If you have to ask, you can't afford them.
With SolarCity you don't buy the system, you just choose to use them as a service provider and pay them per kw/hr for what they generate. They own the panels and all the other hardware, installation, and maintenance. The upside is you aren't responsible for doing anything other than paying your monthly generation bill. There are two major downsides.
It's a 20-year contractual commitment. If you sign up and then sell your house, you're still on the hook. The best case scenario is the new owner could elect to take over the contract. Alternatively, you could pay them to move the system to your new digs. If none of that works, you have to buy out the remainder of the contract.
Also note you're paying them per kw/hr for what's generated, not for what's used. In theory you size the system so that you sell excess generation during the summer to your standard power generation provider, but the fine print is they typically only buy power back at the lowest-tier rate.