Solar Watch Charging: The Debate Rages!


Casio G-SHOCK solar watch

I’ve only been in the Casio game only a few months but I’ve discovered that enthusiasts have more opinions on the “right” way to charge a solar watch than Baptists have about baptism. On the forums and social media groups, solar watch charging has been debated for decades. When a newbie posts concerns about his new watch arriving with a low charge, a battle of solar wits ensues . . .

The Charge of the Light Brigade

When a Casio needs a charge to to keep on keeping on is not up for discussion. Here’s the official and universally accepted 411 from one of my G-SHOCK manuals:

The time required to bring a Casio up to speed? Also not a hot topic.

Here’s what Casio’s wonks recommend re: the amount of charging needed to power solar G-SHOCK watches for one day. To “break even” operations-wise.

You’d be amazed at how many Casio buyers never RTFM. Here’s another salient chart, revealing the requirements to raise their solar watch battery charge from one functional level to the next.

Nature vs nurture

From here, we go straight into the weeds. In the ongoing debate over how to charge their solar watch, Casiophiles divide into two distinct camps: naturalists vs gizmologists.

Naturalists are adamant! Use natural sunlight to charge your solar watch. For one thing, it’s free. For another, it’s green. Using an artificial light source to charge your solar watch is “wasteful” – contrary to the natural helio-horological creed. Not so fast, Mr. Bond . . .

The practice assumes you’re a one-watch man or woman. Its success also depends on where you spend most of your day – outdoors or a fluorescently-lit office. And whether or not you wear long sleeves.

Naturalists have an answer, of course. Put your watch collection outside for an occasional sunbathing session. To quote Meatloaf, you got to do what you can and let Mother Nature do the rest.

Sunbathing solar watch or 24
Courtesy of “judg69” on the WUS forum

The issue here: the potential for your watch to overheat. So some naturalists prefer to leave their watches on an indoor window sill.

Sunbathing solar watch or 12
Courtesy of “Sir-Guy” on the WUS forum

Either way, the “natural approach” to charging a solar watch battery works. Ah, but how well? Let’s look at the “free” options from a “scientific” perspective.

Solar charging by the numbers



The angle of the dangle

In case you didn’t make it to Physics class, the intensity of natural sunlight reaching any given stationary point is in constant flux due to moving clouds. And the Earth’s rotation; as the planet spins, the incidental angle of solar exposure is changing constantly (casting shadows). This holds true whether the location is a window sill or in the middle of your backyard.

The following photos were taken seconds apart in the same location. Only the angle is changed.

There’s another issue with leaving your solar watch outside. Depending on where you live, there’s the possibility of four-legged critters, winged critters or two-legged critters running off with your precious timepieces. Crows, raccoons and people are attracted to shiny objects.

Technology, bitches!

Gizmologists claim to have a better (i.e. easier) way to charge their solar watch: technology! One clear favorite: the USB-powered LED CoolFire Solar Watch Charger. Available on Amazon for $30 (no commission on link), it’s designed to charge a single solar watch.

Solar watch recharger

When naturalists aren’t harping on about global warming, they object to the $30 cost. Others point out that the gizmo isn’t as bright as the sun. Who knew? Maybe they’re waiting for a cold fusion charger.

The LED CoolFire comes close. The company claims their invention puts out 30k lux, which is about the equivalent of direct sun through a window. In fact, TTAW testing reveals that Timechant undersold their light’s output.

The CoolFire puts out almost as much light power as a solar watch receives in direct sunlight, and far more energy than it would absorb in direct sunlight in a window. [Note: after many hours on the charger, a solar watch gets barely perceptibly warm.  Not hot.  Just warm.]

No question: the CoolFire is more convenient and reliable than Mother Nature. You can throw your watch on it and leave it there for hours, cozy and secure, while you’re at work or out living your life (wearing a different G-SHOCK, of course). But what if you want to charge multiple watches at one time and don’t want to pay $30 a throw? Scotty, I need more power!

Farm livin’ is the life for me

Hands up. I’m a gizmologist. A G-SHOCK owner who addresses the multiple solar watch charging issue without worrying one wit about rising sea levels (and I live on the coast of Florida!).

Unsatisfied with the current state of play in the technosphere, I’ve invented a “solar farm” to maintain the “H” charge on my solar watches using an LED “sun therapy” lamp (no commission on link).

Solar watch solar farm

It’s all on auto-pilot with a timer. I’ve found that one hour per day, three days per week is adequate to maintain the charge.

How effective is this light? With the light sensor placed in the center of my watch box under the glass lid, directly under the sun therapy lamp, it registers slightly better (24,100 lux) than the measurement I got in the window with direct sunlight (21,300 lux).

solar watch farm charge test

My collection quickly grew into a second box, so I pushed it up next to the first box and took a measurement at the closest edge. To demonstrate how quickly the light intensity falls off as you move away from the source, I put the sensor in the middle of the second box and took a measurement there, too.

I’d be remiss if I didn’t mention that the whole recharging issue is something of a storm in a teacup.Many of Casio’s solar watches have a “Power Saving” function. Once activated, it turns the display off (stops analog hands) to conserve power. In this dormant state, in pitch black, they can maintain a charge for months.

It should be known that allowing the batteries to deep-discharge significantly degrades their lifespan. Keeping them topped off or nearly fully charged, with either the naturalist or gizmologist approach, will prolong the life of the solar-rechargeable batteries.

Why can’t we all just get along?

You can safely file this post under first world problems. In a free country, you can choose whatever solar watch charging method affirms your green credentials and/or caters to your ongoing desire for budget, practicality and efficiency. But I will say this about that: if man were meant to charge his watch in the sun, he wouldn’t have invented the United Kingdom.

Here’s a summary of the relative intensity of various light sources as measured by my light meter:


  1. I had no idea this issue existed, much more that it had opposing factions. Kudos on using the light meter, which may have a fancier name, for quantifiable results instead of some anecdotal finger-in-the-wind YMMV drivel.
    Layers of solutions for such a tiny problem! A couple of mornings a year I notice that I’m wearing a dead quartz watch. At lunch I go down the road to pick up a new cell for a buck or so, pop it in and am back in action.

    • Thanks for reading it! Of course, I had to use a light meter. I bought it just so I could write this article! 🙂 It’s the scientist (and pedant) in me.

      It’s not really a problem. But, the question is brought up a lot on the forums… usually by someone who is new to solar watches.

      If you’re a one-watch guy, or a few watches that get decent wrist time, then no worries at all. Just wearing a solar watch (like an automatic) will power it just fine.

      I think of this “problem” or “debate” as analogous to the question of whether to use an auto-winder when you have a collection of automatic watches large enough to allow inactive watches to wind down. There’s more than one way to skin the cat, as they say.

      I was inspired to write this (and learn about it) by the many threads on this topic seen in forums on a perennial basis.

  2. Have you considered testing commonly-available T8 LED bulbs? They’re much cheaper than your light therapy solution, and can cover a wider area. I did a little searching and couldn’t find much on the subject.

    • Are you talking about “fluorescent” LED tubes? Funny…. just yesterday I tried those out in our laundry room. Took them right back out…. HUGE difference… not even close to regular fluorescent tubes.

      As compared to my sun lamp at 7 inches away? Very little will compare to the 24,100 lux that’s putting out. If I recall correctly, fluorescent lighting (like in an office) comes in at about 500 lux. The lux value is related to the brightness of the source AND the distance from the source. And, it falls off exponentially as distance increases. 1 lux = 1 lumen / square meter.

      The sun lamps used to employ fluorescent tubes but have converted over to LEDs. The matrix of LEDs creates a more uniform output and last longer.

    • Hi! Thanks for reading my article. Yes. I’ve seen the suggestion of putting the watch in a glass of water and failed to mention that. Using a windowsill was mentioned and demonstrated multiple times in the article.

  3. This is a great atricle. I’ve been searching for a way to keep my solar watches alive while living in the midwest! And, I believe your suggestions in this article were just what I was looking for. You answered all of the questions that I had about using an artificial light source to help out on those less than sunny days here (which is most of them!).

    Thank you for all your research! Thank you for the links!! And thank you for putting this together!!!

    • Thanks! And, thanks for visiting the blog and taking time to comment. I had fun researching and writing it. It’s a rather contentious topic for some odd reason. Of course, that made it even more fun to write, knowing it would ruffle some feathers. I’m glad you found it useful!

    • To my knowledge, it cannot be overcharged. Also, it is said that battery life is prolonged by keeping it topped off rather than letting it discharge deeply and then recharge.

      As for leaving it in the sun… the primary risk is cooking / overheating the watch. I wouldn’t leave it in direct sun for extended periods.

  4. Real talk, the whole point of a solar watch is to not have to think about the battery. Just park your watch in not-the-dark and actually wear it and it’ll be fine. If you have to devote any brainpower to your watch beyond “don’t throw it in a drawer forever” you done goofed.

    • Thanks for reading and commenting!

      In my case, the top of my dresser is pretty much in nearly complete darkness all day long. It’s not near any window. That’s where I want to keep my watches, and they would get no natural light there. So, I came up with a system after which I expend zero brainpower towards it. It works a charm!

  5. The Amazon Ad states that the LX1330B is not suitable for LED lights. Perhaps this accounts for the discrepancy between the manufacturer’s stated output of 30,000 lux and your measurement of 108,600 lux?

    Amazon ad link (look at the pictures on the upper left of the Amazon ad:

    • Hello! Thanks for reading my article and commenting. For the life of me, I cannot find what you’re describing on the Amazon product page for the light meter.

      In any case, I was using the same meter to measure all light sources. I could imagine a light meter being LESS sensitive to certain wavelengths, but not MORE sensitive. If it had a problem reading the LED light, it would have been LOW, I think.

      Nevertheless, the purpose of my experiment was not necessarily to accurately measure light sources – but rather to simply lend some support to the relative effectiveness of various sources to charge a solar watch. My experience with those sources (including the LED watch charger) reflected the findings with the light meter. The “CoolFire” charger flat-out works. It works very well, and it doesn’t cause the watch to overheat (like it can in direct sunlight).

      Thanks again! I had fun putting that article together. It’s an oddly controversial subject.

  6. Hi, Go to the Amazon link I listed in my previous post. Under the large picture of the item at the upper left are seven small pictures. Click on the last picture to enlarge it. There is a note on that picture that says:

    “Warm Tips, This light meter can only use for normal light’s measurement, with a measuring range 0-200,000 Lux. It’s not applicable for LED light, photography light and plant grow light.”

    According to this scientific presentation, the Extech LT300 light meter can accurately measure white LED light. However, the LT300 is 5X the price of the LX1330B.

    Presentation Link (see slides 24 and 28):

    Thank you for your informative evaluation. It played a large part in my decision to order two of the Coolfire chargers.

  7. Update:

    I received two Coolfire chargers and an Extech LT300 lux meter today. I powered the Coolfire chargers from a single RavPower dual USB output, 24 watt wall charger (2.4 Amps max each output). The plastic ring surrounding the LT300’s light sensor is the same diameter as the rubber watch rings of the Coolfire.
    I aligned the LT300’s light sensor so that no light was leaking, then held the sensor in place with a two pound brass weight on the sensor’s back.

    The instruction sheet that came with each charger claims an output of 40,000 Lux.


    1. Sensor pointed directly at the sun at 3:45 PM on 11/02/22, meter hand held 6 feet from ground – 101,100 Lux.

    2. Sensor cap in place – 00.00 Lux.

    3. Coofire charger #1 with “man size” watch ring – 88,400 Lux

    4. Coolfire charger #2 with “man size” watch ring – 99,300 Lux.

    5. Surface of kitchen island – Ten foot ceiling, eight 1130 lumen, 75 watt recessed halogen flood lights – 298.6 Lux.

    6. Desk in home office – Nine foot ceiling, four 1490 lumen, 72 watt halogen bulbs in ceiling fan light fixture – 181.3 Lux.

    7. Reading lamp in home office – Fifteen inches above desk, one 700 lumen 50 watt T4 halogen bulb – 960 Lux.

  8. Thank you for doing the research with proper equipment. lol. I just bought a new casio solar and reading through your article makes me sure that, since I only have one solar watch, I shouldn’t be worry.

    great article by the way.

Leave a Reply