What power reserve means exactly
The power reserve describes how long a mechanical watch keeps running after its mainspring has been fully wound and no further winding takes place. The energy is stored entirely in this wound spring in the so-called barrel. From there the movement releases it in a controlled way until the spring is relaxed and the watch stops.
In an automatic watch, the mainspring is rewound in everyday use by the movement of your wrist: a rotor turns and tops up the spring little by little. Once you take the watch off, though, all that counts is how much energy is stored in the spring at that moment. This remaining running time is the power reserve, and it is the reason an automatic eventually stops after a day or two in the bedside drawer.
You can think of it as the watch's battery, only purely mechanical. Fully wound, the watch runs for the longest time, and as the tension falls it approaches a standstill. It is important to know that in most movements the rate accuracy only suffers right at the end of the power reserve, when the spring force drops off noticeably.
A common misunderstanding is worth clearing up: power reserve and rate accuracy are two different things. The power reserve tells you how long the watch runs at all, while accuracy tells you how precisely it keeps time while doing so. In many movements the accuracy stays stable over most of the running time and only drops noticeably shortly before the watch stops, when the spring force fades.
Typical values: from classic to modern
How high the power reserve is depends heavily on the calibre, and the range is wide. The following figures are meant as a guide, not as guaranteed values for every individual watch. The binding figure is always in the manufacturer data for your specific movement.
| Movement generation | Examples | Power reserve (guide) |
|---|---|---|
| Classic standard movements | older ETA and Sellita base calibres | around 38 to 42 h |
| Established in-house movements | many classic automatic watches | around 42 to 48 h |
| Modern long-runners | Powermatic 80, Seiko 6R55 and similar | around 70 to 80 h |
| Movements with multiple barrels | Designs for very long running time | in some cases well over 80 h |
The trend of recent years clearly points towards longer running times. Where around two days used to be common, many new movements offer three days or more, often thanks to optimised mainsprings or a second barrel. Bear in mind, though, that a higher power reserve does not automatically mean a better watch; it is mainly a question of convenience and design.
A special case is purely hand-wound watches, which have no rotor. Their power reserve works on the same principle, except that you fill the spring solely via the crown. Here too the range runs from around two days in classic movements to several days in modern long-runners, often with an easy-to-read indicator, because deliberate winding is part of the daily ritual with these watches.
The power reserve indicator and how to read it
Some watches carry a power reserve indicator right on the dial, often as a small auxiliary hand or an arc-shaped scale. It shows how much energy is currently left in the mainspring. When you wind the watch the hand moves up, and as the energy is used it sinks again.
The practical benefit is obvious: you can see at a glance whether the watch is about to stop in the next few hours or still has plenty of reserve. Especially with watches you do not wear every day, the indicator saves you the guesswork. Not every movement has this complication; it is a nice but not essential extra.
Technically, the indicator is driven by an additional gear train that picks up the tension state of the barrel. That is why it is a lovely detail but also makes the movement somewhat more complex and expensive. For everyday use, many wearers simply find that experience with their calibre is enough to know when the watch needs winding, with no indicator on the dial at all.
Without an indicator, experience with your calibre simply helps you. If you know your movement manages around 40 hours, you can estimate well when you need to top it up. How to wind by hand properly without overwinding the crown is explained in the article Automatikuhr richtig aufziehen.
Why a long power reserve is practical over the weekend
The most noticeable advantage of a long power reserve shows at the weekend. If you take off a watch with around 40 hours of reserve on Friday evening, it will very likely have stopped by Monday morning. A watch with 70 or 80 hours, by contrast, easily gets through the same break and still shows the right time on Monday.
This is more than mere convenience, because a stopped movement means resetting. Especially with watches that have a date, day of the week or moon phase, resetting is a nuisance and has to be done at the right time of day so as not to strain the mechanism. Anyone who rotates between several watches knows this small annoyance well.
Another everyday advantage shows with watches that you deliberately wear only on certain occasions. With a watch that has plenty of reserve, you can put it on in between without having to rewind and reset it each time. This lowers the barrier to actually wearing a beautiful watch instead of leaving it unused in the drawer.
This is exactly where the idea of the watch winder comes in. Whether such a device is worthwhile for you, or whether your watch's long power reserve is enough anyway, is answered honestly in the guide Braucht meine Uhr einen Uhrenbeweger?. The short answer up front: it is a question of convenience, not a technical necessity.
Hand winding, automatic and watch winder working together
The power reserve can be filled in three ways, and it helps to keep them clearly apart. With hand winding you tension the spring via the crown, which many automatic watches also allow in addition to the rotor. When worn, the automatic rotor provides the supply as soon as your wrist moves.
When you are not wearing the watch, the watch winder steps in. It turns the watch in a gentle rhythm so that the rotor keeps the spring wound, just as if the watch were on your wrist. It is important to understand that a winder only keeps the existing power reserve topped up; it is not a must, and merely replaces wearing during the rest period.
In practice this means: if you wear your watch daily, your wrist alone fills the power reserve, and you need neither hand winding nor a winder. If, on the other hand, you switch often between several watches, each one sits idle more often than its reserve lasts. It is precisely in this situation that a winder shows its convenience, because it keeps the watches you have taken off wound and ready to wear straight away.
A widespread misconception is the worry that a winder could overwind the mainspring. It cannot, because the barrel contains a slipping clutch that simply slips when the spring is fully tensioned. The winder thus keeps the power reserve at the upper end without harming the movement in any way.
For this to work, the winder has to deliver the right number of turns, otherwise the watch will stop anyway. Which values suit which calibre is listed in the sourced table at Umdrehungen pro Tag. You will find suitable devices for every collection size in the Uhrenbeweger-Kaufberatung. If you are looking for a quiet model for the bedroom, the guide on Uhrenbeweger Lautstärke is the place to look.
Factors: barrel, barrels and efficiency
Why one watch manages 38 hours and another 80 hours has several technical reasons. The first is the mainspring itself: a longer or specially shaped spring stores more energy and releases it for longer. Modern materials allow considerably more here than before, without the spring having to be built thicker.
The second factor is the number of barrels. Many movements have one barrel, while some long-runners work with two, which then deliver considerably more running time in series. The third factor is the efficiency of the gear train and the escapement, because the less friction a movement has, the more economically it uses the stored energy.
Besides the design, external influences also play a role. Position and temperature slightly affect the spring force and the friction in the movement, which is why the same watch can run for minimally different lengths of time on different days. These variations are entirely normal and are usually within a small range as long as the movement is in good condition.
Last but not least, the condition of your watch matters. An aged mainspring, old oil or an overdue service show up first in a shrinking power reserve, often long before the watch stops working altogether. If the running time falls well below the manufacturer's figure, that is a good reason to show the watch to a specialist workshop. More basics on movements and winding are also collected in the Uhrenwissen von TopWatchReviews.