PERF_RATIO_NREL

This function computes the NREL weather-corrected Performance Ratio (PR) of a PV system.

It aggregates incident irradiance, ambient temperature, wind speed, and AC power over the provided data ranges. The calculation uses the SAPM temperature model and a weighted reference temperature derived from the data to weather-correct the PR, allowing for more stable long-term performance tracking.

Excel Usage

=PERF_RATIO_NREL(poa_global, temp_air, wind_speed, pac, pdc_ref, a, b, delta_t, gamma_pdc)

poa_global (list[list], required): Total incident irradiance (W/m^2).
temp_air (list[list], required): Ambient dry bulb temperature (C).
wind_speed (list[list], required): Wind speed at 10 meters height (m/s).
pac (list[list], required): AC power output (kW).
pdc_ref (float, required): PV array STC nameplate rating (kW).
a (float, optional, default: -3.56): SAPM temperature model parameter a (unitless).
b (float, optional, default: -0.075): SAPM temperature model parameter b (s/m).
delta_t (float, optional, default: 3): SAPM temperature model parameter deltaT (C).
gamma_pdc (float, optional, default: -0.00433): Power temperature coefficient (1/C).

Returns (float): Weather-corrected Performance Ratio (float), or an error string.

Example 1: Calculate PR with nominal arrays

Inputs:

poa_global	temp_air	wind_speed	pac	pdc_ref
1000	25	2	9.5	10
800	20	1	7.6
900	22	3	8.5

Excel formula:

=PERF_RATIO_NREL({1000;800;900}, {25;20;22}, {2;1;3}, {9.5;7.6;8.5}, 10)

Expected output:

0.948148

Example 2: Calculate PR with custom SAPM and temperature coefficient parameters

Inputs:

poa_global	temp_air	wind_speed	pac	pdc_ref	a	b	delta_t	gamma_pdc
950	30	1.5	8.9	10	-3.7	-0.08	2.5	-0.004
850	28	2.5	8
780	24	3	7.1

Excel formula:

=PERF_RATIO_NREL({950;850;780}, {30;28;24}, {1.5;2.5;3}, {8.9;8;7.1}, 10, -3.7, -0.08, 2.5, -0.004)

Expected output:

0.930233

Example 3: Handle scalar weather and power inputs

Inputs:

poa_global	temp_air	wind_speed	pac	pdc_ref
1000	25	2	9.2	10

Excel formula:

=PERF_RATIO_NREL(1000, 25, 2, 9.2, 10)

Expected output:

0.92

Example 4: Process row-vector arrays for PR calculation

Inputs:

poa_global			temp_air			wind_speed			pac			pdc_ref
700	900	1000	18	22	27	2	2.5	1.5	6.1	8.2	9	10

Excel formula:

=PERF_RATIO_NREL({700,900,1000}, {18,22,27}, {2,2.5,1.5}, {6.1,8.2,9}, 10)

Expected output:

0.896154

Python Code

Show Code

import pandas as pd
from pvanalytics.metrics import performance_ratio_nrel as result_func

def perf_ratio_nrel(poa_global, temp_air, wind_speed, pac, pdc_ref, a=-3.56, b=-0.075, delta_t=3, gamma_pdc=-0.00433):
    """
    Calculate the NREL weather-corrected performance ratio from irradiance, temperature, and power data.

    See: https://pvanalytics.readthedocs.io/en/stable/generated/pvanalytics.metrics.performance_ratio_nrel.html

    This example function is provided as-is without any representation of accuracy.

    Args:
        poa_global (list[list]): Total incident irradiance (W/m^2).
        temp_air (list[list]): Ambient dry bulb temperature (C).
        wind_speed (list[list]): Wind speed at 10 meters height (m/s).
        pac (list[list]): AC power output (kW).
        pdc_ref (float): PV array STC nameplate rating (kW).
        a (float, optional): SAPM temperature model parameter a (unitless). Default is -3.56.
        b (float, optional): SAPM temperature model parameter b (s/m). Default is -0.075.
        delta_t (float, optional): SAPM temperature model parameter deltaT (C). Default is 3.
        gamma_pdc (float, optional): Power temperature coefficient (1/C). Default is -0.00433.

    Returns:
        float: Weather-corrected Performance Ratio (float), or an error string.
    """
    try:
        def flatten(data):
            if not isinstance(data, list):
                return [float(data)]
            flat = []
            for row in data:
                if isinstance(row, list):
                    for val in row:
                        if val != "":
                            flat.append(float(val))
                elif row != "":
                    flat.append(float(row))
            return flat

        # Convert inputs to pandas Series for pvanalytics
        poa_s = pd.Series(flatten(poa_global))
        temp_s = pd.Series(flatten(temp_air))
        wind_s = pd.Series(flatten(wind_speed))
        pac_s = pd.Series(flatten(pac))

        n = len(poa_s)
        if not (len(temp_s) == n and len(wind_s) == n and len(pac_s) == n):
            return "Error: Input ranges must have the same number of valid elements"

        if n == 0:
            return "Error: Input arrays cannot be empty"

        pdc_ref_val = float(pdc_ref)
        a_val = float(a) if a is not None else -3.56
        b_val = float(b) if b is not None else -0.075
        dT_val = float(delta_t) if delta_t is not None else 3.0
        gamma_val = float(gamma_pdc) if gamma_pdc is not None else -0.00433

        pr = result_func(
            poa_s, temp_s, wind_s, pac_s, pdc_ref_val,
            a=a_val, b=b_val, deltaT=dT_val, gamma_pdc=gamma_val
        )
        return float(pr)
    except Exception as e:
        return f"Error: {str(e)}"

Online Calculator

poa_global *

Total incident irradiance (W/m^2).

temp_air *

Ambient dry bulb temperature (C).

wind_speed *

Wind speed at 10 meters height (m/s).

pac *

AC power output (kW).

pdc_ref *

PV array STC nameplate rating (kW).

SAPM temperature model parameter a (unitless).

SAPM temperature model parameter b (s/m).

delta_t

SAPM temperature model parameter deltaT (C).

gamma_pdc

Power temperature coefficient (1/C).