The Bench · N° 09

What a longer power reserve changes inside a watch

More running time requires stored energy to be increased or used more slowly. The solution affects mainsprings, barrels, frequency, torque and the movement's behaviour as it winds down.

Hen & Mills Editorial · 4 min read · 17 August 2026

A longer power reserve is not free running time. A movement must store more energy, consume less per hour or combine both approaches.

Manufacturers can fit a longer mainspring, use multiple barrels, reduce the balance frequency, improve efficiency or lower the energy needed by the oscillator. Each choice affects space, torque, rate stability and service.

The number on the specification sheet tells you duration. It does not explain how evenly the movement performs across that duration.

Energy begins in the mainspring

Winding stores energy by tightening the mainspring inside its barrel. As the spring relaxes, the barrel supplies torque to the gear train.

A longer or stronger spring can store more energy, but the barrel and surrounding movement must accommodate it. Changing spring dimensions also changes how torque is delivered.

The design must avoid excessive force when fully wound and inadequate force near the end of the reserve.

A larger barrel takes space

One route is a wider barrel containing a longer mainspring. That can extend running time while keeping the mechanism straightforward.

The cost is diameter. A large barrel competes with the balance, automatic winding system, calendar and other components for room on the movement plate.

It may suit a broad modern calibre better than a small or traditionally proportioned one.

Multiple barrels

Two or more barrels can be connected in series to extend duration and smooth the delivery of torque. They can also be arranged in parallel when greater torque is the objective.

Multiple barrels add wheels, arbors, bearings and assembly work. They take space and create more components to inspect during service.

Their presence does not by itself reveal the exact benefit. The connection and the rest of the movement design determine what they achieve.

Lower frequency

A balance oscillating fewer times per hour can require less energy, all else being equal. Reducing frequency can therefore contribute to a longer reserve.

Frequency also influences the escapement, seconds-hand motion and response to disturbance. A higher beat rate is not automatically more accurate, and a lower one is not automatically more efficient in every design.

The oscillator, escapement and gear train must be developed as a system.

A more efficient oscillator

Lightweight components, low-friction pivots, optimised escapement geometry and a suitable hairspring can reduce energy loss. A smaller balance may require less energy to maintain amplitude, though its inertia and shock behaviour also change.

Modern materials can help, but efficiency claims should be tied to the complete calibre rather than one silicon or low-friction component.

Energy saved in one area can be spent by a date change, chronograph or other complication.

Automatic winding and the reserve

A long reserve can make an automatic watch more tolerant of a quiet weekend, but it also takes more winding to refill from empty.

The rotor system needs enough efficiency to build reserve during normal wear. A watch may have a 120-hour maximum yet remain partly wound if the owner's movement is limited.

Hand-winding from stopped can give the automatic system a useful starting point, following the maker's instructions.

Torque is not constant

A mainspring generally delivers different torque when fully wound, midway through its reserve and close to empty. The balance amplitude can change as a result.

Movement designers use spring geometry, barrel arrangements and regulating systems to keep performance within an acceptable range. Some high-end watches add a constant-force mechanism, but that brings extra complexity.

The last advertised hours may not produce the same timing numbers as the middle of the reserve.

How reserve is measured

Manufacturers usually quote the time from fully wound until the movement stops under specified conditions. The displayed functions, position and use of complications can affect real results.

A chronograph left running consumes energy. Frequent calendar changes, alarms or striking mechanisms can also alter duration depending on the design.

A modest difference from the nominal figure is not automatically a defect, but a large repeatable shortfall deserves investigation.

Power-reserve indications

A power-reserve hand estimates how much winding remains. It normally tracks the state of the barrel through gearing rather than measuring energy electronically.

The scale may be linear-looking even when the relationship between hand position, spring torque and remaining hours is not perfectly linear.

Its practical value is warning the owner before the movement enters the weakest part of its wind-down.

Service implications

Long springs and multiple barrels still depend on correct lubrication and clean bearings. Old grease or a slipping bridle can reduce winding efficiency and usable duration.

A replacement mainspring must match the calibre specification. Choosing one merely because it fits can alter torque and amplitude.

After service, reserve should be tested alongside rate and amplitude over time, not only at full wind.

What is enough?

A reserve around two days is sufficient for daily wear but may stop over a weekend. Three to five days offers more flexibility. Beyond that, the benefit depends on how the watch is used.

Long duration can be convenient in a watch with a complicated calendar because resetting it takes time. It matters less if the watch is deliberately set each time it is worn.

The better question is whether the movement remains stable and winds effectively across the reserve you will actually use.

Read the number with its design

A longer power reserve can represent clever energy management, a larger movement or a deliberate reduction in frequency. It can also add thickness and service complexity.

Duration is only one performance measure. Rate stability, winding efficiency, robustness and ease of maintenance belong beside it.

The engineering achievement is not simply keeping the hands moving for more hours. It is doing so without compromising how the watch runs while those hours pass.