Power reserve

Power reserve is the time a fully wound mechanical movement continues to run without further energy input. It is a function of mainspring length, spring tension and the gear-train and torque profile. In modern manufacture movements it typically sits between 38 and 80 hours; high-performance designs with twin barrels or additional springs reach seven to ten days.

Construction basics

The mainspring in the barrel stores the movement's entire energy. Fully wound, it delivers a decreasing torque through the gear train to the escapement. Three structural levers determine power reserve:

• Spring length and height. A longer, wider spring stores more energy. The barrel volume sets the upper limit.
• Spring material. Modern alloys such as Nivarox or proprietary equivalents hold higher tension across more turns.
• Twin springs or multiple barrels. A. Lange & Söhne Lange 31 (31-day reserve), Hublot MP-05 (50 days), Vacheron Constantin Reference 57260 (60 days in a specific configuration) — exceptional constructions with cascading barrels.

Reserve indication

A power-reserve indicator (French réserve de marche) displays the current wind state on the dial or through a window on the movement. It is a complication in its own right, translating barrel torque into a linear display motion. Classical forms:

• Sector indicator with a hand sweeping a 0 to 100 percent arc, common on the A. Lange & Söhne Lange 1 and Patek Philippe Calatrava reserve models.
• Vertical window with a sliding scale, e.g. Jaeger-LeCoultre Master Reserve.
• 24-hour or 7-day scale, depending on movement capacity.

On automatic movements the reserve indicator is less common because regular wear keeps the watch constantly near full wind. On hand-wound movements it is functionally more useful and more deeply rooted in tradition.

Torque curve and rate accuracy

A fully wound movement runs at higher torque, a nearly empty one at lower. Because the escapement is designed for constant torque, rate deviation varies across the reserve. Premium maisons counter this with:

• Torque compensation via auxiliary mechanisms such as a stop spring or constant-force device (Lange 31, Romain Gauthier).
• Tighter spring characteristic through optimised spring geometry.
• Auxiliary gear trains such as the carrousel architecture in Antoine LeCoultre constructions.

Master Chronometer movements are explicitly also tested at low wind state — a movement that runs within tolerance only during the first half of its reserve does not pass the METAS standard.

Practical relevance

• Weekend wearers. A power reserve over 60 hours means the watch is still running on Monday morning if it was left fully wound on Friday. Brands like Rolex extended to 70 hours with the current 3235 caliber for this reason.
• Complication movements. Perpetual calendars, chronographs and minute repeaters consume additional energy. A reserve over 50 hours makes it easier to wear them without resetting.
• Watch winders. A watch with about 40 hours of reserve may justify a winder to keep it set between wearing days — particularly for complications whose setting procedure is involved.

At our atelier in Munich we test actual reserve at intake by winding to full and timing to stop — a movement well below specification can point to spring fatigue, lubricant ageing or gear-train wear.