The battery life of the Garmin fēnix® 8 (51 mm, stainless steel) varies depending on the operating mode:

- Smartwatch mode: up to 29 days without solar charging and up to 48 days with solar charging, assuming everyday wear with 3 hours per day outdoors in 50,000 lux conditions.
- GPS mode: Up to 149 hours with solar charging, assuming continuous use under the same conditions.

It is important to note that these values were measured under optimal conditions and actual battery life may vary depending on usage and environment.