Power Reserve Explained: How Long an Automatic Watch Runs Off the Wrist
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Take an automatic watch off on Friday night and it may well have stopped by Sunday. That is not a fault. It is the power reserve running out. Here is what that number means and how to live with it.
What is power reserve?
An automatic watch stores energy in a coiled mainspring. Power reserve is how long the watch keeps running on a fully wound spring once it is no longer being moved. It is measured in hours.
How long is typical?
- Around 38 to 42 hours for most classic automatic movements. The Swiss Sellita SW200-1 in our Elemental and Air Force has 38 hours; the Miyota 82S5 in our field watches about 42.
- Around 70 to 80 hours for some modern movements built for a full weekend off the wrist.
- Several days or more for specialist movements with multiple barrels.
A longer reserve is convenient, but it is not a measure of quality on its own. A 38-hour movement worn daily never stops.
Why your watch may stop sooner than the number says
The quoted figure assumes a fully wound mainspring. A quiet day at a desk may only partly wind it, so the watch stops earlier at night. A full hand wind of 30 to 40 turns in the morning fixes it. See automatic watch losing time or stopping overnight.
Power reserve and accuracy
A mechanical watch keeps its best time when the mainspring is well wound. In the last hours of its reserve, the energy reaching the balance drops and many watches start to lose time. Keeping the watch well wound keeps it more accurate.
How to measure yours
- Wind the watch fully by hand and set it to the exact time.
- Leave it still, dial up, on a table.
- Note when it stops. If it reaches roughly the stated reserve, the movement is healthy. If it stops well short, it may be due for a service.
Living with a 38 to 42 hour reserve
- Worn daily, it never stops.
- Off for a weekend, expect to wind and set it on Monday. It takes two minutes: here's how.
- Rotating several watches? That is when a watch winder starts to make sense.