An automatic watch typically runs for about 38 to 80 hours once it is taken off the wrist, depending on its movement — this is its power reserve. Many common calibres stop after a day and a half to two days, while longer-reserve movements last a full weekend. It is the figure that decides whether you need a watch winder or not.
What happens inside the barrel
A mechanical watch stores its energy in a coiled spring, housed in a cylindrical drum called the barrel. The mainspring unwinds slowly and passes its force to the gear train, then to the escapement, which releases it in regular beats.
On an automatic watch, an oscillating weight — the rotor — rewinds this spring with every movement of the wrist. Left on a shelf, the watch receives nothing: the spring unwinds until it stops.
Power reserve by calibre
| ETA 2824-2 | about 38 hours |
| Sellita SW200-1 | 38 to 41 hours |
| Seiko NH35 | about 41 hours |
| Miyota 9015 | about 42 hours |
| Rolex 3135 | about 48 hours |
| Valjoux 7750 | about 48 hours |
| Omega 8800 | about 55 hours |
| Omega 8500 | about 60 hours |
| Rolex 3235 | about 70 hours |
| Tissot Powermatic 80 | about 80 hours |
The weekend threshold
This is the practical test that matters. Between 6 pm on Friday and 8 am on Monday, sixty-two hours go by.
A watch with a 38-hour reserve has therefore stopped by Sunday morning. A watch with 48 hours has stopped by Sunday evening. A watch with 70 or 80 hours gets through the weekend without difficulty.
If you wear the same watch every day and it has a long reserve, a winder will not add much. We would rather tell you so.
Why a short power reserve is not a flaw
Extending the power reserve means a longer mainspring, a larger barrel, or two barrels mounted in series. Each of these solutions costs space, weight or complexity.
An ETA 2824-2 with 38 hours is not a bad movement: it is a movement designed in the 1970s to be worn daily, which was the norm at the time. The issue only appeared with multi-watch collections.
What the power reserve doesn't tell you
A watch does not stop dead at the end of its stated reserve. In the final hours, the amplitude of the balance drops and accuracy deteriorates before it stops.
A watch regularly left running down to the end of its reserve therefore spends part of its time in its least accurate range. It is a technical argument for a winder that is rarely mentioned: it is not just about avoiding stopping, but about keeping the movement in its optimal operating zone.
How to measure your watch's power reserve
Wind the watch fully — by the crown if it allows it, otherwise by wearing it for a full day. Note the time, put it down, and record when it stops.
A difference of ten to fifteen per cent from the stated figure is normal. A larger gap — a watch rated at 48 hours that stops at 30 — usually signals that it needs a service: ageing lubricants or a tired mainspring. If your watch has already been sitting idle for a long time, see automatic watch stopped for months.
Where a watch winder comes in
A watch winder keeps the mainspring wound at all times. The watch stays on time, the date does not need resetting, and the movement runs at high amplitude.
It still has to turn in the right direction, at the right rate — we publish the settings for forty-one calibres.