Tesla Motors Inc has won US antitrust approval to buy solar panel installer SolarCity Corp, moving closer to its goal of creating a carbon-free energy and transportation company.

The Federal Trade Commission said on Thursday that the deal was approved. It was on a list of proposed transactions that were granted quickly because the merging partners have few or no overlaps.

Tesla said in July after it made its first offer that by acquiring SolarCity the two companies would form a one-stop shop for clean energy, offering consumers solar panels, home battery storage and electric cars under a single brand. The $2.6 billion deal was announced on August 1.

Tesla shares closed down 0.7 per cent at $220.96 and SolarCity shares ended 0.6 per cent lower at $22.36 on Thursday.

SolarCity has come under pressure from rivals offering low-cost solar energy through large, utility-scale installations, and because some state governments have reined in subsidies that encouraged rooftop solar.

A SolarCity spokeswoman said the company hopes to close the deal by the end of the year.

Tesla has yet to file a merger-related form to be reviewed by the US Securities and Exchange Commission, after which it will set a date for shareholders to vote. - Reuters