Power reserve, and why it matters.
Power reserve is the one specification a beginner can put to work on day one, and it is the one most reviews mention last, somewhere near the bottom, in a list. That ordering is backwards and this lesson is about why.
What the number measures
The reserve is how long a fully wound mechanical watch keeps running with no further winding. The energy sits in a coiled mainspring housed in a barrel, and the escapement lets it back out at a fixed rate.
There is a wrinkle worth knowing. When the spring unwinds past the point where it can drive the balance evenly, the watch stops, and shortly before it stops it starts losing time. The last few hours of any reserve are not hours you should trust.
So read the figure as a description of a habit, not a brag. A watch quoted at 38 hours is a watch you wind or wear daily. A watch quoted at 70 or 80 hours is a watch that survives being put down.
The trade every maker makes
Reserve is bought, and the currency is usually beat rate. A movement running at 28,800vph eats its spring faster than one running at 21,600vph, all else equal.
You can watch that decision happen in two watches on this site. Tissot’s Powermatic 80 reaches 80 hours partly by beating slower than the Sellita SW200-1 inside Christopher Ward’s The Twelve, which banks 38 hours at 28,800vph. Neither maker is wrong. One optimised for the watch still being alive on Monday, the other for the sweep of the seconds hand and a slightly more robust rate.
The other lever is barrel design: a longer or wider mainspring, or two barrels in series. That costs space, which is why long-reserve movements often end up in thicker cases.
The weekend test
Here is the only rule that matters in practice. Count the hours from taking the watch off on Friday evening to putting it back on Monday morning, and you get about 60. A watch with a 38-hour reserve will be stopped and want resetting. A watch with 70 or 80 will be running and roughly right.
If you own one mechanical watch and wear it most days, reserve barely matters and anyone telling you otherwise is selling something. If you own three, it quietly decides which one you actually reach for.