Hello,
The inverter efficiency input in PVWatts is a nominal value, which is the inverter's nominal AC power divided by the nominal DC power. That is what you would expect the inverter's DC-to-AC conversion efficiency to be when the output power is the same as the inverter's rated power. For example, if a manufacturer claims that the nominal conversion efficiency of a 5 kW inverter is 96%, you would expect the efficiency to by 96% when the output power is 5 kW.
There are a number of factors that affect the actual conversion efficiency of an inverter at any given moment. That means that the actual conversion efficiency is going to change throughout the day. Inverters are less efficient at lower power levels than at higher power levels, so on a clear day, a properly sized inverter will be most efficient in the middle of the day when the PV array output is at its highest. Inverters are also less efficient at higher temperatures than lower temperatures. Most inverter data sheets show an efficiency curve that shows the relationship between the conversion efficiency and the inverter's output power and temperature.
By "nominal," I mean the rated values that appear on the data sheet provided by the inverter's manufacturer. Because there is not a standard for the information that appears on the data sheets, different manufacturers provide different information, but they usually provide enough information to determine the invereter's nominal efficiency. If the nominal efficiency (or nominal AC and DC power) does not appear on the data sheet, you can use the efficiency curve to determine the nominal efficiency.
During simulations, PVWatts uses a quadratic equation to calculate the inverter efficiency. The equation was empirically derived from inverter performance data measured from thousands of inverters in the California Energy Commission database. For details, see Chapter 12 of the PVWatts Reference Manual, which you can download from the
Performance Model Documentation
page of the SAM website.
Best regards,
Paul.
