USB to MagSafe 2 Adapter In case of an emergency, I'd like to use my solar powerbank to charge my Macbook 2015 13". It outputs 2.1A 5V, and I learned that the MBP can be charged with that, but I don't know if that's actually true (is it?). It would take about 12 hours to fully charge it I think, which is okay

Now next problem is finding a USB to MagSafe 2 Adapter. Any ideas?

What if I use USB to USB-C, and USB-C to Magsafe 2 adapters? :) Those are readily available on the market.

Edit. I've found this which simplifies things.

Do you think this is safe? (charging mbp with 11W)
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.