How long do solar watches last? Three kinds of solar, and what each maker actually promises

Three things "solar" means

1. Casio Tough Solar. A panel under the dial charges a rechargeable cell that runs a digital module drawing microamps. Casio's guides give an "approximate battery operating time" in the dark from a full charge, list the usage assumptions, and describe cell replacement as a job for a service center. No cable exists.
2. Citizen Eco-Drive. The same idea in an analog: Citizen's page says the movement is "powered by any light source, continuously and sustainably, eliminating the need to replace watch batteries." Citizen's page for the BN0151 does not state a dark-running reserve, so we print none.
3. Garmin solar lens. A transparent solar layer in the crystal that tops up a lithium cell driving a GPS chip and a screen. Garmin's tables give with-and-without figures and a footnote: the solar column assumes all-day wear with three hours a day outside at 50,000 lux.

How long a solar watch lasts without sunlight

The Casio figures describe a watch in a drawer; on a wrist that sees daylight, the cell never runs down. The Garmin figures describe a watch worn indoors; outdoors, Garmin says unlimited (Instinct) or 48 days (fenix 8 51mm). Same word, two different physics.

The cell underneath, and how it ages

Years of service: what can be said

No maker we cite publishes a cell life in years, and we will not invent one. What the documents support: a Casio Tough Solar cell is a serviceable part, and Casio's guides describe the replacement path; Citizen markets Eco-Drive as eliminating battery replacement; Garmin describes a "rechargeable, built-in lithium-ion battery" and no user replacement. So the honest ordering for a decade is Casio and Citizen first (slow aging, serviceable), then a solar Garmin worn outdoors (slow aging, sealed), then a solar Garmin worn indoors (normal aging, sealed). The Casio side is detailed on G-Shock solar battery replacement, the Garmin side on how long Garmin watches last.

The picks

How to make yours last

• Let it see light. Casio's guides say to leave the watch where it gets light when not worn and not to let a sleeve cover the face.
• Turn off auto-light on a Casio; the guides name it as the drain.
• On a Garmin, avoid always-on and wear it outdoors; both cut cycles, per Garmin's tables.
• Charge a Garmin within its stated range (0°C to 45°C on the Instinct 3).
• Have a Casio cell replaced by a service center, which checks the gaskets; do not open the back at home.

The ranking that follows from all this is best solar watches.