Let's be really clear about what these batteries do -- they store trivially small amounts of power to smooth out short spikes, until slow-ramping generators can catch up. These AES units, nor any other batteries in use in the grid today, are designed to shift bulk quantities of MWh from night to day. There are technologies to do this: chief among them, pumped storage (which uses two reservoirs, pumping water up when there's cheap power and generating like a hydro facility when power is more valuable). The scale needed to move meaningful amounts of power between night and day is really vast: 20,000 Model S 300-mile batteries, fully charged, could provide about 2 minutes of the power consumed in California on a peak summer hour. They could also operate at 170 MW of output for 10 solid hours. The batteries cost $510 million (at a conservative $300/kWh), while the 170 MW peaking gas-fired unit costs about $150 million. Operation costs will be similar between the two units, as the batteries will be charged by electricity priced by the natural-gas-fired units that are marginal in California overnight.
So, until battery costs drop, or renewables completely displace gas overnight (and so drop the overnight price to close to zero), batteries are a horrible way to store bulk power.
The AES project could create value because NY has ambitions to install jaw-dropping amounts of wind, and overnight the grid has many fewer resources that can freely ramp up and down to balance large fluctuations in wind output.