How long can your mechanical watch last on its own and when is the best time to wind it up?
A. Lange & Söhne Lange 31
Like it or not, horology has a certain number of basic principles. One of them states that if you don’t wind a mechanical timepiece, it eventually stops and you have to set the time once again. Everyone knows it, and still there’s hardly a single watch-lover around not guilty of forgetting to tweak the crown once in a while. In a nutshell, the power reserve – or reserve de march in French – is the remaining amount of energy i.e. the time it takes the mainspring hidden in the barrel to straighten to the point it stops pushing the wheel train.
A nice pair of Panerai Luminor pieces with 8-day power reserve
The majority of modern mechanical timepieces feature nearly 30-cm long mainspring and can work for the period of over 40 hours. This means you’re supposed to wind your watch on a daily basis and always have an extra bit of power for occasional force majeure. The harder mainspring is wound, the more precisely the movement functions. It is advised not to let it go beyond the 30% mark, especially for the timepieces with a week-long power reserve and above. Of course in case of any respected brand this fault is really insignificant in terms of everyday use. However, the whole story of Swiss watchmaking is an endless odyssey to the land of perfection and technology continues to evolve as we speak trying to tame those lost couple of seconds. 
Jaquet Droz Loving Butterfly Automaton
While the power reserve indicator was first developed as a necessity to remind the owner of his regular yet pleasing duties, it soon evolved into a charming complication in its own right, full of aesthetic and sometimes even philosophical value. The module in its pure classical form is fairly simple and is still widely used in the industry. A train of gears connected to the ratchet wheel or to the barrel arbour drives the power reserve indicator. By having the other side of this differential train also connected to the teeth of the barrel, the indicator moves in the opposite direction as mainspring energy is consumed. It all started from the maritime traffic needs. Back then, marine chronometers were a crucial part of navigation and keeping them properly wound was a matter of life and death. When in 1930s-1940s the same principle was adapted to wristwatches, it quickly became clear that such straightforward technical approach gives the watchmakers a real carte blanche in designs. The indicators can be executed in a form of a hand, an aperture, or even a colored dot; it can count hours, days or just moves between the "+" and "–" signs. If a watch runs longer than usual, chances are, we’ll see a power reserve indicator somewhere, and the more sophisticated the barrel engineering, the more daring and artistic will be the complication.
Hublot MP-05 LaFerrari
There are two obvious ways of making a movement run longer: make a longer mainspring or use several barrels working in parallel or independently. The mighty A. Lange & Söhne Lange 31 can run for a whole month from a single winding, all thanks to the enormous barrel taking a dominant part in the movement real estate. Jaquet Droz Loving Butterfly Automaton features two barrels serving different purposes: one pushes the main wheel train, while the other is fully dedicated to the energy consuming automaton involving the carriage spinning wheels and some serious wing swinging.
Rebellion Timepieces T2M
Meanwhile the real heavy-hitters always tend to strive for quantity and boldness in order to show their power in the most obvious, aggressive manner. The famous Hublot MP-05 LaFerrari raises the number of barrels to 11, gaining a stunning endurance of 50 days. And because winding such a monster with a crown would surely be a nightmare, the timepiece comes with a special tool – some hi-tech luxury analog of electric screwdriver. The movement of Rebellion Timepieces T2M looks almost like a spacecraft engine and comprises 8 barrels featuring the 1400-hour or 2-month power reserve. And again, no one expects you to mess with the crown, just pull back and forth a trademark clinch-looking lever at the back.
Vacheron Constantin Traditionnelle Twin Beat
This year’s big shot at the power reserve territory was the Vacheron Constantin presented Traditionnelle Twin Beat – a perpetual calendar with a system for lossless timekeeping transition between high-frequency and low-frequency modes, and a perpetual calendar. A pusher at the 8 o’clock position allows the wearer to easily switch from one frequency to another, depending on activity level. When the watch is on the wrist, the wearer will get the most out of it by keeping it in active mode, which features a 5Hz (36,000 vph) balance and 4 days of power reserve. If the watch is to be taken off and left unworn for some time, it can be switched to standby mode, which runs on a second balance with a drastically reduced frequency of 1.2Hz (8,640 vph). This allows the maximum power reserve to be extended to at least 65 days. As expected, the power reserve indication has a double scale, so we can instantly know the barrel status regardless of the current mode.