Your Watch’s “Power Reserve” Explained

If you wear a quartz watch, carry on. At some point, your battery will run out of juice. Remove, replace, repeat. If you wear a smart watch, there’s no charge for reading this article. If you’re new to watches, welcome to Power Reserve for Intelligent People Who Don’t Like A Lot of Technical Jargon. Let’s start with something simple – and keep it simple . . .

What is it?

Mechanical watches – a term that includes both manual winding and self-winding (a.k.a., automatic) watches – are powered by a wound spring. The spring unwinds, motivating the hands, date and whatever else the watch does. When the spring is fully unwound, the watch stops.

A watch’s official power reserve is the amount of time a mechanical watch will run after it’s fully wound. In other words, if you wind you watch all the way, how long would it be before it stops? That’s the Réserve de Marche (power reserve or PR).

Who cares?

Some people don’t enjoy setting their watch – especially if it has a date window. (To be fair, date setting is a major PITA if your watch doesn’t have a separate setting for rolling the date.) If your watch has a long power reserve, you can leave it for a while, pick it up and keep on keeping on. At least in theory.

Read between the lines!

Notice the words “fully wound” above. If you’re wearing an automatic watch, it winds as you wear it. That does not mean it’s always fully wound while on your wrist. Your automatic timeiece will wind down (lose power) as you go. The rate depends on how much you move your hand, which spins the rotor that sits above the movement (winding your watch).

Obviously enough, a hand-wound watch loses power between winds. In both cases, when you leave your watch, your timepiece may not be wound to its full PR. So when you read that a watch like the Seiko Diver Automatic Orange has a 40-hour power reserve, that doesn’t necessarily mean you can leave it for almost two days without having to reset it. The maximum TOWBND (Time Off-Wrist But Not Dead) depends on how much PR is left.

How much power reserve is enough? 

The longer the power reserve, the longer you can leave your watch between wearing or winding – regardless of how much power reserve is left when you leave it. How much PR you “need” depends on a) whether you give a damn and b) your watch wearing habits. Generally speaking, most mechanical watches have a power reserve between 40 and 50 hours.

Without getting technical, it’s possible to design a movement to increase a watch’s power reserve. The IWC Big Pilot’s watch has a seven-day power reserve. The Hublot MP-05 LaFerrari will run 50 days without winding, but it uses a special winding drill and costs $300k. So there is that.

Power Reserve Indicator?

Some watches have a little gauge on the dial that tells you the amount of tension/power left in the mainspring at any particular moment. Is this useful? That’s up to you. Does it clutter the dial? Some watchmakers are better at incorporating a PR indicator’s design on the dial than others – especially those who put the indicator on the back. But yes. Yes it does clutter the dial. Damn you Seiko Spring Drive!

A Better Answer

If you want to leave your automatic (self-winding) watch for a full weekend or longer, pop it on a watch winder. Not only does a winder maintain the time/date/etc. for as long as it’s on, it helps preserve your watch’s health (as with any mechanical device, regular use keeps the bits from seizing-up). Click here to read our independent advice on winder selection.

If you have a hand-wound watch, you’re SOL, winder-wise. You’re going to have to wind it on a regular basis, no matter what. If you’re anti-setting, buy a watch with a long PR and wind it before you set it down. And after. [Note: wind your hand-wound watch at least once-a-month. Put your finger under the crown and roll it back and forth, careful not to overwind the movement.]

A lot of horophiles consider the need to manually wind a watch a feature, not a bug – establishing an intimate connection with their watch. Bonus! Hand-wound watches don’t have a rotor (the spinny thing that winds an automatic). If your manual wind watch has a transparent caseback, you can see the entire movement in all its glory. Purists reckon it’s worth the hassle.