Cost per watt (without incentives): $3.95
Cost per watt (after federal and state incentives): $1.82
Cost per watt after all incentives received to date: $1.56
Note: The payback was achieved quicker than expected as utility energy costs rose in the latter years. Payback was probably achieved after about 12 years, far longer than most systems installed today.
The early generation of micro-inverter I used (m-190 for the first array) suffered some quality control issues that resulted in premature failure, and extended periods of non-production for some PV modules while waiting for the arrival of replacement inverters. Fortunately, enphase has honored the warranties on the equipment, but I have probably lost significant production as a result of these equipment failures. These kind of technology issues can be one of the challenges of being an early adopter; newer technology is more reliable.
I have not experienced any malfunctions so far with the newer enphase M-215 micro-inverters used in the second array, and they deliver more power, too.
Given that the system should last at least 25 years, I believe that we'll still have a good return on our investment.
For comparison, materials for a similarly sized system (5.7 kw) can be bought in 2017 for under $10,000 BEFORE incentives and SRECs.
Summary of Project Costs (Estimates as of 2018)