Solar Calibration Calculations

Calibration can be used when the available cloud cover does not line up with the solar resource or to adjust the engineering results for non-fixed panel installations.

The regression equation used to calibrate an interval of the day (i) is specified as follows:

The calibration algorithm works by estimating the above equation and then “throwing out” observations with negative residuals (ε_d<0). This serves as a mechanism for automatically eliminating cloudy days from the regression dataset. This process iterates until the specified tolerance of negative residuals is reached, e.g., if 10.0 is entered the process will continue until fewer than 10% of the residuals in the calibration equation are negative.

When forecasting a calibrated Solar Generation model, the following adjustment is made to the calculated cloud adjusted PV generation using the clear sky engineering equation and the predicted values from the interval (i) estimated calibration equations.

To calibrate a fixed panel when good cloud cover information is not available, it is recommended to calibrate to a single interval that is as close as possible to solar noon for the location. This is accomplished using the following parameters:

  • First Interval. The interval to calibrate, e.g. 12:30

  • Last Interval. The last interval should be null for a single interval calibration.

  • Tolerance. The percentage of residuals that are negative at the completion of the process.

By specifying a range of intervals, the calibration equation can be used to adjust the engineering for non-fixed panel. For each interval in the range specified by First and Last Interval, a calibration equation is estimated. When forecasting intervals outside of the calibration range, the nearest interval is used in the adjustment. For example, if the calibration is computed over the range 9:00 to 16:00. If the predicted value of the ClearSkyPV for the forecast interval is greater than 0, the 9:00 calibration adjustment will be applied for intervals earlier than 9:00. For intervals later than 16:00, the 16:00 calibration adjustment will be applied. If the ClearSkyPV potential is computed as 0, then the forecast will be 0.