Power Reserve
The length of time a mechanical watch can continue running after its mainspring has been fully wound.
Also known as: running reserve, reserve de marche, réserve de marche
Definition
Power reserve is the running time available from a mechanical watch after its mainspring has been fully wound. It begins with the energy stored in the mainspring and ends when the remaining torque is no longer sufficient to keep the movement operating. Capacity depends on factors including mainspring length and material, barrel design, movement frequency and energy consumption. Multiple barrels or more efficient escapements can extend reserve, but a long quoted reserve does not by itself guarantee better rate stability throughout the entire running period.
Why it matters
Power reserve determines how long a mechanical watch can be left unworn before it stops. It matters especially when rotating several watches and when comparing high-frequency or complication-heavy movements with more efficient designs.