Sounds like a reasonable option. According to Wikipedia, the 11" MBA has a 35 Wh battery, so additional 60 Wh would indeed be a big improvement. From their Amazon specs (
http://amzn.com/B00449I41K):
High capacity 60-Watt-hour lithium ion battery allows you to use your MacBook, MacBook Pro, or MacBook Air for up to 20 hours
That page also gives the weight as 362 grams (12.8 ounces). But then you'll also need the MacBook airline adapter which is another 49$. The battery itself is 168$ (and the same amount in €).
Here is a review: http://www.slashgear.com/hyperjuice-external-battery-for-mac...
Seems like a valid option if I decide to stay with MBAs after February.
Update, Amazon reviews don't seem to agree:
So although it won't charge my MBA battery it looks like it will power it for over 3 hours (three stars)
i believed the 20 hour run time they posted. right. sure. i bought it so i could use my macbook at the cabin (no electricity). what i got was, at best, 3 1/2 hours of power from it (one star)
If I use the HyperMac with just the MacBook Pro 13 inch I get probably about 2 hours of use out of this external battery. (five stars)