You can read the prospectuses here:
https://solarbonds.solarcity.com/available-bonds/This strikes me as a horrendous investment (as most bonds pitched directly to retail investors are).
You'd be lending to a company in an industry notorious for frequent bankruptcies, that makes no profit, for 2-4% maximum return. Not only that, you can't sell these bonds and must hold them to maturity, making liquidity as bad as possible. Seriously, why? These bonds are sold to retail investors because institutional investors would laugh these terms out of town.
Much more attractive is buying the SCTY convertible bonds which yield 3.15% and have an embedded call option, so you can participate in the upside scenario for the company. Or just buying the equity outright if you're a true believer in the company.