V_FROM_I

This function computes the device voltage corresponding to a specified operating current under the single-diode model. It is useful when a current setpoint is already known and only the matching voltage on the IV curve is required.

The calculation inverts the single-diode relationship

I = I_L - I_0 \left[\exp\left(\frac{V + I R_s}{n N_s V_{th}}\right) - 1\right] - \frac{V + I R_s}{R_{sh}}

to solve for V at the requested current. When shunt resistance is effectively infinite the solution can simplify, while finite-shunt cases use the selected numerical method.

Excel Usage

=V_FROM_I(current, photocurrent, saturation_current, resistance_series, resistance_shunt, nNsVth, vfi_solver)
  • current (float, required): Operating current (A).
  • photocurrent (float, required): Light-generated current (A).
  • saturation_current (float, required): Diode saturation current (A).
  • resistance_series (float, required): Series resistance (ohms).
  • resistance_shunt (float, required): Shunt resistance (ohms). Use a large value for infinite.
  • nNsVth (float, required): Product of ideality factor, cells in series, and thermal voltage (V).
  • vfi_solver (str, optional, default: “lambertw”): Numerical solver method.

Returns (float): The device voltage (V), or an error string.

Example 1: Voltage at typical Imp

Inputs:

current photocurrent saturation_current resistance_series resistance_shunt nNsVth vfi_solver
5.09341 5.5 2e-10 0.5 300 1.5 lambertw

Excel formula:

=V_FROM_I(5.09341, 5.5, 2e-10, 0.5, 300, 1.5, "lambertw")

Expected output:

29.151

Example 2: Voltage at zero current approximates Voc

Inputs:

current photocurrent saturation_current resistance_series resistance_shunt nNsVth vfi_solver
0 5.5 2e-10 0.5 300 1.5 lambertw

Excel formula:

=V_FROM_I(0, 5.5, 2e-10, 0.5, 300, 1.5, "lambertw")

Expected output:

36.0231

Example 3: Voltage at typical Imp using Newton solver

Inputs:

current photocurrent saturation_current resistance_series resistance_shunt nNsVth vfi_solver
5.09341 5.5 2e-10 0.5 300 1.5 newton

Excel formula:

=V_FROM_I(5.09341, 5.5, 2e-10, 0.5, 300, 1.5, "newton")

Expected output:

29.151

Example 4: Voltage at operating current with higher series resistance

Inputs:

current photocurrent saturation_current resistance_series resistance_shunt nNsVth vfi_solver
5.09341 5.5 2e-10 0.9 300 1.5 lambertw

Excel formula:

=V_FROM_I(5.09341, 5.5, 2e-10, 0.9, 300, 1.5, "lambertw")

Expected output:

27.1137

Python Code

Show Code 
from pvlib.pvsystem import v_from_i as result_func

def v_from_i(current, photocurrent, saturation_current, resistance_series, resistance_shunt, nNsVth, vfi_solver='lambertw'):
    """
    Calculate device voltage at a given current for the single-diode model.

    See: https://pvlib-python.readthedocs.io/en/stable/reference/generated/pvlib.pvsystem.v_from_i.html

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

    Args:
        current (float): Operating current (A).
        photocurrent (float): Light-generated current (A).
        saturation_current (float): Diode saturation current (A).
        resistance_series (float): Series resistance (ohms).
        resistance_shunt (float): Shunt resistance (ohms). Use a large value for infinite.
        nNsVth (float): Product of ideality factor, cells in series, and thermal voltage (V).
        vfi_solver (str, optional): Numerical solver method. Valid options: Lambert W, Newton, Brentq, Chandrupatla. Default is 'lambertw'.

    Returns:
        float: The device voltage (V), or an error string.
    """
    try:
        i = float(current)
        il = float(photocurrent)
        i0 = float(saturation_current)
        rs_v = float(resistance_series)
        rsh = float(resistance_shunt)
        nv = float(nNsVth)

        meth = str(vfi_solver) if vfi_solver is not None else "lambertw"

        res = result_func(
            current=i,
            photocurrent=il,
            saturation_current=i0,
            resistance_series=rs_v,
            resistance_shunt=rsh,
            nNsVth=nv,
            method=meth
        )

        return float(res)
    except Exception as e:
        return f"Error: {str(e)}"

Online Calculator

Operating current (A).
Light-generated current (A).
Diode saturation current (A).
Series resistance (ohms).
Shunt resistance (ohms). Use a large value for infinite.
Product of ideality factor, cells in series, and thermal voltage (V).
Numerical solver method.