Tesla Motors on Monday announced that it has reached an agreement to purchase SolarCity. The deal could create the world's first vertically integrated sustainable energy company. The merger would drive efforts to develop rooftop solar systems, and better storage batteries for homes and businesses, as well as electric vehicles. It would, in essence, tie together Elon Musk's sustainable energy businesses. The purchase of SolarCity would be an all-stock transaction with an equity value of $2.6 billion, according to Tesla.