Calculations

Site Energy

Depending on the available tags, site energy is calculated using one of the following methods:

Using the daily energy tag: The daily energy tag contains the value 0 at the beginning of a day and accumulates energy data every millisecond. After a day ends, the value is reset to 0.
Energy produced in a day is the last value of the daily energy tag on that day.
Using the lifetime energy accumulation tag: The lifetime energy accumulation tag begins with the value 0 around site COD and accumulates data as long as the tag exists. It is not reset to 0 every day. The energy data is accumulated every millisecond.
Energy produced in a day is the difference between the last values of the lifetime energy accumulation tag of the current day and the previous day.
Using the AC Power tag: The AC Power tag collects data every five minutes from the site.
Energy produced in an hour is the sum of the individual AC Power values in the hour divided by 12.
Note: This method is not preferred because it lacks granularity of changing production.

This topic describes how the capacity factor (CF) and yield values of a site are calculated per day.

This topic describes how the corrected Performance Ratio (PR) is calculated.

where:

PR is the performance ratio (no units)
PR_corr is the corrected performance ratio (no units)
EN_AC is the measured AC power generation in kW
P_STC is the sum of the power rating of installed modules (from flash test data) in kW
G_POA is the measured plan of array (POA) irradiance in kW/m²
i is a given point in time
G_STC is the irradiance at standard test conditions (STC) in 1,000 W/m²
T_cell is the cell temperature computed from measured meteorological data is °C (refer to the following calculation)
T_{cell_typ_avg} is the average cell temperature in C computed from one year of weather data using the project weather file. It is calculated as an average module temperature reported by the site in the last 30 days from the day of calculation.
δ is the temperature coefficient for power in percentage per °C (negative in sign) that corresponds to the installed modules

where:

T_cell is the predicted operating cell temperature in °C
T_m is the predicted module surface temperature in °C as determined by equation 3
G_POA is the POA irradiance in W/m²
G_STC is the reference irradiation for the correlation, which is constant at 1,000 W/m²

ΔTcnd is the conduction temperature drop as described in the following table

Table 1. Empirical Convective Heat Transfer Coefficients
Module Type	Mount	a	b	ΔTcnd (°C)
Glass/cell/glass	Open rack	-3.47	-0.0594	3
Glass/cell/glass	Close-roof mount	-2.98	-0.0471	1
Glass/cell/polymer sheet	Open rack	-3.56	-0.0750	3
Glass/cell/polymer sheet	Insulated back	-2.81	-0.0455	0
Polymer/thin-film/steel	Open rack	-3.58	-0.1130	3