Before we dive deep into the differences and similarities between a solar vs. an automatic watch we first need to explain what a watch movement is.
A watch movement, also known as a caliber, is the motor that makes the watch and its complications work. The movement is really the heart of the watch. A complication is something additional besides the hands, like a day or date function. There are other complications in watches, but the day and date functions are the most common in dive watches.
In a nutshell, solar and quartz watches are battery powered electronic movements, where an automatic movement is mechanical and relies on a wound spring to turn the hands. Much more detail is provided below.
Solar
A solar watch is a completely electric movement. Sun passes through the crystal and is collected on the dial, which turns the sun into electronic energy that powers the movement. There is a solar cell under the dial that gathers the light.
They work by sending an electronic current through a quartz crystal. This vibrates and drives the hands and complications. Quartz movements are highly accurate. Some gain or lose a few seconds per year.
The solar function was invented by Citizen in 1976. There is a solar cell under the dial that generates electricity and recharges the battery. The benefit is this is a watch that can go years without service or battery changes.
This is a great feature in a dive watch, because there is potential for mishap every time the case back is opened to replace a battery of for service. According to Citizen the battery will maintain an 80% rate of charge after 20 years. I’ll let you know when I get to that point with my solar watch.
Citizen claims that their solar watch can last six months in the dark, and if it does run out just put it in the light to recharge it. Even though it’s called a solar watch, it will recharge in artificial light.
Automatic
An automatic watch is purely mechanical and is self winding, thus it is automatic. There are no batteries inside of the watch, and it can keep going for years without any winding whatsoever. There are gears inside of the watch that turn the hands and power the complications. These gears are turned by a spring called the mainspring.
The mainspring is constantly winding down while driving the gears so it needs to be rewound. The way the mainspring gets rewound is from an oscillating weight that swings on a pivot. The weight moves around from the person wearing the watch. As you raise your arm to drink a beer or eat a taco the weight moves around inside of the watch and winds the main spring. If you stop drinking beer and eating tacos you stop winding the mainspring, so don’t stop.
The mainspring does have a power reserve, and depending on the watchmaker it may be longer or shorter, but most of them have a reserve of around one or two days.
Many automatic watches have a hand winding feature, so if the watch stops because you didn’t wear it for a week it allows you to wind the spring to start working. It is recommended to wind it 20 or 30 turns to wind the mainspring.
Are they accurate? Yes, they are very accurate. Most automatic watches lose or gain a few seconds a day. However, there are very high end automatic movements that have even better accuracy.
Automatic watches do need servicing from time to time, usually every five years or so but it depends on the watchmaker. Like a truck engine, they have moving parts that have wear. They do require a little oil on the gears. So it’s best to find out what service interval your watch requires and have it serviced on that schedule. Kind of like changing the oil in your truck.
Differences
The differences between solar and automatic watch movements are considerable. They are basically nothing alike. A solar watch is an electric powered watch and the automatic is a collection of gears and springs. It’s like comparing an old rotary phone with a cell phone, kind of.
The quartz movement solar watch keeps better time. The mechanical watch requires some more service. They are two completely different types of technology.
Similarities
The similarities between Solar and Automatic watches are that both are designed to last a long time without service. The benefit of either of these movements inside of a dive watch means you don’t have to crack open the case back to change the battery every two to four years.
In my opinion every time you remove the case back you are adding an opportunity of failure to the water resistance of the watch. The other added benefit is the likelihood the watch is going to fail on a dive is miniscule.
Every time you or a watch maker opens the back of the dive watch there is a chance during reassembly that something happens to the o-ring. I will admit that it is small, especially if you take it to a reputable watchmaker, but it does increase the chances of failure.
The fewer amount of times your watch has to be opened the better.
I would add the likelihood a battery operated quartz movement will fail on a dive is also miniscule, but it is even more unlikely with an automatic or solar movement.
Which One is Better?
Well that is a matter of debate. I don’t think there is any debate that a quality automatic movement is more desirable for watch enthusiasts and collectors, but I don’t know that it is better. The fact that a solar watch can basically run for your entire lifetime is pretty spectacular.
I own both, and I like my solar watches just as well as my automatics. Automatics are a feat of engineering and really a work of art. I’m pretty sure I will have both in my collection for years to come. If you like a solar watch that you’ll have for the rest of your life with no service issues, get one. If you want an auto because of the nature of what it is, get one. I think you should get one of each.
If you think there is something I overlooked or left out of this post please drop me a line on my contact form.