Tesla's goal is to make affordable electric-solar vehicles that everyone can afford - - eventually. Any technology that is new, the prices are high, until they are driven down with higher sale volume and lower prices. Solar-electric is a sustainable alternative to hydro carbon, and possible that a driver can put more energy back in the system that used in transportation.
After hearing about the new advancements in Tesla's technologies, I am a believer that only more great things are to come. How amazing would that be to have more technology that is environmentally friendly. If only we could have such technologies duplicated at a smaller cost so that more people had the opportunity to have access to them. But yes, Tesla has done exceptionally well so far and I believe they have the capability to continue.
For Tesla, the purchase of SolarCity provides the promise of greater economies of scale in electrical energy management systems, battery production and marketing. Although it faces the challenge of managing a high-risk ramp-up of vehicle production and a merger simultaneously, it probably Elon Musks best shot to create a carbon free energy company.
For Tesla, benefiting from SolarCity provides them with the guarantee of more noticeably substantial economies of scale in energy source administration technologies, electric battery manufacturing as well as advertising. They also have the short-term constraint of going on to manage a higher than average risk intensification of automobile manufacturing in addition to a merger at the same time.