What’s a Power Reserve?

Article By: Logan Hannen


Dec 11, 2018

What is up, watchfam?! Today, we’re going to take a look at power reserves.

If you’ve been in this hobby for more than a minute, chances are you’re familiar with the words “power reserve.” If not, or if you still don’t know what a power reserve is, we’re going to go over everything you need to know about them. Let’s get into it:

NOMOS Tangente Power Reserve
Source: NOMOS

So, when we talk about power reserves, there are really two different things that we could be discussing.

Topic 1: The Amount Of Time A Watch Can Run Before It Runs Out Of Juice And Needs To Be Wound.

In the case of a quartz watch, the power reserve is usually something like two or more years. For mechanical pieces, which is where the majority of this discussion takes place, the power reserve can vary. It is determined by a few different factors, including things like the size of the mainspring, how many complications are involved in the movement’s construction, etc. While the actual acquisition and distribution of power involves a ton of physics and other science that would be beyond the scope of this article, the basic gist of this process is that the mainspring, being the thing that is wound tight when you hand wind a watch, gives off power as it slowly unwinds, until eventually it has no more energy left to expel, because it’s fully unwound. Once it runs out of power, the watch will stop ticking. The size of the mainspring, as well as how quickly the piece consumes energy all play a role in determining just how long a power reserve truly is.

Jaeger-LeCoultre Master Ultra Thin Reserve de Marche
Source: Jaeger-LeCoultre

Topic 2: Power Reserve Indicators

A power reserve indicator is exactly what it sounds like – some indication of how much power is left in the watch, and thus how much longer it can be worn without needing to be wound. This indicator is often displayed clearly on the watch’s dial. While this can be done poorly, many brands have now come to recognize the value of a power reserve indicator that is designed tastefully and in a way that does not hamper the design of the dial. It is most often displayed as some kind of arcing scale from zero to the maximum estimated power reserve, with a hand that moves along the arc to trace indicate how many hours of power reserve remain.

Most noteworthy, power reserve indicators are generally kind of useless on automatic timepieces with hand-winding capabilities. That is because not only will the watch wind while being worn on your wrist, but because it can be hand wound as well, it can be topped off at a moment’s notice if need be. A hand-wind only watch, however, does not feature anything to keep a watch wound in perpetuity so long as it is on your wrist. As such, it would be infinitely more useful here (or, in the alternative, any movements which do not feature hand-winding at all, such as the legendary Seiko 7S26). In both situations, the use of a power reserve indicator can be truly helpful to the wearer of a watch since it will allow him or her to quickly glance at their watch and know whether or not the watch is keeping the best time it can and how long it has until the power reserve runs dry.

Alright geeks, bit of a quick one on power reserves today, but hopefully you got something out of it and, if not, at least be sure to keep it classy, watchfam.