That right there is my biggest problem with SolarCity. They are using solar efficiency as a hedge. You're not buying free power, you're buying a contract for a particular amount of power at a guaranteed cost for the term. It's a much better deal if you can finance your own solar power installation, but people get sucked into the no-money-down sales spiel that SolarCity has. The contract is transferred with the property and can be difficult to price into the property value since that value, based on the remaining portion of the contract term, diminishes as time passes. To say nothing of the difficult of getting buyer attention for a property that seems to be priced high for the market.
"So the house costs more because I'd be buying a solar system?" "No, it costs more because there are 18 years left on a contract for solar power from SolarCity, who have installed their equipment on the house." "So I wouldn't *own* the solar panels if I bought the house?" "No, it's more like you're buying an option to short electricity prices backed by the advantage of SolarCity's equipment, so it will almost certainly be a better deal for you over the remaining term of the contract."
You have to dig around on SolarCity's site to even find info about the option to purchase. The one thing I think they do right - and anyone purchasing their own solar system would be well advised to copy - is to plan in the cost of long term maintenance and repair when financing a system.