Over a short timeframe storage can be a sink or a source of energy. The grid needs to be managed both over short and long timeframes - you can't just say "100MW

Over a short timeframe storage can be a sink or a source of energy. The grid needs to be managed both over short and long timeframes - you can't just say "100MWh in = 100MWh out over the last 24h", it also needs to be balanced so that the input and output is constant on a per-millisecond basis.