What is the price of a residential battery? Let's say you need 10 kilowatt hours to last from sunset to dawn (you might need more some nights, but if you're still on the grid, 10 kwh might be enough most of the time, which is what matters).
The Tesla model S has a 60 or 85 kwh battery. I don't know what the cost of the battery alone is, but it's probably pretty expensive. For fixed residential, you wouldn't need to use the most energy dense cells available, so it might be cheaper.
Wikipedia has a price per kwh comparison, showing lead acid considerably cheaper, though that might not be up-to-date with the price Solar City and Tesla pay for Lithium Ion cells.
If lead-acid is $250 per kilowatt hour, that puts a 10 kwh battery at $2500. Not too bad, but if they only last six years or so, that's a pretty big part of the ongoing cost of maintaining the system. How does lithium ion compare on price and longevity?