Anyone who’s interested in watches has certainly come across the term “power reserve.” That said, many in the community only have a vague idea of what it really means, despite its importance in everyday watch-wearing life. With that in mind, let’s go ahead and dig a bit deeper.
What does power reserve mean for watches?
Watches need energy to work. This energy is stored in the watch and gradually released to power the movement. In mechanical wristwatches and pocket watches, the mainspring usually performs this task. The mainspring is a spiral-shaped steel band located in the barrel of the movement. The spring is tensioned by winding the crown, thus supplying it with kinetic energy. The tighter the spring is wound, the more energy it stores. Energy can also be stored in other ways, like in the battery of a quartz watch or the weighted pulley of a grandfather clock, for example.
The power reserve is the time it takes for the fully charged energy store to completely release its energy, without being supplied with new energy in the meantime. In other words, the power reserve is the time from when the mainspring is fully wound to the moment the watch stops. For calculating the power reserve of an automatic watch, it’s important for it to remain stationary, as the winding rotor would otherwise supply the mainspring with more energy with every movement. The same applies to solar-powered quartz watches: To determine their power reserve, no light should hit the solar cells, in order to prevent the battery from recharging during the measurement.
A watch’s power reserve saves you from having to wind your watch constantly. You can put it aside for the weekend, and it will still be ticking when you pick it up again on Monday morning.
What’s a good power reserve for a watch?
Mechanical watches have a standard power reserve of between 38 and 42 hours, but many manufacturers have been scrambling to increase these numbers over the years. For example, Omega developed their Co-Axial Master Chronometer caliber to offer over 50 hours of power reserve, while Rolex’s modern calibers push 70 hours. And if that’s not enough, you can check out the Panerai Radiomir Otto Giorni, which can tick away for 8 days undisturbed. The Hublot Big Bang MP-11 takes it one step further, offering a staggering 14-day power reserve. However, the gold medal goes to the A. Lange & Söhne Lange 31, which only needs to be wound every 31 days.
You might be wondering how such impressive results are possible. Well, in the case of the latter three examples, the manufacturers simply increased the number of barrels in their movements. The Panerai and Lange 31 have two barrels, while the MP-11 has seven. How it works is quite straightforward: When the energy of the first mainspring is almost depleted, the next, still fully wound mainspring takes over, and so on. However, there are some drawbacks to increasing the power reserve in this way, since mainsprings don’t release their energy at a constant rate. The more the spring slackens, the less power it transmits to the movement, which in turn affects the accuracy of the watch. Multiple barrels amplify this effect, since each spring begins with its full strength and then slowly weakens.
Rolex opted for a different method, namely optimizing their calibers as a means to improve the power reserve. Their Chronergy escapement boasts a particularly high degree of efficiency, i.e., significantly less friction, meaning Rolex watches consume less energy. Rolex also experimented with the mainspring itself and succeeded in producing the spring from progressively thinner material. This increases the length of the spring and thus how much energy it can store.
Watches With Power Reserve Indicators
Power reserve indicators are considered simple complications in watchmaking. As you might have guessed, they are used to show the wearer how much longer the watch will keep ticking before it needs to be wound again. It’s therefore the watch equivalent to the battery percentage on phones, tablets, and the like.
Abraham-Louis Breguet was the first to incorporate this display in a watch in around 1800. The complication quickly became a standard feature of marine chronometers, as they had to be kept running at all times.
With the arrival of the automatic caliber in the early 1930s, the power reserve indicator started appearing on more and more wristwatches. Customers were skeptical of the automatic winding system, so manufacturers included the indicator as a kind of reassurance. The first brand to feature a power reserve indicator as standard was Jaeger-LeCoultre in 1948.
Nowadays, power reserve indicators are relatively rare. Some well-known models are the A. Lange & Söhne Zeitwerk, the IWC Big Pilot, and the Hamilton Jazzmaster Power Reserve Auto. The display itself is usually executed as an off-center subdial with a separate hand – kind of like the fuel gauge in a car. A lot of quartz watches take a different approach, doing without a dedicated display and instead using the second hand to signal when the battery needs to be changed. If the battery power drops into a critical range, the second hand jumps in 2-second increments.
The power reserve plays a practical role for every watch wearer. The longer the power reserve, the less often you’ll have to wind your watch. But beware: timepieces that can go extremely long periods without a power supply run the risk of loosing accuracy as a result. It’s better to be on the safe side and go for watches in the mid-range between 40 and 80 hours.