A common - and very helpful - complication one might find on a mechanical timepiece is a power reserve indicator. As its name suggests, a power reserve indicator acts as a fuel gauge, informing the wearer of how many hours of run time the movement has left in it before it will need winding again.
Power reserve indicators are most common on manual wind watches, which need to be wound by hand in order to continue functioning. When the power reserve indicator shows that power has reached a critical level (often indicated by a red bar or a tapering gauge) the wearer is advised to remove the watch and to wind it fully.
It is important to note that the power reserve indicator not only tells the wearer when the power is about to run out, but also often indicates that they should keep the watch fully wound. A fully wound watch is the best way to keep your watch running accurately. That is because accuracy can fade as the power given out by the mainspring (the mechanical watch's power source) starts to wane. With less power traveling through the escapement (the watch's regulating organ responsible for timekeeping accuracy), any physical poising errors in the hairspring or balance wheel (the beating heart of the watch) can result in faulty time measurements.
In automatic or self-winding watches, those that are wound by the motion of the wearer's wrist by way of an oscillating weight mounted on the movement, power reserve indicators are decidedly less useful but still play a role. That is because an automatic watch will remain fully wound if worn daily, but if a wearer chooses to remove it for a day or two it could run low on power. The power reserve indicator will show the wearer how much power remains when the watch is picked up to be worn again. If the power is running low or has run out completely, it provides a gentle reminder to give the crown a few winds to get the watch back up and running at an optimal rate.
A secondary function of a power reserve indicator on an automatic watch could be as an error indicator. If a problem occurs with the automatic winding mechanism, the watch will begin to act as if it were a manual wind watch. That means it will not be charging automatically. While the watch may still be operational for a time, it is a good indication that the watch requires a service and should be sent back to an authorized dealer or repair center as soon as possible.
Power reserve indicators are most often found on the dial, but this is not always the case. Notably, Panerai features power reserve indicators on the movement side of the watch, visible through the sapphire crystal display back.
While the majority of power reserve indicators deliver their information via a hand pointing at a +/- scale, there are other ways the same information can be displayed. Some brands use discs, which change color (normally turning red when power reaches dangerously low levels), while others use a "loading bar" or linear style of indication like the Moritz Grossmann Power Reserve model.
Power reserve indicators have been around for a long time. They even featured on some of the very first marine chronometers, in which they were an imperative addition. Used for navigation at sea, the marine chronometers needed to remain running at a reliable daily rate so that the vessel could stay true to its course.
This technology eventually made its way into wristwatches. In 1948, Swiss brand Jaeger-LeCoultre pioneered the complication in a range of models called the Powermatic, which were fitted with the LeCoultre Caliber 481. This release marked the first series production of a power reserve wristwatch and was a timely development for the industry.
The power reserve indicator is linked directly to the barrel, which contains the mainspring that functions as a mechanical watch's power source. Power is generated in a wristwatch by the mainspring attempting to uncoil. Winding the crown has the effect of coiling the mainspring around the central post that runs from top to bottom through the barrel (a component known as the barrel arbor). In a manual wind watch, the mainspring can only be wound so far until the crown stops and the watch is fully wound. The power reserve indicator is thus geared to track the mainspring's coiling and uncoiling by being in constant engagement with the barrel via a series of small pinions.
Over time, consumer expectations have risen in regards to what constitutes a sufficient power reserve. The bare minimum one would expect to find in a modern automatic is around 38 hours. For manual wind watches one would expect a minimum of about 42 hours since the watches need to be manually wound. Manufacturers often take great pains to increase the minimum power reserve, especially when it comes to manual watches.
A "weekender" watch is one that has a three day power reserve, so named because it can be laid down on a Friday after work and picked up Monday morning still running. These lengthy power reserves are not wholly uncommon these days. Many high-end companies have experimented with multiple barrels and highly-efficient power transfer mechanisms (often by employing space-age materials that enable entirely new regulation systems) to stretch the reserve out even further. Some very fancy watches can run for weeks, even, in a couple of truly remarkable case, months.
Many nice power reserve watches are available in the entry-level price bracket, with some particularly nice examples on offer from NOMOS Glashütte (the Metro Power Reserve specifically), Hamilton (especially the Jazzmaster range), and Maurice Lacroix.