Tesla Inc. has another financial worry: Its home solar-panel business is facing slowing installations and could be on the hook for financial promises it made to some investors, just as the U.S. tax law presents new risks to the industry.

SolarCity, which the electric-car maker bought in 2016, is undergoing a sweeping sales revamp that reduced the number of panels the company installed last year. Now, investors and analysts say they are scrutinizing guarantees on investment returns and other promises the company made to firms...