NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

Pyranometers and Reference 

Cells, What’s the Difference? 

Preprint

Jenya Meydbray

PV Evolution Labs

Keith Emery and Sarah Kurtz 

National Renewable Energy Laboratory

To be published in PV Magazine

April 2012

Journal Article

NREL/JA-5200-54498

March 2012
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1 

Part 1 of 2 

Pyranometers and Reference Cells, What’s the Difference?

Jenya Meydbray, Keith Emery, and Sarah Kurtz 

As the photovoltaic industry has grown exponentially in the past decade, large photovoltaic (PV) 

fields have become more common. The investors for these projects calculate the expected return 

on investment based on expected electricity generation and adjust the interest rates and other 

financial terms according to the perceived risk. These calculations usually assume worst case 

according to the listed warranty and any uncertainty in the measurement is translated directly 

into a reduced predicted performance. Because a 1% difference in predicted output could 

represent a large fraction of the expected return on investment, a small reduction in uncertainty 

translates into a much larger value to the entity making investment decisions. To reduce 

perceived risk in large-scale solar investments power plant performance (or production) 

guarantees have become increasingly common. This two part article explores some subtleties of 

accurately measuring PV efficiency in the field.

Weather versus PV efficiency

PV output is determined by two factors: 

1. Weather (solar resource)

2. PV system efficiency 

Investors often seek guarantees from the EPC (Engineering, Procurement and Construction) firm 

and/or PV module manufacturer to protect themselves against underperforming PV modules. 

The PV module manufacturer is in a good position to guarantee the PV module efficiency with 

respect to standard test conditions defined as 25 °C PV temperature, 1000 Wm-2 total irradiance 

and the spectral irradiance given in IEC 60904. The module manufacturer may be able to predict 

the system efficiency but can’t be expected to guarantee the weather and resulting energy 

delivered. These performance guarantees require two pieces of information to accurately assess 

the performance of the PV modules: the amount of useable solar resource (fuel in) and energy 

out. Additionally, the investors typically require measurement of the PV performance 

(efficiency) before full payment. The question then arises about the best way to perform 

measurement of the PV system performance. This article addresses one small part of that 

question: what irradiance sensor will give the lowest uncertainty in characterizing PV 

performance?

As the weather varies, the output of the PV system changes. Key factors affecting the PV system 

performance include not only the irradiance, but also the spectrum, angle of the irradiance, 

temperature, wind speed, and wind direction. Much of this variability is predictable because of 

how all solar cells work, but variations in efficiency over time can occur because of 

manufacturing defects in any of the system components. For example, if the PV module 

develops a poor electrical connection, the associated resistive losses will reduce the efficiency 

the most at high irradiance conditions. Whereas, the emergence of localized shunts in the solar 

cell will result in disproportionately reduced low-light performance. The challenge of 

identifying whether the PV manufacturer meets the terms of the contract is in quantifying 
2 

whether the efficiency varies with the weather in the way we expect it to or in some other way. 

Thus, the ideal irradiance sensor for monitoring PV performance would also vary with the 

weather in a way that mimics how a “good” or “defect-free” PV product is expected to vary. In 

other words, if one can accurately measure the total useable incident irradiance under any 

environmental conditions (fuel in) one can determine if the PV system is generating electricity 

per expectations (energy out). 

Pyranometers

Meteorologists around the world use pyranometers to quantify the sunshine. Pyranometers 

respond to the change in temperature when the sunlight heats a black surface. The pyranometer 

gives a voltage signal that is directly proportional to the irradiance as measured in watts per 

square meter. The pyranometer is specially designed to accept light from all angles, to have a 

flat response to light from the ultraviolet to the far infrared, and to have a stable output regardless 

of sky conditions and changing ambient conditions. A variety of products are on the market 

today, typically with the most stable instruments having the highest prices. Typically

pyranometers are recalibrated about once per year, since the output can drift. A photograph is 

shown in Fig. 1 of a typical state-of-the-art commercial unit. 

Reference cells

PV reference cells can also be used to measure irradiance. However, they work in a very 

different way: photons with energy above the band gap of the PV material are converted directly 

into positive and negative charges that can be collected and used in an external circuit. The 

reference cell generates a current that is dependent on the number and spectral distribution of the 

photons. Typically, the current of the reference cell is measured by measuring the voltage across 

a small resistor that is included in the reference cell package. This voltage is calibrated under the 

reference spectrum [ASTM G173, or IEC 60904-3 spectrum] at 1000 W/m2

, 25 °C using 

standard techniques [IEC 60904-1]. Figure 2 shows an ESTI type reference cell where two half 

cells are utilized to measure irradiance and cell temperature. Figure 3 compares the responsivity 

of thermal based pyranometers with PV sensors and the reference spectrum. Like a PV module a 

reference cell responds to light from all angles, but typically shows an increased reflectance, and, 

therefore, a decreased efficiency for light that arrives at a glancing angle. If a solar reference cell 

is constructed with typical PV cells, glass, encapsulant, and backsheet the spectral and angular 

response will closely match that of the PV modules generating energy in the power plant. The 

reference cell does not respond to photons with energy less than the band gap, implying that they 

will be insensitive to changes in this part of the spectrum when used for quantifying the 

broadband meteorological irradiance. Or, in other words, a reference cell is designed to measure 

the irradiance that is available to a PV module for conversion into electricity (fuel in) rather than 

being designed to measure the broadband irradiance. The close spectral match of the reference 

cell to a PV system minimizes scatter in the data due to variable spectral conditions. The 

attributes that introduce uncertainty when reference cells are used to characterize the weather are 

the same as the attributes that make them more ideal for characterizing the PV system 

performance.
3 

Comparison

Table 1 compares pyranometers and reference cells when they are used for measuring the 

efficiency of PV at reference conditions and when they are used as radiometers (to measure 

weather). Reference cells are especially useful for precise characterization of the PV 

performance, allowing better detection of changes in PV system performance with time and 

shorter time assessment of the PV operating efficiency. However, because predictions of PV 

performance are almost universally based on irradiance data that were derived from pyranometer 

measurements, it may be most useful to simultaneously use reference cells to characterize the PV 

efficiency and pyranometers to compare performance relative to historical-type weather data. 

Both reference cells and pyranometers are calibrated to international standards. ISO 17025 

accredited reference cell and pyranometer calibrations are commercially available. An in-depth 

comparison will be presented in part 2 of this article. Figure 3 shows the output and efficiency of 

different devices across the spectrum of sunlight. Thermopile pyranometers clearly show a very 

flat or broadband output but do not respond in the IR past the transmittance of quartz about 2500 

nm. When utilizing this device for measuring useable incident energy for a PV power plant 

uncertainty is introduced when the spectrum changes because the pyranometer and PV plant 

respond differently to different spectra. Part 2 of this article will explore this uncertainty. 

Financial implications of this uncertainty will also be presented.
4 

Table 1. Comparison of use of solar reference cell and thermopile pyranometer for two 

types of measurements. 

Measurement of PV 

efficiency at reference 

conditions 

Solar Radiometric 

measurement (Weather) 

Reference cell Thermopile 

pyranometer 

Reference Cell Thermopile 

pyranometer 

Spectral 

response

Can be made 

to closely 

match solar 

panel

Broadband 

response needs to 

be corrected

Narrow 

wavelength 

response treated 

as an error

Broadband response 

requires essentially 

no correction

Angle of 

incidence

Can be made 

to closely 

match solar 

panel

Response to all 

angles 

Response falls off 

at > 80°

Glass dome 

provides wide angle 

acceptance 

(especially useful for 

> 80°)

Temperature 

response

Temperature 

response is 

similar to PV 

system; Linear 

temperature

response 

allows for 

corrections

Are designed to 

minimize sensitivity 

to temperature. 

Not corrected for 

temperature

Need to be 

corrected for 

temperature; 

usually treated as 

an error

Are designed to 

minimize sensitivity 

to temperature; 

uncertainty may be 

~1%

Time 

response

< milliseconds; 

matched to PV 

response

Up to 30 

seconds; Can be 

problematic for 

measuring PV 

performance

< milliseconds Slow (up to 30 

seconds) response 

is fine when using 

average irradiance

Other issues Emission to cold 

sky and transients 

in ambient 

temperature affect 

output

Emission to cold sky 

and transients in 

ambient temperature 

affect output

International 

standards 

for 

calibration

IEC 60904 ISO 9847, ISO 

9845, ISO 9846

IEC 60904 ISO 9847, ISO 9845, 

ISO 9846
5 

Fig. 1. Photograph of thermopile pyranometer. Source: NREL

Fig. 2. Photograph of Suntech ESTI type reference cell built and calibrated by PV 

Evolution Labs. Source: PV Evolution Labs.

Fig. 3. Left: The spectral responsivity of several photosensitive devices in units of signal 

per incident watt at a given wavelength. The spectral irradiance is in units of power per 

unit area per unit wavelength.

Right: The quantum efficiency with which each of these devices converts incident

photons to electrons is compared.