Nope, the main issue is indeed the fact that it can't store a whole lot of energy (also important that they can't maintain a high output for too long), but we s

Nope, the main issue is indeed the fact that it can't store a whole lot of energy (also important that they can't maintain a high output for too long), but we still talk in MW as that is what we're primarily concerned with (MW flow on a line going over the line's rating...not having enough generation (MW) to meet load (MW)...etc). Saying a battery can't store enough MW isn't scientifically accurate, but it is the slang of the industry if you will. If you worked in the water industry they're probably very concerned with water flow rates. In high voltage transmission we're concerned with the flow of power (MW (real power) & MVAR (reactive power))