Edit: This will replace the need for peaker plants first (generators of last resort, very expensive, only run a handful of hours a year), and as the cost drops, will slowly push out base load coal and natural gas (by increasing the capacity factor of solar and wind). "Batteries are the new peaker plants", as it were. [1] [2] [3]
This is what it looks like when batteries are used to offset fossil fuel generators (instead of curtailing excess wind and solar, it'll be soaked up by utility batteries such as these). Frequently, depending on renewables output, the spot of price of power can go negative. This means someone gets paid to use that power. This is where utility scale battery storage shines, as its happy to gobble up that power, being paid to do so, and can later be paid to release that power when demand is high.
Edit 2: If Tesla can book this revenue in Q3, combined with their vehicle sales push, they're going to be GAAP profitable for the quarter, which will allow them to close the Solar City acquisition. Well played.
[1] https://en.wikipedia.org/wiki/Peaking_power_plant
[2] http://www.greentechmedia.com/articles/read/dueling-charts-o...
[3] http://www.bloomberg.com/news/articles/2015-12-22/batteries-...