Power reserve explained
How long a mechanical watch runs on a full wind, how to read a power reserve indicator, and why a watch keeps better time when it is well wound.
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What it means
Power reserve is how long a fully wound watch runs before it stops. Many movements manage about 40 hours. Newer ones often reach 70 to 80, enough to leave a watch off for a weekend and find it still running on Monday. Some hand-wound watches run for a week or more.
Reading an indicator
Some watches show the reserve on a small hand or arc, marked in hours or from full to empty. On a hand-wound watch it tells you when to wind. On an automatic it shows how well your day has wound it.
Why it affects accuracy
A mechanical watch keeps its best time in the top half of its reserve. As the mainspring runs down, the balance swings less far and the rate drifts, usually slower. Winding in the morning, or wearing an automatic all day, keeps it in its best range. That is also why a rate check should start from a full wind.
When it drops
If a watch that used to run two days off the wrist now stops overnight, the reserve has dropped. Dried oils and a tired mainspring are the usual causes, and it is a sign the watch is due a service. If it simply stopped after a few days in a drawer, that is normal: here is how to restart it.