Michael Safi:

Wall-mounted, with a sleek design, the lithium-ion batteries are designed to capture and store up to 10kWh of energy from wind or solar panel. The reserves can be drawn on when sunlight is low, during grid outages, or at peak demand times, when electricity costs are highest.
 The smallest “Powerwall” is 1.3m by 68cm, small enough to be hung inside a garage on or an outside wall. Up to eight batteries can be “stacked” in a home, Musk said, to applause from investors and journalists at the much-anticipated event.
 The batteries will initially be manufactured at the electric car company’s factory in California, but will move production to its planned “gigafactory” in Nevada when it opens in 2017.
 The Nevada facility will be the largest producer of lithium-ion batteries in the world, and it is hoped its mass-production scale will help to bring down costs even further.