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There is, in the ritual of the mechanical watch, a poetry that resists time. In the morning, we adjust a crown, we feel the resistance of the spring, we almost listen to the silence of a mechanism starting up again. Behind this simple gesture hides an essential concept, often cited in technical data sheets but rarely explained with nuance: the power reserve. It tells how long your watch will keep ticking, on its own, before stopping. But it also says something about your relationship with the object: daily use or alternation of parts, sought-after precision or absolute comfort, fascination for fine watchmaking or elegant pragmatism. For those who wonder about the inner workings of these marvels, the automatic movement offers a fascinating perspective on the energy that drives each tick.
Power reserve: the definition, without jargon
The power reserve corresponds to the autonomy of a mechanical (or automatic) watch once it is fully wound. Concretely, this is the time during which the movement receives enough energy to function correctly, before the mainspring relaxes to the point of stopping the watch. To ensure this performance, the quality of the oils used in the mechanism plays a crucial role, guaranteeing optimal lubrication of the components.
It is most often expressed in hours: 38 h, 40 h, 70 h, 120 h… Some companies proudly display it on the dial (“Power Reserve”), others discreetly slip it into the product sheet. In all cases, it is an indicator of “energy capacity”: between the quantity of energy stored, the efficiency of its transmission and the way in which it is consumed by the exhaust and the regulating organ.
Two families: manual and automatic winding
On a hand-wound watch, the power reserve depends entirely on your action: you wind the crown, you charge the spring, then the watch gradually draws on this energy.
On an automatic watch, the principle is identical but the energy is, in part, replenished by the movements of the wrist via an oscillating mass (the rotor). An automatic watch can therefore “maintain” above a certain charge level if you wear it regularly. On the other hand, placed on a table, it once again becomes a mechanism with finite autonomy.
What is a high power reserve really for?
You might think that “more is better”. In reality, the power reserve is especially important because it determines your comfort of use. A watch is a companion: it must follow your rhythm, not the other way around.
1) Spend a weekend without thinking about it
The most telling situation is that of Friday evening. You put your watch down, wear something else on Saturday, go out without a watch on Sunday, then put it back on Monday morning. A watch with 38–42 hours of power reserve will often be at a standstill. A 70 hour watch has a good chance of still being alive, and above all, on time.
2) A blessing for collectors
If you alternate between several watches, a more generous power reserve prevents you from having to reset the time and date too often. This may seem anecdotal… until we talk about complete calendars, moon phases or annual dates. Some settings require method, sometimes precautions (in particular avoiding rapid corrections near the date change). A large power reserve then becomes a real convenience.
3) Stability and precision: not always, but sometimes
As the mainspring relaxes, the torque transmitted to the movement varies. Depending on the architecture of the caliber, this variation can influence the amplitude of the balance wheel and therefore the chronometric behavior. Manufactures have been working for decades to smooth out these differences (optimized escapements, modern materials, specific barrels, stop-seconds type devices, etc.).
Be careful though: a longer power reserve does not automatically imply better precision. It all depends on how the energy is delivered and regulated, and the overall quality of the movement.
Extending the power reserve is not a simple matter of “putting in a big spring”. It is a balance between storage capacity, efficiency and consumption.
Barrel(s): the energy reservoir
The barrel houses the mainspring, a real reservoir. To increase autonomy, you can:
- Increase size of the barrel (more room for the spring, more energy).
- Optimize the spring (alloys, geometry, more regular torque curves).
- Multiply the barrels in series, to spread out the energy delivery and gain autonomy.
Efficiency and friction: watchmaking as a science of detail
A movement “consumes” energy because of friction and losses. Polishing, treatments, quality of oils, choice of rubies, architecture of the cogs: everything counts. Improving performance sometimes allows you to gain hours of battery life without increasing the volume.
Frequency: the tempo of the movement
A high frequency watch (for example 4 Hz and above, depending on the caliber) can offer better resistance to disturbances, but it often “eats” more energy. A high power reserve with a high frequency therefore requires a particularly efficient design (or more energy storage).
The power reserve indicator: useful or gadget?
The power reserve indicator (often a hand or an arc display) is not just a stylistic effect. With manual winding, it avoids winding “blindly” and reminds us that the pleasure of the object also comes through its mechanics. On an automatic, it provides information on the actual charge level: practical if you alternate watches or if your daily life is very sedentary.
It is also a cultural sign: historically, this complication has flourished on watches designed for reliability and energy management, particularly in precision watchmaking and certain traditional pieces.
Power reserve and actual use: what we don’t always tell you
The advertised power reserve is generally measured under standard conditions, on a fully loaded movement, until stopping. But between “still working” and “working at the best of its torque range”, there is sometimes a nuance. Some watches remain running even though their amplitude drops, which can affect the accuracy at the end of the reserve.
Another point: an automatic watch worn on the wrist does not necessarily remain at 100% charge. If your activity is moderate, it may move into a middle zone. This is generally provided by brands, but it is one more reason to consider autonomy as a comfort, not as a guarantee of absolute performance.
Which power reserve to choose? A simple guide
For an “everyday” watch
- 40–50 hours : sufficient if you wear it daily and you like the ritual.
- 70 hours : excellent comfort zone (Friday evening → Monday morning).
For rotating several watches
- 70–120 hours : ideal for limiting time discounts, especially with a date.
- Power reserve indicator : highly recommended if you are juggling parts.
For calendar complications
- High reserve : it can avoid tedious adjustments.
- Caution : if the watch stops, follow the correction instructions (“prohibited” zone around the date change depending on the movements).
Which watch has the largest power reserve in the world?
At this time, if I may say so, the watch which holds the record for the largest power reserve is the Vacheron Constantin Traditionnelle Twin Beat Perpetual Calendar with, get this, 65 days of power reserve!
In addition to the hours and minutes, it displays the date, month, leap year (obviously, it’s a “QP”) and its incredible power reserve. His secret? It is possible to slow down the frequency of the caliber 3610 QP to switch from high frequency mode (5 Hz) to low frequency mode (1.2 Hz) while maintaining good timing.
The last word: a technical notion, a very human luxury
Power reserve is not just a line on a data sheet. It’s a bridge between engineering and real life. It reflects the ability of a watch to adapt to your rhythm, your style, your habits. In a world where everything can be recharged at will, where time is displayed everywhere, the mechanical watch continues to exist through this form of controlled fragility: it lives on a finite energy, which it transforms into seconds and stories.
And this is perhaps where the power reserve becomes really important: not because it “gives you more”, but because it elegantly reminds you that time is earned — and that a well-designed object knows how to be forgotten, while remaining present.
And if you really have too many watches (I know that…), use an automatic watch winder! You will find a small selection here.
