Comment by ZeroGravitas

X hour batteries is mostly because battery people think in MWh and grid people talk in MW (and confusingly both call that capacity.)

The hours designation is the MWh / MW i.e. how fast you can empty the battery into the grid at full throttle.

The economics comes in because batteries are expensive and you want to target the highest and lowest prices during the day, charging and discharging totally to maximize revenue via price abitrage. A 1 hour battery is going to be shaving the very peak prices of each day soaking up solar at noon and displacing expensive gas peaker plants in the evening.

As the battery MWh gets bigger, but not the ability to put it all on the grid, that implies you'll be charging and discharging for 4 or 8 hours each day which means you'll be paying and getting paid closer to the average prices in both directions.

It only works economically if the battery is itself cheaper (and/or there's more renewables pushing down prices for longer periods of the day)