The rational is, a grid is run by "main power", which is divided into three layers:
- base load
- load following
- balancing power
The batteries are used for balancing power, because the react much much faster than a load following plant.
Then, behind the "main power", you have "reserve power".
And power means: power not energy.
Reserve power is also divided into three layers:
- seconds reserve, "spins up in few seconds" and only needs to last seconds, usually 90 - perhaps more
- minute reserve, "spins up in minutes", like above, only lasts several minutes, but can be indefinitely in case of a gas turbine for example
- hour reserve, "spins up in hours", lasts hours, or indefinitely in case for example a coal plant
So, you either use the battery for balancing power or in your case for "seconds reserve" power. Bonus point of the battery, same like pumped storage: it balances in both ways.
In other words, if the grid frequency drops because an idiot pushed the button on his aluminium smeltery, without consulting with the power company/grid operator, the balancing power is by far not enough, and the load following power can't react fast enough. So my "seconds reserve" has to buffer the problem until minutes reserve takes over which buffers the problem until "load following" is ramped up to fix it - or hour reserve has to ramp up, if the load following plants are at max capacity.
The only thing interesting in this scenario is the POWER of every short term plant involved because it is a ladder of plants that jump in, and replace the ones that are running out of energy.
In other words, you might have a puny 1MW battery, that only stores 0.2MWh, but I know I can use it for 2 or 5 minutes ... and cover the start up time of my gas turbine.