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1250 mAh at an educated guess, and this thread https://forums.garmin.com/showthread.php?86244-Battery-life-Edge-1000-in-use/page2&highlight=BATTERY+LIFE+1000
and this
The unit houses a powerful, dual-core, 6000mAh lithium-ion battery, capable of recharging an iPhone 5 up to three times, a GoPro Hero3+ four times and a Garmin Edge 1000 four times. It is also compatible with a plethora of other 5V devices such as tablets (including iPad), iPods, digital cameras, sat phones and more.from here
http://www.outdoorchargers.co.uk/powermonkey-explorer-2-portable-charger-6000-mah-436-p.asp
what is the battery capacity of a Garmin Edge 1000 (in mAh)? My google foo is apparently weak