A year after Elon Musk unveiled the Powerwall at Tesla Motors Inc.’s design studio near Los Angeles, the first wave of residential installations has started in the U.S. The 6.4-kilowatt-hour unit stores electricity from home solar systems and provides backup in the case of a conventional outage. Weighing 214 pounds and standing about 4-feet tall, it retails for around $3,000. But hookup by a trained electrician is required, as is something called a bi-directional inverter that converts direct-current electricity into the kind used by dishwashers and refrigerators. The costs add up quickly .
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