How long can an energy storage module keep on powering the grid at its rated output?
The answer to this question is the Energy to Power Ratio:
E/P = MWh/ MW = hours (or minutes or seconds)
A funny thing is that it is such a basic question for all energy storage devices and technologies, but, strangely, it is rarely easily available information.
Most news releases will mention the nameplate capacity or power rating (in MW), without mentioning the energy capacity (in MWh) or the duration over which the power rating can be provided (in seconds, minutes or hours).
If you want to know as much as possible about a storage device in two numbers only, the best way to do it is to ask for its rated output and its energy to power ratio.
I wrote a more detailed discussion of the energy to power ratio here.