This was offered to SolarCity customers in late 2014, I see that the image here http://www.solarcity.com/residential/backup-power-supply now reflects Tesla branding.
Iinteresting how they are branding it as a backup solution, while Tesla is branding it as a complete replacement. "What if we could move the electricity grid off of fossil fuels" (Tesla) vs "Be prepared the next time the power goes out." (Solar City)
10 kWh $3,500 For backup applications
7 kWh $3,000 For daily cycle applications"
From http://www.teslamotors.com/powerwall
Maybe Solar City has been using the ones meant for daily cycle - that would make sense.