(i.e. I have a 1.5 kW system, and I want to add 1.5 kW more.) Similarly, can I apply this credit to used equipment going into a new installation?
This is not entirely clear at present. However, the language would suggest that both scenarios are allowed – the credits apply to the amount of expenditure on solar energy property in a given year. SEIA will work with the IRS to develop regulations favorable to the solar industry. We will pass on additional information as it becomes available.

source: FLASEIA (Florida Solar Energy Industries Association)