Power Reserve Hours
A watch specification field known as Power Reserve Hours, used to describe or compare a measurable or identifying characteristic of a watch.
Also known as: Power reserve, Running time, Autonomy hours
Definition
Power reserve hours is the numerical duration a mechanical watch is specified to continue running from a fully wound mainspring before stopping, expressed in hours. A 72-hour reserve, for example, is roughly three days. The figure is usually measured under defined conditions and does not by itself describe rate stability throughout the entire reserve. Multiple barrels, mainspring design, movement frequency and energy consumption all influence the result.
Why it matters
Longer reserve can keep a watch running through days off the wrist, but duration alone is not a quality measure; accuracy, amplitude and movement design across that reserve also matter.