All Musk said was that they would be competitive with comparable roofing tiles, once the value of the electricity they generate is taken into account. That left the question pretty open, since the value of that electricity itself will vary widely from place to place.
This week, Musk made a more boastful claim.
At a shareholder meeting (in which shareholders approved the Tesla/SolarCity merger with an 85 percent vote), Musk said he had just come from a discussion with Tesla solar engineers.
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"It's looking quite promising that a solar roof actually costs less than normal roof," he said, "before you even take the value of electricity into account."
Tesla is reducing costs, he says, by squeezing inefficiencies out of the "incredibly inefficient" roofing supply chain. Tesla's tiles weigh less than conventional tiles and are made of tempered glass, so they are much more resistant to damage and can be packed more tightly. Those savings offset the slightly higher upfront costs of materials and manufacturing.