Power reserve

Power reserve is the length of time a fully wound mechanical watch will keep running before the mainspring is spent and the watch stops. The same phrase names the indicator fitted to some watches, a hand or aperture driven by a differential that counts turns in against turns out and shows how much winding is left.

Where the hours come from

The mainspring is a flat ribbon of spring alloy coiled inside the barrel. Winding tightens the coil, running unwinds it, and the barrel turns roughly six or seven times from full to spent. What those turns buy in hours is a gearing question, and the escapement sets the spending rate: at 28,800 vibrations per hour a movement draws the barrel down about a third faster than at 21,600, all else being equal.

There are only a few levers. A longer, thinner spring fits more turns into the same barrel at the cost of torque. Two barrels in series add their turns together and extend the reserve, while two in parallel share the load and steady the torque instead.

Typical figures

A conventional single barrel movement running at 4 Hz gives somewhere around 38 to 42 hours, which is why so many watches are quoted at "about 40 hours". Longer figures come from a longer and thinner mainspring in a wider barrel, from two barrels working together, or from dropping the balance frequency to 3 Hz so the movement spends its energy more slowly. Reserves of 70 to 80 hours are now common, and are marketed as covering a weekend off the wrist.

Two figures are often published and they are not the same thing. The minimum is what the caliber must deliver to leave the factory; the typical is what a good example does. Judge a watch on the minimum. The figure also belongs to the movement, not the watch: a chronograph left running drains the same barrel faster.

How to read the spec

The quoted number is measured from full wind to stop, on a new movement, on a bench. Two things follow.

First, the last hours are the weakest. As the spring unwinds, torque falls, amplitude falls and the rate drifts, so the useful accurate reserve is shorter than the headline figure. This is why makers quote rate performance over the first 24 hours and why a watch worn on alternate days can appear less accurate than one worn daily. The property has a name, isochronism, and no mechanical oscillator has it perfectly, which is why almost every rate specification carries the words "at full wind".

Second, an automatic on the wrist rarely reaches full wind through ordinary daily activity. A desk-bound day may put in less than it takes out. If accuracy matters to you, give an automatic 20 or 30 turns of the crown before putting it on rather than relying on the rotor to catch up.

How the indicator works

A reserve display is a subtraction done in gears: a differential compares the winding side against the barrel unwinding, and drives a hand, a disc or an aperture from the difference.

On an automatic the arrangement has to allow for the slipping bridle. The indicator is calibrated to sit at the top of its scale about where the bridle starts to slip, so a reserve hand that never quite touches the end stop, or that sits pinned at full through a whole day of wear, is usually behaving normally.

What a falling reserve tells you

Reserve is one of the few pieces of movement health you can measure at home. Wind the watch fully, note the time, leave it dial-up and see when it stops. A movement running well short of its specification, say 26 hours where 42 is claimed, usually points at degraded lubricant, a mainspring that has taken a set, or, on an automatic, a slipping bridle letting go too early. All three are service findings rather than things to live with, since low torque means low amplitude and unstable timekeeping long before the watch actually halts.

Wind a manual watch to the stop or give an automatic thirty to forty crown turns, note the time, leave it dial up. Compare against the published minimum. Ten percent short is inside the noise of a home test; thirty percent short is a finding, and a service is an indicative $250 to $500 for a three-hand automatic at an independent watchmaker in the United States before sales tax.

Common confusions

Power reserve is not accuracy. A longer reserve does not make a watch keep better time: it makes the watch more likely to be inside its accurate window when you pick it up.

It is not battery life either, although the phrase gets borrowed. A quartz watch runs for years on a cell, and a solar watch such as an Eco-Drive quotes a run time in the dark from a full charge, measured in months.

And winding past full does nothing. On a manual watch you will feel the stop and should respect it; on an automatic the bridle slips and the turns go nowhere. If a watch keeps stopping overnight despite a full day of wear, the short reserve is the symptom and not the disease: why an automatic stops overnight works through the causes.