PVWatts inverter (Dobos 2014)

Method

The method is the inverter model of PVWatts version 5 (Dobos 2014). The load ratio is the DC input over the DC input that gives the AC nameplate at the nominal efficiency (1). The efficiency is a curve in the load ratio, scaled so that it equals the nominal efficiency at full load (2). The nominal efficiency is the inverter's maximum efficiency from the Inverter tab. The efficiency peaks at 0.6035 of full load, at 1.0026 times the nominal efficiency. Below 0.6 % of full load the curve is negative and the output is zero. The efficiency curves, the power threshold and the DC voltage are not read. The operating-limit step cuts the output at the AC nameplate.

Settings

Setting	Value
Nominal efficiency $\eta_{nom}$	the inverter's maximum efficiency (Inverter tab); 98 % if not given; 96 % in the simple request
Reference efficiency $\eta_{ref}$	0.9637
Curve coefficients	−0.0162, −0.0059, 0.9858

The math

1. Load ratio
2. Efficiency and output

$$\zeta = \frac{P_{dc}}{P_{dc0}},\qquad P_{dc0} = \frac{P_{ac0}}{\eta_{nom}} \tag{1}$$ $$\eta = \frac{\eta_{nom}}{\eta_{ref}}\left(-0.0162\,\zeta - \frac{0.0059}{\zeta} + 0.9858\right),\qquad P_{out} = \max(0,\ \eta\,P_{dc}) \tag{2}$$

where

Symbol	Meaning	Unit
$\zeta$	the load ratio	—
$P_{dc}$	the DC input	W
$P_{dc0}$	the DC input that gives the nameplate at the nominal efficiency	W
$P_{ac0}$	the inverter's AC nameplate	W
$\eta_{nom}$	the nominal efficiency	—
$\eta$	the efficiency at the load ratio	—
$\eta_{ref}$	the reference efficiency of the curve, 0.9637	—
$P_{out}$	the AC output before the operating limits	W

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. 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

inverter.efficiency_interpolation = pvwatts

Symbols follow the symbol registry.