In 2016, Tesla Motors acquired SolarCity for $2.6B of TSLA common stock. This deal led to increased vertical integration for the electric vehicle and lithium battery manufacturer. After the deal was finalized, the parent company was renamed to Tesla, Inc.
Terms of the Acquisition
Due to the structure of the deal, along with extensive mutual ownership, the deal has often been considered a merger. Tesla offered $2.6B of common stock for control of SolarCity.  Shareholders of SolarCity voted after 14 September 2016 to determine whether or not to finalize the deal.
The Federal Trade Commission approved the merger, stating that it did not violate Anti-Trust Laws. Due to fluctuations in the underlying assets' value, Tesla renegotiated their offer, reducing the bid by roughly 10%, subject to negotiation. [✔]
Tesla, (formerly Tesla Motors), made the offer in an effort to create the energy needed for their Model S, Model 3 and Model X as well as the company's PowerWall. [✔] In order to support the rapidly expanding Tesla Supercharger network, solar and other renewable energy sources need to be harnessed. Elon Musk claimed there would be a reduced overhead that would result from merging the two companies. Additionally, Elon Musk planed to bundle the sale of solar panels with Tesla vehicles. [✔]
The deal is considered a merger because Tesla bought out the company solely with shares of Common Stock (TSLA); SolarCity (SCTY) Stock converted at a 9:1 ratio, meaning for every 9 SCTY shares held, 1 TSLA share was be granted after the acquisition.  Since ~85% of the shareholders of each company are shareholders in both companies, the acquisition was expected to be overwhelmingly approved on both ends. 
Closing the Deal
The deal finalized on 21 November 2016, with ~85% of Solar City in favor of the deal. Management had argued that it would reduce the corporate overhead while rebranding SolarCity products. Musk believes that their solar tiles will be cheaper than conventional roofing systems.