How long does it take to charge a solar watch?

Two answers circulate. One says a few minutes of sun is all a solar watch ever needs. The other says it takes days. Both come from the same charging tables, read in different columns. A few minutes buys one day of running. Days of light fill an empty cell. Below, 13 published tables from Citizen, Seiko and Casio are set side by side.

One day of running: the upkeep number

This figure tells you whether your routine keeps the watch charged. Citizen calls it "to work for one day", Seiko "time required for charging the watch to last for 1 day". Every figure assumes the dial faces the light continuously.

Casio prints one set of figures across the three Tough Solar modules we read: the 3495 inside the GW-M5610U and GW-5000U (the solar versions of the DW-5600 square), the 5719 inside the GR-B300, and the Oceanus 5600. It uses different light levels: 8 minutes at 50,000 lux (sunny day, outdoors), 30 minutes at 10,000 lux (sunny day, near a window), 48 minutes at 5,000 lux (overcast day, near a window) and 8 hours at 500 lux (indoor fluorescent lighting).

Sun is minutes for everyone; room light is hours for everyone. Taking each caliber's indoor figure against its own sun figure, room light takes between about 30 times as long (Seiko V187, 95 minutes against 3) and 120 times as long (Citizen E168, 4 hours against 2 minutes). Casio's ratio is 60.

Why Casio looks slower, and mostly is not

Casio's 8 minutes against Citizen's 2 or 3 looks like a worse solar cell. Most of the gap is the reference: Citizen and Seiko call 100,000 lux a sunny day, Casio calls 50,000 lux a sunny day. Where all three share a level, 10,000 lux, Citizen and Seiko need 5 to 12 minutes and Casio 30, so some gap remains. Casio's modules also do more: the 3495 figures count daily radio time signal reception, the 5719 four phone syncs a day.

Garmin uses the same 50,000 lux reference: its "unlimited" claim for the Instinct 2 Solar 45 mm "assumes all-day wear, 3 hours per day outside in 50,000 lux conditions". Hours, not minutes, for a smartwatch; our guide to smartwatches against mechanical watches covers that trade.

Restarting a stopped watch: the rescue number

If the watch has stopped, the one-day figure is no help: the flat cell has to recover before the hands move normally. Citizen calls this "to start working normally when the cell is discharged"; Seiko calls it "time required for charging the watch to stable hand movement", meaning the seconds hand is back to one-second steps.

Sun is 100,000 lux for Citizen and Seiko, 50,000 for Casio; indoors is 500 lux, or 700 lux ("general offices") for Seiko. "Not possible" is where the maker prints a dash. Casio's full-charge times add its three stages, and its G-Shock restart figure is to "medium charge" on the battery indicator. Citizen's E81* dark figure assumes power saving is active, the E650 runs up to 1.5 years with power save, and Casio's figures assume 6 hours a day in power saving.

So a watch that died in a drawer needs 6 minutes to 2 hours of sun to run again, and days under room light. Citizen's E81* manual adds: "Continue to subject the watch to light until the second hand moves to the 'SET' position (approximately 30 minutes or more)", then reset the time. If it still will not move after a full day of sun, our page on a stopped quartz watch takes it from there.

A full charge: the storage number

First, indoor light cannot do it. Citizen's E81* manual says so directly: "A fluorescent lamp or interior lighting does not have sufficient illumination to charge the rechargeable cell fully." Of the 14 rows above, 11 print a dash for a full charge under room light, and the E81*'s 810 hours is about 34 days of continuous lamp light.

Second, full-charge time follows the size of the reserve, not the quality of the panel. Seiko's V181 and V187 fill in 126 minutes and 3 hours of sun, the fastest here, and run only about 2 months in the dark. Citizen's E820 takes 6 hours to fill and then runs about 2 years. Comparing full-charge times between models tells you about capacity, not about how well a watch charges.

What routine keeps a solar watch charged

• You are outdoors for part of most days, wrist uncovered. A few minutes of sun covers a day, so the watch stays full without thought. Citizen says most users get "more than enough light" this way.
• You work indoors under a sleeve. Uncovered at a desk it needs 75 minutes to 8 hours of room light to bank one day; under a sleeve it gets far less. Expect it to run down over weeks.
• The watch sits in a drawer between wears. It is running on its reserve, from 2 months on a Seiko V181 to 2 years on a Citizen E820. Citizen's advice is a bright place such as a window sill. Our guide to watch storage covers where.

Citizen's E81* and E650 manuals both give the same top-up rule for a watch that does not see light regularly: direct sunlight "for 5 to 6 hours" once a month, and the E650 manual adds "not through a window".

The window note matters. Casio puts a sunny window at 10,000 lux: 30 minutes for a day of running, against 8 outdoors.

How to tell it needs light

• Seiko: "Two-second interval movement by the seconds hand is a signal that the energy of the watch is nearly depleted."
• Citizen: on the E650, two-second steps and "approximately 3 days" to a stop; the E81* family points the seconds hand at CHARGE and stops within 5 to 13 days.
• Casio 5719: two-second steps for low battery, all hands stopped when dead; leave it in the dark for about one hour between 10 p.m. and 6 a.m. and the seconds hand parks to save power, a power save state, not a fault.

On a battery quartz, a jumping seconds hand means a cell to replace; our answer on how long a watch battery lasts covers that case, and our quartz accuracy answer explains why the solar watches here keep the same ±15 seconds a month as a battery one.

Heat is the limit, not overcharging

Seiko says there is "no need to worry about damage caused by overcharging no matter how much the secondary battery is charged in excess" of the full-charge time, and Citizen's manuals list an overcharge prevention function too.

What damages them is heat. Citizen says do not charge at about 60°C (140°F) or more, keep the watch at least 50 cm (20 inches) from an incandescent bulb, and does "not recommend the use of Halogen bulbs for recharging". Casio names a dashboard in the sun and incandescent, halogen or camera lights; Seiko adds spotlights. Our answer on what temperature a watch can withstand has the measured dashboard figures.

The cell does wear, slowly

Seiko says the cell "does not require periodic replacement" but that "the capacity or charging efficiency may gradually lower" with long use; Casio says much the same. Citizen's E168 manual goes further: "The energy cell should last the life of the watch." None of the three gives a figure in years. What you would notice is the dark-running column shrinking. Our Eco-Drive page and our comparison of Eco-Drive and Seiko Solar cover what replacement involves, and our servicing guide covers the seals, which age whatever powers the watch.

When this answer stops applying

• Your caliber is not in the table. The caliber number is engraved on the case back: on a Citizen it is the first four characters of the case number. Look up that manual; Seiko notes "the required charging time slightly varies depending on the model of the watch."
• It is a smartwatch. These figures are for analog and digital quartz.
• It is a Kinetic or an automatic. Those charge from movement; see power reserve.

For everything else: a few minutes of sun for a day, up to 2 hours to restart, up to two days to fill, and a monthly afternoon outdoors. If you are still choosing a solar watch, our Seiko and Citizen comparison and our Casio brand page are the next stops.