Cap the power, no I–V curve

Method

No current-voltage curve is used. The array delivers its maximum power unless a limit caps it. When the maximum-power current exceeds the inverter's maximum DC current, the DC power is scaled down by their ratio. When the AC output then exceeds the AC limit, the DC power is reduced until the output equals the limit. The MPPT window only bounds the voltage at which the efficiency curves are read, so a maximum-power voltage outside the window costs no energy. The inverter does not start below an effective irradiance of 7 W/m² or at zero output. On east-west racking this option also runs in place of the operating point on the array's I–V curve, because the two faces have no single current-voltage curve.

The math

1. Voltage for the efficiency curves
2. DC current limit
3. AC limit

$$U_w = \min\big(\max(U_{mpp},\ V_{min}),\ V_{max}\big) \tag{1}$$ $$P_{dc} = P_{mpp}\,\min\!\left(1,\ \frac{I_{max}}{I_{mpp}}\right) \tag{2}$$ $$P_{ac} = \min\big(P_{out}(P_{dc},\ U_w),\ P_{lim}\big) \tag{3}$$

where

Symbol	Meaning	Unit
$U_w$	the voltage at which the efficiency curves are read	V
$U_{mpp}$	the array's maximum-power voltage	V
$V_{min}$	the minimum MPP voltage, the bottom of the inverter's MPPT window	V
$V_{max}$	the maximum MPP voltage, the top of the window	V
$P_{dc}$	the DC power after the current limit	W
$P_{mpp}$	the array power at its maximum-power point after the DC losses	W
$I_{max}$	the inverter's maximum DC input current	A
$I_{mpp}$	the array's maximum-power current of the timestep	A
$P_{ac}$	the AC output	W
$P_{out}(P, U)$	the AC output of the efficiency curves at DC power $P$ and voltage $U$	W
$P_{lim}$	the AC limit of one inverter at the timestep: its nameplate, derated with the ambient temperature and lowered by a grid power limitation	W

References

1. Gilman, DiOrio, Freeman, Janzou, Dobos & Ryberg 2018, SAM Photovoltaic Model Technical Reference Update, NREL/TP-6A20-67399

API

inverter.operating_point = power_limits

Symbols follow the symbol registry.