Here comes the sun: how do solar watches actually work?

Jamie Weiss

The history and evolution of wristwatches has, until relatively recently, been all about making telling time more convenient. First, watches went from our shelves to our pockets. We figured out how to wind them without keys, and then they went from our pockets to our wrists. We worked out how to wind them automatically, saving the hassle of winding a watch every day. Then, quartz watches arrived, offering even less day-to-day hassle, more accuracy and increased affordability. Solar watches are the next step: timepieces that do away with the hassle of regular battery changes and are instead powered by the renewable, life-giving sun.

Look, I love mechanical watches, but from a purely practical perspective, it’s hard to argue with the practicality and performance of solar-powered watches. However, I bet you might not have thought that much about how solar watches actually work. Here, I’ll attempt to succinctly explain how these electro-mechanical marvels work, as well as answer some common questions about solar watches.

How do solar-powered watches work?

If you’re looking for an explanation of how solar cells or the photovoltaic effect work, that’s a bit above my pay grade. Long story short, solar cells convert light into electrical energy. Traditionally, solar watches used multiple solar cells on their cases or dials: these days, solar watches more elegantly hide a small solar panel behind their dials (sometimes the solar panel is the dial), a technology pioneered by Citizen by its first Eco-Drive watches in 1996.

Solar watches usually store energy in rechargeable batteries to power themselves when there’s no sunlight, such as during the night, when covered by clothing or when placed in storage. Also, this might be obvious, but all solar watches are quartz watches.

Modern solar watches are extremely efficient: most only need a relatively small amount of exposure to direct sunlight, and do not need constant exposure. Broadly speaking (every solar movement is different), 3-5 hours in direct sunlight will be more than enough to charge a solar watch for 6 months. Most solar watches, when fully charged, can run for multiple years without additional charge. Many feature power-saving modes that will turn off their displays or stop their hands from running if the watch detects persistent low-light conditions.

Usually, a solar watch’s dial will be transparent or translucent or feature apertures through which the solar cell/panel can be exposed. For instance, the Cartier Tank Must Solarbeat exposes its solar cell through the dial’s numerals, an elegant solution that makes the Solarbeat almost indistinguishable from a typical Tank. Another novel modern method can be found in Citizen’s “The Citizen” high-end solar watches, some of which have dials made of thin translucent washi paper (as often used in shoji sliding screens in Japan) that cover the solar cell.

History of solar watches

Solar cells started to become commercially available in the 1950s, although remained prohibitively expensive for most consumers. As early as 1952, Patek Philippe was experimenting with a solar-powered clock, which they exhibited at the Basel Fair. However, it wasn’t until the 1970s when integrated circuit technology improved enough to build quartz watches (leading to the so-called quartz crisis) and small solar cells became affordable enough to be used in small consumer electronic devices that we saw the world’s first proper solar-powered watch, the Synchronar 2100.

Unveiled in 1972 and developed by self-taught American engineer Roger W. Riehl, the Synchronar 2100 was priced at around US$500 (or US$3,770 in today’s money), measured up at 40.8mm in diameter, had a hermetically sealed casket-shaped case, a digital LED display, a perpetual calendar that tracked the date through the year 2100 and was powered by two prominent solar arrays on top of the watch. While technically impressive, its high price tag put it firmly into luxury watch territory, and its unwieldy, sci-fi design made it a tough sell when it hit the market in 1975.

Another formative solar watch was the Uranus Solar-LED, which was also released in 1975. It featured a much more conventional design than the Synchronar, placing its solar cells on the dial of the watch itself which would become the prevailing format for most solar watches going forward. A year later in 1976, Citizen – now the market leader in solar watches – released the Crystron Solar Cell, the world’s first light-powered analogue watch and arguably the world’s first practical solar watch. Other watchmakers who were formative in the early development and popularisation of solar watches include Casio, Junghans, and Seiko.

Do solar watch batteries need to be replaced?

While solar watches typically have spectacular longevity, far more than your average quartz watch, eventually the batteries in a solar watch will need to be replaced (despite marketing to the contrary). On average, a solar watch will need a new battery every 10 years, but some watches can last even longer. Cartier, for instance, claims that the battery in its Tank Must Solarbeat will last “at least” 16 years without battery replacement (versus 8 years for Cartier’s high-autonomy quartz movements, for comparison). TAG Heuer provides a 10-year warranty on the watch and a 15-year warranty on the battery for its Solargraph models. There’s also plenty of anecdotal evidence for Casio solar watches surviving upwards of 20 years without new batteries. This is all to say that solar watch batteries last a long time.

It’s also reasonably straightforward to replace the battery in a solar watch. However, solar watches cannot use standard alkaline button cells like a normal quartz watch: those batteries are not designed to be recharged, so solar watches require specific rechargeable batteries, which is something to keep in mind if you want to change a battery yourself. It’s also worth mentioning that many people write off their solar watches as needing new batteries or being broken when they just need more charge. If you come across a solar watch that’s been lying dormant for years, expose it to direct sunlight for a few days – you might be surprised to find that it’ll spring back to life!

Can solar watches be charged by artificial light?

Yes – solar watches don’t need direct sunlight to charge, so if you live in a cloudy place or a country with harsh winters, you’re not totally precluded from owning a solar watch. Indoor lights and lamps will also charge a solar watch, albeit at a much slower rate. As mentioned earlier, modern solar watches don’t need much time in direct sunlight to keep running for many days.

Are solar watches eco-friendly?

Compared to standard quartz watches? Definitely. Because a solar watch can last so much longer between batteries, they’re better for the planet than your average quartz watch, and therefore are often described or marketed as being ‘eco-friendly’ or ‘sustainable’. On balance, however, solar watches are less eco-friendly or sustainable than mechanical watches, because they still eventually require a battery change, whereas a mechanical watch (if serviced properly) will last decades or even hundreds of years.