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It will be slow, as the voltage is 5v and I think the MacBook takes 14.5 from the official charger. So this will be the same as as iPad charger.
The macbook charger is 29 watt.
This puts out 60 watts (http://www.amazon.com/Sabrent-Famil...8&qid=1430885787&sr=8-17&keywords=usb+charger)
and this one 40 watts (http://www.amazon.com/40-Watt-Deskt...F8&qid=1430885885&sr=8-6&keywords=usb+charger)
Ideally I'd like to get a charger that can charge the macbook, a phone and a watch along with an external battery. (For travel purposes)