Solarcity does not lose money. They finance each install for a 20 year monthly revenue stream."What will it take for Solar City to stop net losing money?"
SolarCity will be profitable once they slow new lease/PPA installs such that the cost of these new installs is less than the margin generated by (1) upfront system sales and (2) previously installed systems.
In addition, Solarcity has a 20 year relationship for upsale for future products such as powerwall, Solarcity management service of customer sided energy sales, and now tesla automobiles and assorted tesla products that may come down the line in the next twenty years.
That approximately 300k customers as of the signing of the deal. Solarcity has a goal of achieving 1 million customers by beginning of 2019 so that is a massive boon for tesla long term product sales/marketing/advertising.