Power reserve, and why it matters.

Power reserve is the one specification a beginner can use immediately, and the one most reviews mention last.

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; the escapement lets it out at a fixed rate. 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, which is why the last few hours of any reserve are not hours you should trust.

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 consumes its spring faster than one running at 21,600vph, all else equal. Tissot’s Powermatic 80 reaches 80 hours partly by beating slower than the Sellita SW200-1 in Christopher Ward’s The Twelve, which banks 38 hours at 28,800vph. Neither choice is wrong. One optimises for the watch still being alive on Monday; the other optimises 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 are often thicker.

The weekend test

Here is the only rule that matters in practice. Count the hours from when you take the watch off on Friday evening to when you put it on again on Monday morning: about 60. A watch with a 38-hour reserve will be stopped and need 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. If you own three, it decides which one you actually reach for.