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Power Reserve

Moser & Cie Pioneer Perpetual Calendar MD with 168 hours Power Reserve

The power reserve of a mainspring-driven mechanical watch or clock generally refers to the amount of time the timekeeping instrument can run autonomously. When provided in the specifications of a watch, the power reserve refers to its capacity after it has been fully wound. In other words, it is the total running time of the watch in between successive windings.

While mechanical spring-driven clocks typically have seven days of power reserve, those in mechanical wristwatches generally have between 36 and 48 hours. 

Even longer power reserves can be found in high-end complication watches such as the Patek Philippe Gondolo 10 Day Power Reserve Ref. 5100J-001 and Jaeger-LeCoultre’s Reverso Grande Date 8 Days.

When worn, the power reserve of an automatic or self-winding movement will be extended and continue running as long as one’s wrist movements cause the winding mechanism, the oscillating weight or rotor, to wind the mainspring. The power reserve will also be maintained if placed and rotated in a watch winder. Conversely, if the wristwatch is left unattended for a long period, on a table for example, its power reserve will gradually be depleted.

A powerful hand-wound engine with minimum 7 day power reserve housed in the H.Moser & Cie. Pioneer Perpetual Calendar

The power reserve indicator of a wristwatch displays the remaining power level in the movement. It may be on the dial side or at the rear of the movement. It is a useful complication especially on mechanical manual-winding watches as a low power reserve indication means that the movement requires winding while an empty one simply means the movement has stopped. 

In a sense, the power reserve display is also an indication of the movement’s accuracy. A higher torque is generated from the mainspring when it is almost fully wound or fully wound. Together with a constant energy distribution to the escapement, the movement will theoretically be more precise at higher energy levels.

On the other hand, when energy from the mainspring is almost depleted, lower torque and an erratic energy supply to the escapement will result in a less precise movement. 

As such, it is advisable to keep the power reserve levels at higher levels rather than at lower levels. This applies for both manual-winding and automatic watches. The best time to wind a watch is in the morning or just before it is worn.