Dear PhilippS,
I can describe one approach to modeling inverter replacements in SAM, but I would not call it an "industry accepted" way to do it. An inverter replacement has two potential effects on the performance and financial modeling: The new inverters may be more efficient than the original inverters, and they may cost less per installed capacity. Although SAM does not explicitly model an inverter replacement, you can approximate those effects as follows:
1. On the
Performance Adjustment page
use an annual schedule to specify the "Percent of annual output" adjustment factor, and assign a value greater than 100% for the year of inverter replacement and later. (If you are using the performance adjustment factor to represent a separate system loss, you would just increase that value by the expected increase in inverter efficency, for example, a 98% availability factor with a 2% increase in inverter efficiency would change the value from 98% to 100% in the replacement year.) That will increase the system's total annual output to represent the new inverter's higher efficiency. (See the section under "Annual Schedules" for instructions.)
UPDATE (APRIL 2015): In SAM 2014.11.24 and later, you use the degradation rate on the Degradation page instead of the percent of annual output adjustment factor. If you expect the new inverter to result in a 2% increase in the system's annual output, assign a negative degradation rate to the inverter replacement year and each subsequent year.
2. On the
System Costs page
, use an annual schedule for one of the O&M cost categories to specify the inverter replacement cost in the replacemement year. You can choose whether to specify the cost as a fixed dollar amount, or as a $/kWdc of array capacity, or $/kWhac of system output. (See the section under "Specifying O&M Costs in Specific Years" for instructions.) In an actual system, you might expect some degradation of the array's DC output over the years, which, when combined with the possible better efficiency of the new inverter(s), would mean that you might be able to install a less inverter capacity in the replacement year than was needed at the beginning of the project. SAM does not model that detail, but you can approximate its effect on the project cash flow using the technique I describe above.
Best regards,
Paul.
