Most of the solar cells made at Tesla’s solar factory in Buffalo, New York, are being exported instead of being installed on U.S. homes as originally planned, as Tesla continues to struggle on the American solar market, Reuters reported on Wednesday, citing sources at the factory and documents it has reviewed. Tesla entered the solar power business with the acquisition of SolarCity in 2016, sparking concern among investors and industry observers that it was stretching itself too thin.    Tesla said in late 2016 that it had finalized…