Depends on the contract, solar city contract is below:, which would answer a lot of the questions asked. End of contract, buy it outright, have them remove it, or new contract and new stuff.
PDF document.
http://www.solarcity.com/downloads/S...act_sample.pdf
They promise a certain output, so if it fails to deliver over the 20 year period, they will fix/replace as needed to keep it running as designed 20 years previous.