PVWatts DC model (Dobos 2014)

Method

The method is the DC model of PVWatts version 5 (Dobos 2014). The module power is the nameplate power times the effective irradiance over 1,000 W/m², corrected linearly for the cell temperature (1). The effective irradiance includes the rear side times the bifaciality factor. The model has no low-light loss, so the irradiance-level row of the loss diagram is 0. The model has no current-voltage curve. The array runs at the inverter's nominal voltage: the middle test voltage of its efficiency curves, else the middle of its MPPT window. The current is the power over that voltage. Without either voltage, the voltage and the current are 0 and the DC wiring loss takes its load-ratio form. The operating-limit step then clips the AC output at the nameplate, also when the I–V curve option is chosen. The module area sets the efficiency the cell-temperature models use (2). The module's current, voltage and resistance fields are not read.

Settings

Setting	Value
Power temperature coefficient $\gamma$	the module's Pmax temperature coefficient (Module tab); −0.40 %/°C if not given
Reference irradiance $G_{ref}$ and cell temperature $T_{ref}$	1,000 W/m² and 25 °C (Module tab)
Module efficiency $\eta$	the module's efficiency (Module tab); else from its width and height; else 19 %, and 18 % for CdTe, CIGS and a-Si (PVWatts version 8's standard and thin-film modules)
Low-light correction	none

The math

1. Module power
2. Module area

$$P = \max\Big(0,\ P_{nom}\,\frac{G}{G_{ref}}\,\big(1 + \gamma\,(T - T_{ref})\big)\Big) \tag{1}$$ $$A = \frac{P_{nom}}{\eta\,G_{ref}} \tag{2}$$

where

Symbol	Meaning	Unit
$P$	the module's DC power	W
$P_{nom}$	the nameplate power at STC	W
$G$	the effective irradiance on the module, front plus bifaciality times rear	W/m²
$G_{ref}$	the reference irradiance, 1,000 W/m²	—
$\gamma$	the power temperature coefficient	1/°C
$T$	the cell temperature	°C
$T_{ref}$	the reference cell temperature, 25 °C	—
$A$	the module area	m²
$\eta$	the module efficiency at STC	—

References

1. Dobos 2014, PVWatts Version 5 Manual, NREL/TP-6A20-62641
2. NREL SAM Simulation Core (SSC) source code, release 2026.7.3, github.com/NatLabRockies/ssc: ssc/cmod_pvwattsv5.cpp, the DC power, the system losses and the inverter efficiency
3. NREL SAM Simulation Core (SSC) source code, release 2026.7.3, github.com/NatLabRockies/ssc: ssc/cmod_pvwattsv8.cpp, the temperature coefficient and the efficiency of each module type
4. pvlib-python 0.16.1 source code (github.com/pvlib/pvlib-python), reference implementation: pvlib/pvsystem.py, pvwatts_dc() and pvwatts_losses(); pvlib/inverter.py, pvwatts()

API

module.model = pvwatts

Symbols follow the symbol registry.