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CHARACTERIZING SHADING LOSSES ON 

PARTIALLY SHADED PV SYSTEMS

Chris Deline

PV Performance 

Modeling Workshop

September 23, 2010

Albuquerque, NM

NREL/PR-520-49504
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Overview

2

Introduction: Shading on PV systems

Theory: Shaded PV power loss

Practical issues with modeling shaded PV

• Shade Estimation

• IV curve analysis

Methods of implementing partially shaded PV modeling

Some experimental results

Current and future work
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Introduction – Shading on PV systems

3

Shading and mismatch occur on 

all types of PV installations.

• Nearby shade obstructions like 

trees and telephone poles

• Horizon shading from faraway 

structures

• Self-shading from adjacent rows

• Imp mismatch from orientation, 

manufacturing tolerance, 

differential aging or soiling

Some types of shading are 

easier to quantify and model 

than others.

1

2

3

1: Lakewood, CO. 2: Maumee, OH. 3: Arlington, VA 

Credit: NREL

Credit: NREL PIX 15617

Credit: NREL PIX 08558
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Introduction – Impact of Shade 

4

Shade impact depends on e.g. 

module type (fill factor, bypass 

diode placement), severity of 

shade, and string configuration.

Power loss occurs from shade, 

also current mismatch within a 

PV string and voltage mismatch 

between parallel strings.

Power lost is greater than 

proportional to the amount of 

shade on the system

‘Shade Impact Factor’ (ratio of 

power lost to area of shade) for a 

single module in a single string PV 

system [1]

[1] C. Deline, IEEE PVSC, 2009
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Bypass diode operation in most modules

5

+

- +

-

-

+

+

-

I

+

V

-

2 N-1 N

1

+

D1

Shade

Bypass diodes typically protect

substrings of 15-20 cells.

Shade on one of these cells can

cause the diode to turn on,

removing those cells electrically

from the string.

Current is continuous in the PV

string; a small amount of shade

can greatly reduce output

power.

On typical Si modules, reducing

1 cell’s irradiance by 25% can

lead to bypass diode turn-on.

1 substring
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I

V

I

mp

Shaded cell

V

rev

Unshaded cell

Theory – Partially shaded substring of cells

6

A shaded cell has reduced Isc. In

order to pass the string current Imp

the cell will operate in reverse

bias. The total substring voltage

is a sum of the various operating

voltages including the reverse

biased cell.

If the total substring voltage < 0, 

the bypass diode turns on and the 

shaded cell will operate near Vrev.

Variability exists in the reversebias characteristics of different

cells – the same shading could

result in different outcomes.

Full I-V curve of a partially shaded 

cell. Current continuity requires the 

shaded cell to operate in reverse 

bias to pass the Imp current of the 

rest of the substring.
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0

10

20

Volts

Current

0 50 100 150

0

500

1000 Power 

Unshaded 

IV + Shaded

IV

Theory – system level IV curve

7

System IV curve is built from 

individual substring IV curves 

in series and parallel.

+ =
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0

10

20

Volts

Current

0 50 100 150

0

500

1000 Power 

Global max [A]

Local max [B]

Theory – system level IV curve

8

System IV curve is built from 

individual substring IV curves 

in series and parallel.

Partial shading can lead to Local 

[B] and Global [A] maxima.

Bypass diode turn-on depends 

on the peak power point chosen. 

For instance, operating at point 

[A] requires shaded bypass 

diode turn-on while point [B] 

does not.
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Practical matters – shade estimation

9

Shading trees

Proposed array

Rooftop survey

Shading site survey typically relies on aerial imagery and

fisheye shade analysis e.g. SunEyeTM

, or Solar PathfinderTM.

Some issues include: foliage changes throughout the year,

spatial resolution requires multiple pictures, shading objects

are considered 100% opaque, nearby objects have more

position uncertainty, 3D CAD modeling is time intensive.

Credit: Chris Deline / NREL

Credit: Chris Deline / NREL
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Methods of modeling substring IV curves

10

Full 5-parameter IV curve

• High accuracy, but slow (when 

calculated 1000’s of times) 

Simplified IV curve (3-parameter)

• Computationally less intense, 

reduced accuracy for V < Vmp.

• I = C1 – C2 exp(C3 * V)

Empirical ‘Shade impact factor’

• System-specific lookup table, based 

on shade % and diffuse / global ratio.

0 5 10 15 20 25 30 35

0

2

4

6

8

Volts [V]

Current [A]

5-parameter

3-parameter

Comparison of full 5-parameter IV 

curve with a simplified 3-parameter 

IV curve for an Evergreen ES-200 

PV module. Accuracy is better for 

V > Vmp .
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Real-world application of shade modeling

11

Site survey conducted on a 

‘typical residential installation’[3]

• Most shade from 6-10am, 2-6pm

• ~21% annual irradiance loss

• 2 strings of 7 mSi modules @ 3kW

Site survey picture taken at each 

PV module substring

• 3 images per module = 42 total

• Provides # of shaded substrings 

for a given hour and date

‘Typical residential installation’. 2x7 mSi panels

Site survey:~20% irradiance loss due to shade [3] R. Levinson, Solar Energy 83, 2009

Credit: Brent Nelson / NREL

Credit: Chris Deline / NREL
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Numerical shade simulation

12

Simulation uses PVWatts with additional shade derating[4]

• Derating based on empirical relationships between shade extent and 

power loss, as determined in a scale experiment at NREL.

• TMY3 weather data and the default PVWatts AC to DC factor (0.77)

Two shade conditions are simulated: 1) both strings are 

shaded as per the survey, and 2) one string is entirely 

unshaded

• Two-string shading is more realistic, but some installations may have 

more limited shading.

[4] C. Deline, IEEE PVSC, 2010
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Σ

Simulation method - overview 

13

Site survey: one image 

for each substring TMY3 database

Experimental Shade 

Impact Factor

Beam/Global Irradiance

PVWatts model

X

DC Derating (hourly)

Annual shaded 

power production

# Shaded Substrings
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Simulation results

14

Annual power produced Power lost to shade

Unshaded baseline 4.4 MWh 0

Site survey estimate -21%

2 strings shaded 3.5 MWh -22%

1 string shaded 3.7 MWh -17%

PVWatts simulation results using site survey data for a two-string PV system.

5 10 15 20

0

500

1000

1500

2000

2500

Time

DC power (W)

March modeled

March actual

Dec. modeled

Dec. actual

Modeled results (dots) and measured data 

(lines) for two representative sunny dates

Modeled results compare favorably 

with measured data on 

representative sunny days.

Annual results show close 

agreement with site survey’s ‘solar 

resource fraction’ (but this is not 

always the case)
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Large commercial installation

15

0 5 10 15 20 25

0

0.2

0.4

0.6

0.8

1

Percent of string shaded

String power

FF = 0.65

FF = 0.73

Observed

6 8 10 12 14 16 18

-0.5

0

0.5

1

1.5

2

2.5

3

Time

String power (kW)

AM shading

30% loss from shade

Morning power loss is monitored with 

DC current transducers. 30% loss is 

coincident with 12.5% string shading

Modeled shade impact for large parallel 

systems. Note that higher FF is more 

sensitive to shade.

This 1MW installation has 16 PV 

modules per string. Periodic shading 

occurs from nearby light poles.

Credit: Chris Deline / NREL
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Current / Future work at NREL

16

• Shade simulation feature going into Solar Advisor Model, 

specifically for inter-row shading of large utility-scale 

systems.

• Further work on developing and validating shaded PV 

models

• Test & Evaluation of DC-DC converter devices and microinverters to determine the performance improvement
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Questions / Comments?

17

Acknowledgments

This work was supported by the U.S. Department of Energy under 

Contract No. DOE-AC36-08GO28308 with the National Renewable 

Energy Laboratory.

Chris Deline

chris.deline@nrel.gov

Ph: (303) 384-6359
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Backup slides
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0 10 20 30 40

0

0.2

0.4

0.6

0.8

1

Cell shade %

Relative substring power

No DC-DC

Bypass diode 

turn-on

0 10 20 30 40

0

0.2

0.4

0.6

0.8

1

Cell shade %

Relative substring power

Bypass diode 

turn-on

With DC-DC

No DC-DC

1 2 3

0

50

100

150

200

250

300

# of M10's substrings shaded

System power loss (W)

21

26

60

41

22

117

78

18

175

M1-M5

M6-M9

M10

Mismatch

(With DC-DC, 50% shade) (No DC-DC)

1 2 3

0

50

100

150

200

250

300

# of M10's substrings shaded

System power loss (W)

M1-M5

M6-M9

M10

Results – single module shading

19

Without DC-DC devices:

• Single-cell shading of 25% 

causes bypass diode turn-on

• Mismatch loss accounts for 

~40% of the total shade loss

With DC-DC devices:

• Bypass diode turn-on can be 

delayed

• Mismatch losses reduced

• Shaded module output 

proportional to shade opacity
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0 10 20 30 40 50 60 70

50

60

70

80

90

100

Single PV string shade(%)

System power(%)

>60% shade

SIF = 1.63

Results – Shade Impact Factor

20

Shade Impact Factor without DC-DC= 1.63

With DC-DC, SIF = shade opacity

With DC-DC

Shade %

Power loss

m = SIF

0 10 20 30 40 50 60 70

50

60

70

80

90

100

Single PV string shade(%)

System power(%)

75% shade

SIF = 0.73

100% shade

SIF = 0.97

50% shade

SIF = 0.48

No DC-DC

SIF = 1.63