USB to MagSafe 2 Adapter
Apple's MBP power cable supplies a maximum of 60W of power in order to both use the machine and recharge the battery at the same time.

If you're not using the MBP, then it's possible that 11W is enough to charge the battery, albeit very slowly, as you indicate. 

It's unlikely that you would be able to use the machine with 0% battery on 11W, and even if you could, then the battery would barely recharge at all.

So, this might well work for the sort of emergency that required you to leave your MBP closed and plugged in for several hours until you could use it a bit.

Also: you will only draw the full power over USB C connector. USB 3.0 A connectors can only draw 0.9 A.