I am about to buy a power supply for my new Raspberry Pi Model B+, since I am getting the low power indicator.
I also saw this model: DELTACO power adapter from 230V to 5V USB, 2,1A But it looks quite small. I wonder if the 2,1A are nominal or Max. And if it will reach 2,1 and still be able to maintain 5V.
Unless you can find a manufactures data sheet, (if exists), then you will just have to try it and see.
Also a good quality USB cable is important. If too thin and/or too long, the the voltage will drop too much even with a good power supply. I am using a 5V 10A supply and still got the power warning until I made my own USB power cable.
Unless specified otherwise my response is based on the latest and fully updated Raspbian Buster w/ Desktop OS.