What does “Power Reserve” mean?

Power reserve is how long a fully wound mechanical watch runs before it stops. Many traditional automatic movements run for roughly 40 to 50 hours, while newer movements often reach 70 hours or more; Rolex's caliber 3235, for example, offers about 70 hours. Some watches show the remaining reserve on the dial with a power reserve indicator.

A longer power reserve means a watch can be taken off on Friday evening and still be running on Monday morning, which buyers appreciate. When testing a watch, a reserve much shorter than specified can point to a worn mainspring or a movement due for service. Note that wearing an automatic for a short time may not wind it fully, so test from a full hand wind where possible. Dealers mention the power reserve as a feature on newer models and as a condition check on older ones.

Example: 'Full 70-hour power reserve confirmed after winding.'