Conference Paper

NREL/CP-550-45380

April 2009

Instrumentation for Evaluating 

PV System Performance Losses 

from Snow 

Preprint

B. Marion, J. Rodri uez, and g J. Pruett

National Renewable Energy Laboratory

To be presented at the American Solar Energy Society (ASES)

2009 National Solar Conference (SOLAR 2009)

Buffalo, New York

May 11–16, 2009
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INSTRUMENTATION FOR EVALUATING 

PV SYSTEM PERFORMANCE LOSSES FROM SNOW

Bill Marion

Jose Rodriguez

Jim Pruett

National Renewable Energy Laboratory

1617 Cole Boulevard

Golden, Colorado 80401

bill_marion@nrel.gov

ABSTRACT

When designing a photovoltaic (PV) system for northern 

climates, the prospective installation should be evaluated 

with respect to the potentially detrimental effects of snow 

preventing solar radiation from reaching the PV cells. The 

extent to which snow impacts performance is difficult to 

determine because snow events also increase the uncertainty 

of the solar radiation measurement, and the presence of 

snow needs to be distinguished from other events that can 

affect performance. This paper describes two instruments 

useful for evaluating PV system performance losses from 

the presence of snow: (1) a pyranometer with a heater to 

prevent buildup of ice and snow, and (2) a digital camera for 

remote retrieval of images to determine the presence of 

snow on the PV array.

1. INTRODUCTION

For northern locations in winter, snow reduces the amount 

of PV energy produced, with the severity of the reduction a 

function of the amount of snow received and how long it 

remains on the PV modules. Snow remains the longest on 

PV modules when sub-freezing temperatures prevail, small 

PV array tilt angles prevent snow from sliding, the PV array 

is closely integrated into the roof, and the roof or other 

structure in the vicinity facilitates snow drifting onto the PV 

modules. Presently, there is little information pertaining to 

predicting performance losses from snow, but it has been 

identified for some locations as a serious loss factor.

1

To precisely determine snow losses would require identical 

systems operating side-by-side, with one system allowing 

snow to accumulate naturally and with the other having the 

snow immediately removed. Of course, this is impractical

for most situations, and removing snow immediately is an 

insurmountable task. However, if an accurate irradiance 

measurement is made with a pyranometer, we can estimate

what the PV performance should be without snow on the PV 

modules. Unfortunately, pyranometer measurements are 

also affected by snow. When covered with snow, the 

measurement will be low and not reflect the true solar 

resource. Additionally, sometimes the measurements are 

erroneously high if the sun melts the snow nonuniformly 

and the sun’s rays are reflected (concentrated) onto the 

pyranometer sensor.

Our approach for instrumentation for evaluating PV system 

performance losses from snow is twofold: (1) a pyranometer 

with a heater to prevent buildup of ice and snow, and (2) a 

digital camera for remote retrieval of images to determine 

the presence of snow on the PV array. This paper describes 

these instruments and their use to determine losses related to 

snow for PV systems located at the National Renewable 

Energy Laboratory (NREL).

2. INSTRUMENTATION

As described in the previous section, a pyranometer with a 

heater and a digital camera with remote image retrieval 

capabilities are the additional instrumentation requirements 

for determining performance losses from snow.

2.1 Pyranometer with Heater

For the irradiance measurement, it is desirable to use a small 

pyranometer to minimize the surface area subject to snow 

accumulation and to minimize the heating requirements. 

Consequently, we used a Li-Cor pyranometer and mounted 

it on top of a vertical rod to further prevent snow 
accumulations from interfering with operations. The top of 

the rod was machined for securing the Li-Cor pyranometer 

with a tilt angle of 40° from horizontal, which is equal to the 

site latitude and matches the tilt angle of most of the PV 

systems located at NREL.

For the heating element, a 3-foot piece of 120-V AC electric 

water pipe heating cable was wrapped around the Li-Cor 

pyranometer and vertical rod, and the ends of the heating 

cable were secured with electrical tape. This is shown in 

Fig. 1. The temperature is regulated with a Johnson Controls 

A419 Series Electronic Temperature Controller, Model No. 

A419 AEC-1, with the temperature sensor located near the 

top of the vertical rod and under the heating cable.

Fig. 1: Heated pyranometer components. (a) Li-Cor 

pyranometer, (b) heating cable, and (c) temperature sensor

for temperature controller. The ends of the heating cable are 

secured with electrical tape.

Fig. 2: Heated pyranometer demonstrating effectiveness in 

preventing snow and ice accumulation.

The controller is set to turn on the power to the heating 

cable if the temperature sensor is equal to or less than 35°F 

(1.7°C) and to turn off the power when the temperature 

sensor reaches 60°F (15.6°C). Raychem heat-shrinkable 

fabric tubing is used to cover the installed heating cable. 

The fully assembled unit is shown in Fig. 2.

2.2 Digital Camera with Remote Image Retrieval

For this work, a CC640 digital camera from Campbell 

Scientific, Inc., was placed on the roof of the building to 

record images of the PV arrays being studied. This setup is 

shown in Fig. 3. The camera may be connected to a 

Campbell Scientific datalogger that is programmed to take 

images at required intervals and then transfer them from the 

camera to the data logger. Any of the available Campbell 

Scientific communication protocols may be used to retrieve 

images from the datalogger.

The camera may also be operated in standalone mode 

without the need for a datalogger. The camera’s clock is 

used to time image acquisitions, and the images are stored 

on a removable compact flash memory card (up to 10,000 

images). We used this method because remote access 

capability was not required for this work. Other remote 

cameras are also available that allow images to be retrieved 

by phone modem, internet, or wirelessly.

Fig. 3: CC640 digital camera from Campbell Scientific, 

Inc., located on building roof.

3. USE AND EXPERIMENTAL RESULTS

For the five PV arrays shown in Fig. 4, the heated 

pyranometer and digital camera were used to determine PV 

system performance losses associated with a snowfall 

beginning about 7 a.m. on the morning of January 26, 2009.
The five PV arrays use crystalline PV modules and are gridconnected. PV array number 1 is building-integrated (BIPV)

with a PV module tilt angle of 15° from horizontal. The

other PV arrays are open-rack mounted with a PV module 

tilt angle of 40° from horizontal.

Fig. 4: PV arrays, identified by numbers, for evaluating 

performance losses from snow. 

The snowfall ended at about 11:00 a.m. on the morning of 

January 26, with a total accumulation of about eight inches.

Figure 5 shows the digital images for January 26–30 for 

9:00 a.m., 12:00 p.m., and 3:00 p.m. PV arrays 3, 4, and 5 

began shedding snow in the afternoon of January 26th, but 

wind during the night re-covered the PV modules with snow 

and drifting also occurred on the roof of the shed where PV 

array 1 was installed. By noon on the 27th, the snow had 

been shed from PV arrays 4 and 5; and by 3 p.m., the snow 

was also gone from PV array 2, and only a small amount 

remained on PV array 3. Beginning in the morning of the 

28th, no snow remained on the rack-mounted PV arrays 2, 3, 

4, and 5.

Snow remained the longest on PV array 1, with one module 

retaining snow until the morning of the 29th. Interestingly, 

the east side of PV array was free of snow about 1 day 

before the west side. To better understand the thermal 

characteristics of BIPV arrays, two different mounting 

methods had been used. The west side was mounted directly 

on the roof sheathing, but the east side was raised about an 

inch using battens to promote air flow. Apparently, the 

increased air flow promoted snow removal.

Figures 6, 7, and 8 show the meteorological and PV system 

performance data for January 26–30, 2009 using data 

recorded at 15-minute intervals. Figure 6 provides dry-bulb 

temperature and wind-speed measurements. Figure 7 shows 

solar radiation values measured with the Li-Cor 

pyranometer with heater and those measured with a Kipp 

and Zonen CM11 pyranometer. Snow on the CM11 

pyranometer significantly reduced measurement values until 

the afternoon of the 27th, at which point the snow had 

melted from the CM11 and both pyranometers, began 

providing comparable measurements.

PV system performance is shown in Fig. 8 using the 

performance ratio (PR) metric.

2 The PR is a dimensionless 

quantity determined as the final PV system yield (kWh/kW 

or hours) divided by the reference yield (equivalent hours at 

the reference irradiance). The PV performance was 

consistent with the images showing snow on the PV 

modules. Even a small amount of snow remaining on the 

PV modules significantly lowered the PR. PV system 1did

not regain its normal PR until January 30th. 

Daily summary data are provided in Table 1. Compared to 

the Li-Cor pyranometer with heater, the Kipp & Zonen 

pyranometer at the same tilt (40°) underreported the solar 

radiation by 58% on the 26th and by 44% on the 27th. For the 

remaining days without snow on the Kipp & Zonen 

pyranometer, the two pyranometers provided daily totals

within 4%.

PR calculations used solar radiation measurements recorded 

by the Li-Cor pyranometer for PV systems 2 through 5. 

Because the tilt angle for PV system 1is less than that for 

the other PV systems, it used solar radiation measurements 

from the Kipp and Zonen with a tilt from horizontal of 15°

for its PR calculations. PR values were near zero for all PV 

systems on the 26th. In fact, slightly negative values

occurred because the PV arrays put out sufficient voltage 

that some inverters came out of sleep mode and increased 

their standby losses.

On the 27th, PR values improved for systems 2 through 5 as 

the snow began to slide from the PV modules and had 

returned to normal values by the 28th. For PV system 1, the 

PR did not return to normal values until the 30th, when the 

last of its snow was gone. For this one snowfall, PV system 

1 lost nearly four days of production. Considering that 

during this period the solar radiation was about 35% greater 

than the January average because of the relatively clear days 

following the snowfall, the “effective” loss in production 

was about 5 days.
Fig. 5: Digital images for January 26–30, 2009, for 9 a.m., 12 p.m., and 3 p.m.
1/26/09 1/27/09 1/28/09 1/29/09 1/30/09 1/31/09

Day

-20

-10

0

10

20

Dry Bulb Temperature (°C)

Dry Bulb Temperature

Wind Speed

0

2

4

6

8

10

Wind Speed (m/s)

Fig. 6: Dry-bulb temperature and wind speed for January 26–30, 2009.

1/26/09 1/27/09 1/28/09 1/29/09 1/30/09 1/31/09

Day

0

400

800

1200

1600

POA Irradiance (W/m2

)

LI-COR w/Heater

Kipp & Zonen

Fig. 7: Plane-of-array (40° tilt) solar radiation measured with Li-Cor pyranometer with heater and Kipp and Zonen CM11. 

1/26/09 1/27/09 1/28/09 1/29/09 1/30/09 1/31/09

Day

0.0

0.4

0.8

1.2

Performance Ratio

#1

#2

#3

#4

#5

Fig. 8: Performance ratio (PR) values for January 26–30, 2009 determined using 15-minute data.
Table 1. Daily POA Solar Radiation and PV System Performance Ratios

Date Solar Radiation (kWh/m2) PV System Performance Ratios

Li-Cor 

w/Heater

(40° Tilt)

Kipp & 

Zonen

(40° Tilt)

Kipp & 

Zonen

(15° Tilt)

System 1 System 2 System 3 System 4 System 5

1/26/09 3.15 1.33 1.25 -0.116 -0.013 0.010 0.047 -0.016

1/27/09 7.38 4.17 4.15* -0.034 0.188 0.216 0.621 0.576

1/28/09 6.73 6.56 4.86 -0.028 0.859 0.857 0.824 0.855

1/29/09 7.42 7.19 5.22 0.194 0.847 0.847 0.799 0.839

1/30/09 7.17 6.92 5.07 0.741 0.820 0.827 0.773 0.804

* Snow was cleaned from this pyranometer at noon on the 27th

4. SUMMARY

A pyranometer with a heater to prevent buildup of ice and 

snow and a digital camera for remote retrieval of images 

were shown to be useful instruments for evaluating PV 

system performance losses from the presence of snow.

Application of the instruments and their use for analysis was 

demonstrated for a snowfall occurring on January 26, 2009 

and subsequent losses in PV system performance were 

determined. Most of the PV systems lost 1 to 2 days of 

energy production. But the BIPV system, with a smaller tilt 

angle, lost 4 days of energy production.

5. ACKNOWLEDGEMENTS

This work was performed under DOE Contract No. DEAC36-08GO28308. 

6. REFERENCES

(1) Ueda, Y., Kurokawa, K., Itou, T., Kitamura, K., 

Miyamoto, Y., Yokota, M., Sugihara, H., Performance Ratio 

and Yield Analysis of Grid Connected Clustered PV 

Systems in Japan, Proceedings of the IEEE 4th World 

Conference on Photovoltaic Energy Conversion, May 2006, 

pp. 2296–2299. 

(2) IEC, “Photovoltaic System Performance MonitoringGuidelines for Measurement, Data Exchange, and Analysis, 

IEC Standard 61724,” Geneva, Switzerland, 1998. 
F1147-E(10/2008) 

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Instrumentation for Evaluating PV System Performance Losses from 

Snow: Preprint

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14. ABSTRACT (Maximum 200 Words)

When designing a photovoltaic (PV) system for northern climates, the prospective installation should be evaluated 

with respect to the potentially detrimental effects of snow preventing solar radiation from reaching the PV cells. The 

extent to which snow impacts performance is difficult to determine because snow events also increase the 

uncertainty of the solar radiation measurement, and the presence of snow needs to be distinguished from other 

events that can affect performance. This paper describes two instruments useful for evaluating PV system 

performance losses from the presence of snow: (1) a pyranometer with a heater to prevent buildup of ice and snow, 

and (2) a digital camera for remote retrieval of images to determine the presence of snow on the PV array.

15. SUBJECT TERMS

PV; system performance; evaluation; snow; solar radiation; pyranometer; digital camera;

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