Generic 20 000mah power bank a ****

I bought a 20 000mah power bank from Dischem Kimberley for R499 but it does not nearly have a 20 000mah capacity as indicated by the product.<br> <br> I am wary of buying ****s from the street but getting ripped of by a chain store is unacceptable. <br> <br> I either want a power bank with a true 20 000mah or I'd like my money back.<br> <br> Don't have the receipt but you do.