hours. A reserve obtained through twin or triple barrels. It mainly concerns pieces worn in rotation.
Moving from 48 to 70 hours is the only difference noticeable without instruments. That is why manufactures generalised it over the past decade.
How it is obtained
Two levers, often combined.
The first is the mainspring. A longer spring, in a low-friction alloy, stores more energy in the same space.
The second is escapement efficiency. A watch that consumes less runs longer on the same energy. Silicon lever escapements and recent surface treatments raise that efficiency by several points.
What it does not tell you
Power reserve says nothing about accuracy, nor about robustness. A 70-hour piece does not keep time better than a 42-hour one.
Nor is it linear: accuracy degrades over the final hours, as the torque delivered by the barrel weakens. The stated figure should therefore be read as autonomy, not as a window of optimal performance.