The power reserve is how long a mechanical watch keeps running after being fully wound, without further winding or wearing. That energy is stored in the mainspring, coiled inside the barrel. For the ETA 2824-2, one of the most widespread automatic calibres, ETA states a typical reserve of 42 hours.
Where the energy comes from
The Fondation Haute Horlogerie describes the barrel as a toothed wheel joined to a cylindrical box closed by a cover. It holds the mainspring, hooked by its outer coil to the barrel and by its inner coil to the arbor. The barrel meshes with the first pinion of the train and turns slowly: between a ninth and a sixth of a turn per hour, according to the same source.
Winding the watch, by the crown or by the rotor, coils this spring around the arbor. As it unwinds, it drives the train, which keeps the balance oscillating. The power reserve is the length of that unwinding.
Reading a maker's figures
The technical documentation of the ETA 2824-2 gives two values: a typical reserve of 42 hours and a minimum of 38 hours. The distinction is worth remembering when comparing specification sheets: a catalogue figure describes one specific calibre, under given conditions.
Tissot explains how the duration can be extended. Its Powermatic 80 calibre, launched in 2013, is presented as an evolution of the 2824 developed with ETA. It reaches up to 80 hours, notably thanks to a frequency lowered from 4 to 3 Hz, which reduces energy consumption. The balance swings more slowly, and the same spring lasts longer.
The power reserve indicator
Some watches show the state of the spring on the dial, like a gauge. The Fondation Haute Horlogerie lists the power reserve indicator among the complications. At A. Lange & Söhne, the AUF/AB display is a house signature: "AUF" means fully wound, "AB" fully unwound. It reminds the owner to wind the watch before it stops.
Living with your reserve
An automatic watch set down on Friday evening may have stopped by Monday morning: with 42 hours of reserve, that is what the arithmetic predicts. Before wearing it, a few turns of the crown give it fresh energy; our article on the crown and its positions explains the gesture. The rotor's work is described in our article on automatic winding.
Frequently asked questions
Where can I find my watch's power reserve?
In the manufacturer's documentation for your reference or calibre.
Does a quartz watch have a power reserve?
Its energy comes from the battery; our article on changing the battery covers that case.
Further reading
The School's Mainspring and barrel module shows how energy is stored and released, and the Barrel and bridle workshop moves on to practice. To follow the full course, join the Décalibre School.