Hi Corey,
In the attached .sam file, I set up a parametric study on DC/AC ratio for a PVWatts case similar to yours (but using the weather file for Des Moines that comes with SAM). To see it, click
Parametrics under the Simulate button. The following plot of the results, which I exported from the Parametrics page to Excel to create the graph, shows that as you increase the DC/AC ratio from 1 to 1.5, the total annual output increases slightly up to a point and then decreases sharply.
PVWatts uses the efficiency curve below (Figure 4 from the
PVWatts Technical Manual
) to model the inverter so that the inverter is slightly more efficient at higher load fractions.
As you increase DC/AC ratio for a given array capacity, the inverter size decreases, which causes the inverter to operate more often at a higher load fraction, causing a slight increase in the total annual output of the system. However, beyond a certain point, decreasing the size of the inverter causes more of the PV array power to be "shed" by the inverter causing the decrease in annual output.
Best regards,
Paul.