Nissan's grand designs for zero-emission cities of the future may be some way off, but you gotta start somewhere. And that somewhere starts with "xStorage," the automaker's home energy hoarding system announced today. Like Tesla's Powerwall and Mercedes' residential battery, the idea is to save you money in the long run by charging the thing up when off-peak electricity rates are in effect. You can then use the cache to power your home when energy is more expensive -- switching between sources by way of smartphone app -- or sell it back to the grid during these times of higher demand. And if you can juice the xStorage unit up using a renewable source like solar, then all the greener.