The production of wind and solar energy for the consumer is quickly approaching (or, depending on your tax regime, has already passed) the cost of traditional energy from the grid. This is great, and we're starting to see wind and solar become major forms of energy production. The problem with both forms of energy is that the energy generally is not generated when it is consumed, so it must be stored.
The problem with storing electricity is it is expensive. The cost of this lithium-based solution finally is cost-competitive with lead-acid. Basically, when the Tesla's Gigafactory is completed (their battery factory, being built near Reno, and expected to be finished in 2017) means that Tesla will be the lowest-cost provider of lithium batteries. Unfortunately, the size of the factory also means that they will have enormous excess capacity. Unless that capacity is utilized, their costs will eat away at their revenues.
This is their answer to this situation: they have excess battery capacity, and costs that are about 1/2 that of their nearest competitor, and there is a growing need for energy storage.
The real target of this announcement, if you ask me, is ERCOT, the regulatory body governing the Texas power grid. In general, Texas has enough power generation capacity for average load days. Unfortunately, they are running out of power plants for peak power days. Traditionally, these were natural gas-fired generators, while "normal" load was handled by hydroelectric, coal, and nuclear (because natural gas used to be more expensive than the others, and also because natural gas generators can be turned on and off over the course of minutes instead of hours or days, like the others). However, as natural gas prices have dropped, most of these power plants that used to be used for reserve power have been reallocated towards baseline power--this is why Texas has enough baseline power generation capacity, but not enough peak power capacity. ERCOT released a study late last year that indicated that it would be cost effective for ERCOT to deploy regional battery storage devices instead of building new power plants to handle these peak power periods. According to financial estimates, ERCOT's power demands will result in >$4 billion/year in revenue for Tesla. Basically, it would appear that Tesla might end up making more revenue selling these solutions to power companies than they can selling cars to consumers.
All of this will lead to economies of scale, further driving down Tesla's costs and entrenching their position as the lowest-cost provider of battery storage. All of this also means that this battery becomes an attractive device for home use, as well. Basically, this is what a lot of people are thinking about when they talk about a "smart grid" that can push power back on the grid during peak demand times.
Here's a couple of articles about the ERCOT annoucement:
http://www.texastribune.org/2014/12/15/state-law-could-short...
http://www.eenews.net/stories/1060008712