Through the datasheet MPP

Method

The ideality factor is the one at which the curve at standard test conditions carries the datasheet current at the datasheet voltage (1). The curve's maximum power at standard test conditions (2) can differ from the nameplate. The array nominal energy is counted from that maximum (3), and the irradiance-level loss row is the difference between that energy and the curve's power at 25 °C. The nameplate remains the system's DC power in the results. A module without a maximum-power voltage or current, or whose point no ideality factor between 0.6 and 3 reaches, takes a maximum equal to the nameplate. A module rated 540 W with the datasheet point 31.2 V and 17.33 A has a maximum of 540.70 W at standard test conditions.

The math

1. The third point
2. The curve's maximum at standard test conditions
3. The array nominal energy

$$I(V_{mp}) = I_{mp} \quad \text{at } G_{ref},\ T_{ref} \tag{1}$$ $$P_{STC} = \max_{V}\ V\,I(V) \quad \text{at } G_{ref},\ T_{ref} \tag{2}$$ $$P_{nom,arr} = N\,P_{STC}\,\frac{G}{G_{ref}} \tag{3}$$

where

Symbol	Meaning	Unit
$I(V)$	the module current at voltage $V$ on the fitted curve	A
$V_{mp}$	the datasheet maximum-power voltage	V
$I_{mp}$	the datasheet maximum-power current	A
$G_{ref}$	the irradiance of standard test conditions, 1000 W/m²	W/m²
$T_{ref}$	the cell temperature of standard test conditions, 25 °C	K
$P_{STC}$	the maximum power of the fitted curve at standard test conditions	W
$P_{nom,arr}$	the array nominal power of the timestep	W
$N$	the number of modules	—
$G$	the effective irradiance	W/m²

References

1. Mermoud & Lejeune 2010, Performance assessment of a simulation model for PV modules of any available technology, 25th European Photovoltaic Solar Energy Conference
2. PVsyst 8 help, Standard one-diode model
3. Analysis of PVsyst 8.1.5 outputs shows this convention in its results.

API

options.module_stc_point = datasheet_mpp

Symbols follow the symbol registry.