Battery endurance has always been a thing to look after when coming to Garmin watches. Most problems comes from decreasing battery life.

The Forerunner 910XT battery should last for at least 20 hours, which is enough even for the longest endurance races.
Of course battery life depends on many things such as how many functions you are using and so on but the 20 hours is supposedly to be a minimum time of battery life.

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Garmin has also introduced Garmin solar power charger, which basically is a small solar panel, which charges your watch. Solar panel charges the external battery charger or the battery itself. To charge your watch you’ll need an extra battery or a USB mini input to charge your battery. Solar panels should be useful for longer tour but is not really practical for example for runners. 20 hour battery should be enough for any average sportsman.