I don't think you can count on just one letting you forget about your wall charger unless you only tend to need to recharge your BB every other day; you may need 2 or 3 or more so you when you drain one, you can do another recharge with the next if it's before the first gets enough sunlight to get recharged.

And keep an eye on the spec - a BB is happiest with 5V at 700-800 mA - the battery inside the solar gizmo has to be able to deliver that.

The brand that's easiest to find to buy is probably Solio, so start by looking into theirs.

(Solio - if you see this - you owe one to Max - and I should probably remind you of your promise to get me one to review).