Depends on who you ask. For the grid operators since they balance instantaneous supply and demand they only care about power available. Expecting the producers

Depends on who you ask. For the grid operators since they balance instantaneous supply and demand they only care about power available. Expecting the producers to maximize their profit and hence save the batteries until they are most needed. When explicitly designing an electrical system, say an off-grid cabin, both the energy stored and instantaneous matching of supply needs to pencil out. In the large scale grid example we expect the market to handle the energy balance. Adding transmission links, secondary markets and so on when the market does not adequately match demand. Generally speaking today batteries used for grid scale storage store 4 GWh per 1 GW. This number comes from maximizing the utilization of the grid connection and matching the hours per day with highest electricity rates.