The efficiency of an inverter, which determines how much of the DC power generated by a solar array is converted to AC power, is generally not a fixed value. Instead, this parameter varies with input DC power and voltage, and the amount of variation is specific to the inverter. The California Energy Commission (CEC) maintains a database of experimental test results on a number of inverters, expressing the efficiency as a function of DC power for three different voltages within each inverter’s operating voltage window. An example of the CEC data for a string inverter is shown below.
Instead of applying a fixed, weighted efficiency to calculate the DC/AC conversion losses, Aurora’s performance simulation engine models the full inverter efficiency curve for inverters that have CEC test data. Aurora fits a parabolic curve to the power- and voltage-dependent efficiency data, following a model developed by Sandia National Labs. An inverter in Aurora's component database that has CEC data will automatically use an efficiency curve in the simulation. If an inverter in Aurora's database does not have the CEC data and therefore cannot simulate an efficiency curve, the simulation will use a fixed efficiency (the smallest of the CEC, European, and maximum efficiencies from the datasheet). Custom inverters will also simulate with the lowest fixed efficiency value specified in the datasheet. A log message will be included in the performance simulation report detailing whether an efficiency curve model is applied or which fixed efficiency is used if the efficiency curve is not available. If no efficiency data is available for the component, Aurora will raise an error.
For inverters with more than one maximum power point tracker (MPPT), each MPPT effectively operates with its own efficiency. An MPPT with shorter strings or less input power may operate at a lower efficiency than another MPPT in the same inverter that has a larger input voltage and/or power.
Impact on Energy Production
Inverter efficiency tends to be lower when the DC input power is low, relative to the nameplate rating of the inverter. As such, watch out for the following:
- Designs that are under-sized (in input voltage, but especially in input power) for the chosen inverter will tend to experience a lower overall DC/AC conversion efficiency than those that are properly-sized for the inverter. Aurora’s string sizing validation checks can help ensure the designs you create are compatible with the inverter you’ve chosen.
- Hours in which the array output power is low, perhaps due to shading or very early/late in the day, will also tend to experience a lower inverter efficiency than hours in which the array is operating under full irradiance with no shading. This is expected behavior, but since input power is low, this will usually not make a big difference in the performance of the system.