PVWATTS_LOSSES

This function combines multiple PV system loss assumptions into a single total derate percentage using the PVWatts compounding-loss formulation. Each input is a percentage loss for one mechanism such as soiling, shading, mismatch, or equipment availability.

PVWatts multiplies the retained fractions for each loss component and converts the remaining derate back to a total loss percentage:

L_{total}(\%) = 100 \left[1 - \prod_i \left(1 - \frac{L_i}{100}\right)\right]

Because the model compounds retained performance, the total loss is not simply the arithmetic sum of the inputs except in very small-loss approximations.

Excel Usage

=PVWATTS_LOSSES(soiling, shading, snow, mismatch, wiring, connections, lid, nameplate_rating, age, availability)

• soiling (float, optional, default: 2): Loss due to soiling (%).
• shading (float, optional, default: 3): Loss due to shading (%).
• snow (float, optional, default: 0): Loss due to snow (%).
• mismatch (float, optional, default: 2): Loss due to mismatch (%).
• wiring (float, optional, default: 2): Loss due to wiring (%).
• connections (float, optional, default: 0.5): Loss due to connections (%).
• lid (float, optional, default: 1.5): Light induced degradation (%).
• nameplate_rating (float, optional, default: 1): Nameplate rating loss (%).
• age (float, optional, default: 0): Loss due to age (%).
• availability (float, optional, default: 3): Loss due to availability (%).

Returns (float): Total system losses (%), or an error string.

Example 1: Standard PVWatts default losses

Inputs:

soiling	shading	snow	mismatch	wiring	connections	lid	nameplate_rating	age	availability
2	3	0	2	2	0.5	1.5	1	0	3

Excel formula:

=PVWATTS_LOSSES(2, 3, 0, 2, 2, 0.5, 1.5, 1, 0, 3)

Expected output:

14.0757

Example 2: No modeled losses

Inputs:

soiling	shading	snow	mismatch	wiring	connections	lid	nameplate_rating	age	availability
0	0	0	0	0	0	0	0	0	0

Excel formula:

=PVWATTS_LOSSES(0, 0, 0, 0, 0, 0, 0, 0, 0, 0)

Expected output:

0

Example 3: Single age loss component

Inputs:

soiling	shading	snow	mismatch	wiring	connections	lid	nameplate_rating	age	availability
0	0	0	0	0	0	0	0	5	0

Excel formula:

=PVWATTS_LOSSES(0, 0, 0, 0, 0, 0, 0, 0, 5, 0)

Expected output:

5

Example 4: Higher aggregate project losses

Inputs:

soiling	shading	snow	mismatch	wiring	connections	lid	nameplate_rating	age	availability
5	8	1	3	2.5	1	2	1.5	1	4

Excel formula:

=PVWATTS_LOSSES(5, 8, 1, 3, 2.5, 1, 2, 1.5, 1, 4)

Expected output:

25.6764

Python Code

Show Code

from pvlib.pvsystem import pvwatts_losses as result_func

def pvwatts_losses(soiling=2, shading=3, snow=0, mismatch=2, wiring=2, connections=0.5, lid=1.5, nameplate_rating=1, age=0, availability=3):
    """
    Implement NREL's PVWatts system loss model.

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

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

    Args:
        soiling (float, optional): Loss due to soiling (%). Default is 2.
        shading (float, optional): Loss due to shading (%). Default is 3.
        snow (float, optional): Loss due to snow (%). Default is 0.
        mismatch (float, optional): Loss due to mismatch (%). Default is 2.
        wiring (float, optional): Loss due to wiring (%). Default is 2.
        connections (float, optional): Loss due to connections (%). Default is 0.5.
        lid (float, optional): Light induced degradation (%). Default is 1.5.
        nameplate_rating (float, optional): Nameplate rating loss (%). Default is 1.
        age (float, optional): Loss due to age (%). Default is 0.
        availability (float, optional): Loss due to availability (%). Default is 3.

    Returns:
        float: Total system losses (%), or an error string.
    """
    try:
        l_soiling = float(soiling) if soiling is not None else 2.0
        l_shading = float(shading) if shading is not None else 3.0
        l_snow = float(snow) if snow is not None else 0.0
        l_mismatch = float(mismatch) if mismatch is not None else 2.0
        l_wiring = float(wiring) if wiring is not None else 2.0
        l_conn = float(connections) if connections is not None else 0.5
        l_lid = float(lid) if lid is not None else 1.5
        l_nameplate = float(nameplate_rating) if nameplate_rating is not None else 1.0
        l_age = float(age) if age is not None else 0.0
        l_avail = float(availability) if availability is not None else 3.0

        res = result_func(
            soiling=l_soiling,
            shading=l_shading,
            snow=l_snow,
            mismatch=l_mismatch,
            wiring=l_wiring,
            connections=l_conn,
            lid=l_lid,
            nameplate_rating=l_nameplate,
            age=l_age,
            availability=l_avail
        )

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

Online Calculator

soiling

Loss due to soiling (%).

shading

Loss due to shading (%).

snow

Loss due to snow (%).

mismatch

Loss due to mismatch (%).

wiring

Loss due to wiring (%).

connections

Loss due to connections (%).

lid

Light induced degradation (%).

nameplate_rating

Nameplate rating loss (%).

age

Loss due to age (%).

availability

Loss due to availability (%).