And there are other promising technologies being developed.

Aquion is interesting because their batteries would be easily for retail side storage. Solar reaches grid parity much sooner at the retail side than at the wholesale side. We could see people installing home electricity storage units, buying cheap wind off-peak and using it to replace more expensive peak hour power.

5,000 cycles, one cycle per day, yields 13+ years life.
Additionally their batteries show no self discharge or problems in high heat conditions. They are smaller and lighter than lead acid, easier/cheaper to ship. And 100% recyclable.

Aquion should be in commercial production in 2013, they are now completing their factory.

If what Aquion is reporting turns out to be true then the standard for grid storage is going to be set fairly high.

It's likely that Ambri's liquid metal battery is going to provide even better performance per dollar.

So with 50% upto 70% (if free heat is close by) needs to be a good bit cheaper than batteries. This look a lot like the March 2011 post, but with less data.
Yet another innovative way to store renewable energy is being tested in the UK by a cryogenic company, Highview Power Storage, whose liquid air energy storage could be up to 70% efficient and cost just $1,000 per kilowatt.
Clean Technica (http://s.tt/12tMh)
In that post “Next, Highview plans to build a 3.5 MW, commercial-scale system by late 2012, building it up into an 8 to 10 megawatt storage plant by early 2014″And checking their web site, you see they have already build some. A quick look I didn’t see their current cost structure.