Every brand quotes a power reserve. No authority publicly defines it.
What was checked. The published catalogue of ISO/TC 114, the technical committee for horology, contains no standard on power reserve, autonomy or running time. ISO does standardise autonomy for battery-powered watches (ISO 12819:2009, on evaluating the battery life of a battery-powered watch), which makes the absence of a mainspring equivalent more conspicuous, not less. BIPM does not define it; it is not a metrological quantity. METAS tests it, listing "the validation of the power reserve" among its certification criteria, without defining the term.
The one gap that remains. ISO 6426-2:2026, Horological vocabulary — Part 2: Technical and commercial definitions, published February 2026, "defines the principal terms used in horology". Its term list is paywalled and could not be read, so it may contain an entry. That single document would settle the question.
Why this entry exists at all. A glossary that silently omits the terms it could not source is indistinguishable from one that never looked. The honest position is that in common use a power reserve is the period a fully wound watch will run unaided, that this usage is universal, and that no public authority has fixed its meaning or its method of measurement.
Sources
Every statement above traces to one of these. Read on the date shown.
- Tier 1 ISO/TC 114 Horology, published standards catalogue, source read 2026-09-26
- Tier 1 ISO 6426-2:2026, Horological vocabulary — Part 2: Technical and commercial definitions, edition 3, published February 2026 — term list not public, source read 2026-09-26
- Tier 1 Federal Institute of Metrology METAS, "Watch certification", uses the term without defining it, source read 2026-09-26