Conference Paper

NREL/CP-550-45374

March 2009

Relative Accuracy of 1-Minute 

and Daily Total Solar Radiation 

Data for 12 Global and 4 Direct 

Beam Solar Radiometers 

Preprint 

D.R. Myers and S.M. Wilcox

National Renewable Energy Laboratory

To be presented at the American Solar Energy Society 

Annual Conference 

Buffalo, New York

May 11–16, 2009
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1 

RELATIVE ACCURACY OF 1-MINUTE AND DAILY TOTAL SOLAR RADIATION DATA 

FOR 12 GLOBAL AND 4 DIRECT BEAM SOLAR RADIOMETERS

Daryl Myers 

National Renewable Energy Laboratory

1617 Cole Boulevard, Golden CO

daryl.myers@nrel.gov

Stephen M. Wilcox

National Renewable Energy Laboratory

1617 Cole Boulevard Golden, CO 80401

stephen.wilcox@nrel.gov

We evaluated the relative performance of 12 global and 

four direct beam solar radiometers deployed at a single site

over a 12-month period. Test radiometer irradiances were 

compared with a reference irradiance consisting of either an 

absolute cavity radiometer (during calibrations) or a low 

uncertainty thermopile pyrheliometer (during the 

evaluation period) for pyrheliometers; and for pyranometers 

a reference global irradiance computed from the reference 

pyrheliometer and diffuse irradiance from a shaded 

pyranometer. One minute averages of 3-second data for 12 

months from the test instrument measurements were 

compared with the computed reference data set. Combined 

uncertainty in the computed reference irradiance is 1.8% 

±0.5%. Total uncertainty in the pyranometer comparisons is 

±2.5%. We show mean percent difference between 

reference global irradiance and test pyranometer 1 minute 

data as a function of zenith angle, and percent differences 

between daily totals for the reference and test irradiances as 

a function of day number. We offer no explicit conclusion 

about the performance of instrument models, as a general 

array of applications with a wide range of instrumentation 

and accuracy requirements could be addressed with any of 

the radiometers.

ABSTRACT

1. BACKGROUND

The National Renewable Energy Laboratory (NREL) Solar 

Radiation Research Laboratory (SRRL) has a complement 

of 12 solar radiometers of various designs for measuring 

global horizontal irradiance, and four radiometers which 

report direct beam irradiance1

 

1 See http://www.nrel.gov/midc/srrl_bms

. The 12 pyranometers for 

measuring global horizontal radiation are installed in 

various configurations (unventilated and ventilated) and 

some have empirical corrections applied for cosine response 

[1, 2] and thermal offsets [3, 4] to produce engineering 

corrected data. Table 1 lists radiometer designations, 

manufacturers2

2.

, whether any empirical corrections are 

applied, and the sensor type. Eight pyranometers and all of 

the pyrheliometers use thermopile sensors. Two Yankee 

Environmental System (YES) radiometers use platinum 

resistance thermal detectors (RTD). The remaining 

radiometers, including an Irradiance, Inc. RSR-2 Model 

rotating shadowband radiometer (RSR2), have silicon diode 

sensors. The RSR2 global (unshaded) and diffuse (shaded) 

irradiances were reported as uncorrected, and with 

corrections for cosine and spectral effects described 

elsewhere [5, 6]. The evaluation is subdivided into two 

sections: pyrheliometers (direct beam measurements) and 

pyranometers (global horizontal and diffuse horizontal 

measurements). Unless otherwise noted in table 1, the 

instruments listed measure global horizontal radiation; or 

are designed for Diffuse (Dif) or Direct (Dir) radiation 

sensing. 

The evaluation data set was retrieved from the SRRL

Baseline Measurement System recorded at 1-minute 

averages of 3 sec (0.33 Hz, 20 samples per minute) sampled 

data using two Campbell Scientific CR23X data loggers 

with factory calibrations. The manufacturer’s specified 

accuracy is ±0.025% of Full Scale Range (FSR). 

UNCERTAINTY IN EVALUATION

 

2 No endorsement of manufacturers mentioned is implied
2 

*period of record (4 months) ** period of record (11 months) 

TABLE 1: TEST INSTRUMENTS CONFIGURATION

*** Unventilated pyranometer corrected using ventilated pyrgeometer

FSR is typically ±50 mV, meaning FSR is 100 mV. So in 

the 50 mV range accuracy is ±0.025 mV or 25 microvolts 

(μV) with 1.67 μV resolution. Thus, the data logger 

contribution to uncertainty is ~ ±25 μV ± 1.67 μV which is 

approximately ±26.7 μV. Radiometer responsivities are 

generally about 5.0 μV/ Wm-2to 10.0 μV/ Wm-2, i.e., 1.0 

Wm-2 generates 5 to 10 μV of signal. The 26.7 μV logger 

uncertainty equates to approximately 5 watts to 2.7 watts 

(absolute) in both reference and test instrument data, or 

1.0% to 0.5% of reading at midrange values of 500 Wm-2, 

and 2% or 1% of 1000 Wm-2 readings. We will be 

conservative and select the average of the larger 

uncertainties to be 1.5%, for the data logger overall 

uncertainty.

Combined uncertainty in the computed reference irradiance 

is estimated from the weighted contribution of the beam 

and diffuse components, ranging from ±1.0 % for 

extremely clear (nearly all direct beam) to ±2.5% overcast 

(all diffuse, or high zenith angles). The mean of the 

250,981 1-minute reference uncertainties was 1.8% ±0.5%. 

Total uncertainty in the comparisons is the square root of 

the mean squared uncertainties, plus twice the standard 

deviation squared, plus the logger uncertainty contribution 

squared or (1.82

+(2*0.5)2 + 1.52)

0.5 = ±2.5% . Thus 

reference and data irradiances within ±2.5% of each other 

are considered identical.

3. EVALUATION OF PYRHELIOMETERS

The absolute cavity radiometer (Eppley Automatic Hickey 

–Freidan, AHF) is the reference for NREL calibrations. For 

this work the AHF is presumed to be the pyrheliometer 

with the lowest uncertainty of ±0.4% [1, 7, 8]. Although a 

full year of data is desired, it is not practical to deploy the 

cavity full time because its aperture is open to the elements 

and internal components are subject to water or soiling. We 

collected reference cavity radiometer data from all NREL 

calibration events from 2002 to 2007 and merged this 

reference data with three pyrheliometers from the SRRL 

baseline system (Eppley NIPs 25792E6 and 31137E6 and 

Kipp & Zonen CH1 010256). Known and accepted 

uncertainties in the NREL calibrations are similar to 

expected differences among pyrheliometers, so if there 

were different responsivities in use for an instrument in the 

different calibration events over the study period, these 

responsivities were averaged. This removed noise effects of 

calibration uncertainty and also provided the opportunity to 

detect sensitivity drift with time (none was observed). 

The first portion of the pyrheliometer study used only clear 

sky conditions as mandated by the calibration events. Then 

RSR2 and NIP 1-minute direct beam data were compared to 

the CH1 beam data for the whole year.

Instrument Manufacturer Configuration Sensor 

CM22_V Kipp & Zonen Ventilated Thermopile

CM6B Kipp & Zonen Unventilated Thermopile

CM3 Kipp & Zonen Unventilated Thermopile 

PSP_V_C Eplab Model 

PSP

pyranometer 

Ventilated 

thermal offset; 

cosine corrected

Thermopile

PSP_C Eplab Unventilated 

thermal offset; 

cosine corrected

Thermopile

PSP_V_U Eplab PSP Ventilated 

Uncorrected 

Thermopile

PSP_U Eplab Uncorrected Thermopile

TSP700 YES Ventilated RTD

TSP1 YES Ventilated RTD

SPN1 (Glo)*

Delta-T 

Uncorrected Multiple

SPN1 (Dir)* Thermopiles 

SPN1 (Dif)*

SP_LITE Kipp & Zonen Uncorrected Silicon (Si) 

LI200_TOT Licor Uncorrected Si diode

RSR2_C 

(Glo)** 

Irradiance 

Licor

Corrected 

Global 

Si diode

RSR2_C 

(Dir)** 

Computed from 

corrected Glo & 

Diffuse 

RSR2_C 

(Dif) 

Corrected

RSR2_U 

(Glo)** 

Uncorrected 

RSR2_U 

(Dir)** 

Computed from 

uncorrected Glo 

& Dif

RSR2_U 

(Dif)** 

Uncorrected

31137E6 Eplab NIP DIR Uncorrected Thermopile

25792E6 Eplab NIP DIR Uncorrected Thermopile

CH1 010256 Kipp Zonen

DIR

Uncorrected

Thermopile
3 

Fig. 1. Percent difference from AHF Pyrheliometer for three 

test pyrheliometers during clear sky calibrations

Table 2 summarizes the mean percent difference of each of 

the three pyrheliometers from the reference cavity 

pyrheliometer and standard deviation (in percent) as a 

function of zenith angle. The zenith angle numbers represent 

the center of 10° wide zenith angle bins. The smaller values 

at 45° Z result because the responsivity at Z=45° is used to 

convert the pyrheliometer signal to irradiance. The larger 

deviation of the NIP pyrheliometer at higher zenith angles 

as opposed to the relatively constant CH1 response is 

apparent. Figure 2 plots the data in table 2. 

3.1 Results for Pyrheliometer Cavity Comparisons

Figure 1 shows a composite of all deviations, in percent, of 

test pyrheliometers from the AHF reference cavity during 

calibrations in the period from 2002 to 2007. Note the CH1 

deviations in the lowest layer of the plot are relatively flat, while 

the NIP deviations show a greater deviation throughout a day. 

Zenith 

Bin

TABLE 2: AVERAGE PERCENT DIFFERENCE AND 

STANDARD DEVIATION BETWEEN TEST AND 

CAVITY PYRHELIOMETERS

CH1 

Avg% 

σ

% 

25792E6 

Avg % 

σ

% 

31137E6 

Avg% 

σ

% 

15 0.24 0.40 -0.29 0.49 -0.53 0.35

25 0.12 0.42 -0.17 0.48 -0.44 0.42

35 0.12 0.42 -0.13 0.47 -0.22 0.47

45 0.18 0.43 0.01 0.54 -0.06 0.55

55 0.15 0.44 0.13 0.54 0.06 0.55

65 0.10 0.46 0.39 0.56 0.27 0.56

75 0.18 0.45 0.79 0.70 0.46 0.60

85 0.22 0.48 1.04 0.80 0.48 0.82

Fig 2. Zenith angle dependence of pyrheliometer percent 

differences and standard deviations with respect to reference 

AHF pyrheliometer.

3.2 Direct Comparison with CH1 Pyrheliometer 

This study used the 1-minute monitored data for all sky 

conditions over a year. Eppley NIP 31137, Kipp and Zonen 

CH1 010256, direct beam from the RSR2 and direct beam 

from a Delta-T, Inc. SPN1 radiometer were studied. The 

RSR2 data is provided in two forms, with and without 

corrections for spectral and cosine effects. The correction 

scheme is provided by the manufacturer based on published 

results [5, 6]. The SPN1 direct beam data is computed from 

shaded (diffuse) and unshaded (global) sensors mounted in 

the SPN1 instrument, using the manufacturer’s calibration 

factor. Table 3 shows the difference between the CH1 and 

NIP pyrheliometer 25792E6 1-minute data over the year, as 

a function of zenith angle. Table 4 summarizes the mean 

percent difference and standard deviation for the corrected 

and uncorrected RSR2 computed 1-minute direct beam data.

Zenith bin 

TABLE 3: AVERAGE PERCENT DIFFERENCE 

BETWEEN CH 1 AND NIP 25792E6 ONE MINUTE 

DATA BY ZENITH ANGLE

Count 

(N)

Mean 

% σ %

15 4447 -0.20 1.71 

25 14162 0.01 2.21 

35 17701 0.11 2.13 

45 21586 0.14 1.91 

55 28766 0.23 1.96 

65 36135 0.33 2.06 

75 27069 0.37 1.83 

85 19672 0.06 1.83 
4 

TABLE 4: UNCORRECTED (U) AND CORRECTED

(C) PERCENT DIFFERENCE BETWEEN RSR2 and 

CH1 DIRECT BEAM VERSUS 

Zenith 

Angle

Z. 

Count 

(N) 

RSR2

U 

Mean

StDev 

(sd) 

%

Count 

(N) 

RSR2 

C 

Mean 

StDev 

(sd) 

%

15 2599 1.25 16.8 2504 -3.51 17.0 

25 17944 0.50 20.2 12387 -6.90 19.4 

35 17944 0.50 20.2 16643 -7.51 18.4 

45 22645 0.98 18.9 20957 -7.25 17.1 

55 31499 1.02 17.6 29369 -6.61 15.9 

65 40494 0.63 18.6 37428 -6.19 16.3 

75 30449 2.87 20.4 27961 -5.02 16.5 

85 22925 -2.24 33.0 18818 -9.56 25.1 

Next, monthly mean daily totals (MMDT) for the reference 

pyrheliometers and the test direct beam data were computed. 

Table 5 shows the percent difference between the reference 

(CH1) MMDT and NIP 25792E6, the RSR2 corrected (C) 

and uncorrected (U) beam data, and the SPN1 beam data for 

the months available.

Month

TABLE 5: NIP, RSR2, AND SPN1 MMDT PERCENT 

DIFFERENCE FROM CH1 MMDT

CH1 

MMDT 

Whm-2

NIP % ∆ RSR2 U 

% ∆

RSR2

C 

% ∆

SPN1 

DIR 

% ∆ 

Jun-07 7063 -0.33 ---- ---- ----

Jul-07 6109 1.15 3.37 -0.59 ----

Aug-07 5540 -0.28 2.74 -3.74 ----

Sep-07 5322 -0.18 1.46 -4.58 ----

Oct-07 5936 -0.03 1.47 -4.19 ----

Nov-07 5021 0.42 0.45 -5.13 ----

Dec-07 4072 0.05 0.76 -3.86 ----

Jan-08 4922 -0.21 2.99 -3.90 ----

Feb-08 4799 0.27 -2.07 -7.28 -7.6

Mar-08 5140 0.39 -1.46 -8.81 3.9

Apr-08 6648 -0.33 -0.64 -4.60 15.4

May-08 5189 ---- 1.80 -4.00 6.0

These results show that the basic accuracy of pyrheliometers 

under the very best of conditions (clear sky calibration 

conditions) can be less than 0.5% with respect to an absolute 

cavity radiometer. Under general conditions, bias errors may 

be around 0.5% between thermopile instruments, but the 

standard deviation of differences can approach 2%, and total 

uncertainty about 2.5%. The RSR2 computed direct beam 

irradiances are on the order of 3% to 6% different. The 

larger RSR2 standard deviations (~ 15% to 20%) may be

due to the orders of magnitude difference in time response 

of the silicon detectors (milliseconds) and the thermopile 

reference unit (1 to 2 seconds) under variable sky 

conditions. MMDT for thermopile pyrheliometers are 

comparable to better than 1%; however the RSR2 MMDT 

difference with the reference pyrheliometer was 3 to 9 times 

larger. The SPN1 direct data could be off by as much as 

15%. 

4. PYRANOMETER COMPARISONS

Data for the pyranometer study were collected under all sky 

conditions throughout a year (data quality filtering removed 

about 5% of the data, and some instruments as noted had a 

short period of record). Two independently evaluated 

instruments were used to create a computed global 

hemispherical irradiance data set against which the 

pyranometers’ measured global hemispherical irradiance 

was compared. A second set of two independently evaluated 

instruments were used to validate and quality assess (QA) 

the reference data set and reject data inconsistent among 

those four instruments (computed global and QA 

instruments, respectively)[9]. The reference irradiance was 

computed from direct beam * cos(Z) + diffuse, with Direct 

Beam from Kipp & Zonen CH1 Serial No. 010256 

(uncertainty +0.60% - 0.56% for 30°≤ Z ≤60°) and for 

diffuse: Eppley 8-48 Serial No. 32331 (Uncertainty: +1.95% 

-1.89% for 0°≤ Z ≤60°).

4.1 Pyranometer 1-minute and MMDT Results

Table 6 is a summary of the percent difference between the 

computed reference irradiances and the instrument 1 minute 

and MMDT data. The first two columns of Table 6 are for 

the 1-minute data for each instrument or instrument 

configuration over all zenith angle ranges. Columns 3 and 4 

of Table 6 show the range of percent bias error in MMDT

(over 12 possible months) and the annual average daily total 

with respect to the computed reference irradiance. Since the 

bias and standard deviation changed as a function of zenith 

angle (and hence by the month of the year) ranges are

shown, from minimum to maximum magnitude. Note the 

RSR2 direct and diffuse, and SPN1 diffuse data are also 

included in this table. Typically the minimum bias with 

respect to zenith angle is for the low zenith angle data 

(summer), and maximum bias occurs with the largest zenith 

angle bin (winter) data. 

The majority of the pyranometers (9 of the 13, counting 

RSR2 as 1 radiometer) have a bias between ±1% and ±5% 

from the reference irradiance. These are the CM22 

Ventilated, CM6B, PSP Ventilated and Corrected, TSP1, 

TSP700, SP LITE, LI200, and RSR2 Corrected and 

Uncorrected. Standard deviation of the differences from the 

1-minute reference irradiance is less than ±6% for 6 units: 
5 

ventilated CM22, TSP700, RSR2 corrected and uncorrected, 

and PSP ventilated and corrected.

Unit

TABLE 6: ONE MINUTE AND MMDT MEAN % 

DIFFERENCE AND STANDARD DEVIATIONS

1-min 

Mean 

% ∆

1-min 

Sigma

% 

Monthly 

% Bias 

Range

Annual 

Mean

% Bias

CM22_V +0.6 

+0.4

2 to 3 +0.8

+0.2

+0.5

CM6B +0.8

-0.3

4 to 7 +0.9

-2.0

+0.2

CM3 +0.1

-7.4

2 to 7 +0.4

-1.8

-1.2

PSPV_C *** +0.0

-3.1

2 to 6 -0.1

-5.1

-2.1

PSP_C +5.7

-0.3

2 to 11 +1.2

-3.3

+0.8

PSP_V_U +1.6

-11.3

2 to 8 +0.0

-8.4

-3.8

PSP_U +2.5

-7.5

2 to 9 +0.4

-10.3

-2.8

TSP700 -0.8

-2.5

4 to 6 +1.5

-0.0

+1.1

TSP1 +3.2 

+0.4

2 to 12 +4.6

-0.4

+2.3

SPN1 

(Glo)* 

-0.3

-3.7

4 to 7 -0.7

-6.0

N/A

SPN1 (Dif)* -13.8

-4.3

7 to 11 -12.9

-10.4

N/A

SP_LITE +1.4 

+0.8

4 to 7 +1.4

+0.0

+1.4

LI200_TOT +2.8

-2.0

3 to 8 +1.9

-2.0

+0.9

RSR2_C 

(Glo)** 

+1.0

-1.2

4 to 6 +0.5

-2.5

-0.7

RSR2_C 

(Dif) 

-0.2

+3.0

5 to 6 -1.1

+3.0

+1.0

RSR2_U 

(Glo)** 

-0.8

-2.5

4 to 6 -0.8

-3.6

-2.2

RSR2_U 

(Dif)**

-12.0

-15.0

13 to 

16

-19.3

-19.6

-14.1

25792E6 +0.8

-0.3

0.5 to

0.7 N/A (wrt Cavity)

31137E6 +0.5

-0.5

0.4 to 

0.6

N/A (wrt Cavity)

CH1 10256 +0.2 

+0.1

0.4 to 

0.5

N/A (wrt Cavity)

*period of record (4 months) ** period of record (11 months) 

*** Unventilated pyranometer corrected using ventilated pyrgeometer

We compared the daily total global irradiance for the 

reference irradiance, and 14 test global irradiances. The 

percent mean difference and standard deviations of the 

differences are shown in Table 7. Note the RSR and SPN1 

direct beam data are also shown, with respect to the CH1 

daily totals. 

TABLE 7: PERCENT DFFERENCES BETWEEN 

TEST AND REFERENCE DAILY IRRADIANCE 

TOTAL

Unit

S

N Mean 

% Bias

Sigma % 

CM22_V 366 0.5 0.8

CM6B 366 0.2 2.8 

CM3 366 -1.2 2.8 

PSP_V_C 366 -2.1 2.1 

PSP_C 366 0.8 5.9 

PSP_V_U 366 -3.9 4.8 

PSP_U 366 -2.8 6.1 

TSP700 366 1.1 2.2

TSP1 366 2.3 4.5

GLO_SPN1 152 -2.9 3.1 

SP_LITE 366 1.3 2.7 

LI200_TOT 366 0.9 2.9 

DIR_SPN1* 144 4.1 17.8

RSR2_U (Glo)** 341 -2.2 2.8 

RSR2_U (Dir)** 329 1.0 9.3

RSR2_C (Glo)** 341 -0.7 3.0

RSR2_C (Dir)** 330 -4.6 10.5

*period of record (4 months) ** period of record (11 months)

Example plots of individual instrument percent differences 

between 1 minute data from the reference irradiance as a 

function of zenith angle and the percent difference in daily 

totals as a function of day of the year are shown in Figure 3. 

Comparable plots for each radiometer are provided in the 

NREL technical report which is the basis of this paper [10].

Table 8 shows the percent difference between monthly daily 

totals for each global radiometer with respect to the 

reference irradiance by month. Those cells highlighted in 

gray are where the difference between the test and reference 

monthly daily total exceeds 2.5%, the assumed uncertainty 

in our evaluation process from section 2. Averaging (hourly, 

daily totals and averages, monthly mean and annual mean 

daily totals) may help cancel out some of the random

variability between instruments. However, as Figure 3 and 

table 8 demonstrate, there is still the possibility of 

systematic deviations from the best measurements 

exceeding 5% by members of this family of instruments.
6 

Fig. 3. Example plots of percent difference between 1-minute reference irradiance and test instrument irradiance as a 

function of zenith angle and monthly daily total percent differences as a function of day of the year.
7 

TABLE 8: MONTHLY MEAN DAILY TOTAL PERCENT DIFFERENCES FROM REF GLOBAL HORIZONTAL 

(CH1 + DIFF) 

JUN 

2007 

JUL

2007 

AUG 

2007 

SEP

2007 

OCT

2007 

NOV

2007 

DEC

2007 

JAN

2008 

FEB

2008 

MAR 

2008 

APR

2008 

MAY

2008 

GLO_REF 

Wh/m-2 6749 6473 5386 4503 3881 2306 2211 2500 3467 4597 6133 6001

CM22_Vent 0.27 0.20 0.18 0.32 0.37 0.20 0.06 0.32 0.53 0.77 0.61 0.69

CM6B 0.29 0.23 0.12 0.25 0.14 0.26 -1.96 -0.24 0.75 0.87 0.23 0.55 

CM3 -1.61 -1.24 -1.58 -1.34 -1.21 -1.84 -1.64 -0.58 -0.19 0.39 0.35 0.23 

PSP_V_C -0.03 -0.06 -0.63 -1.39 -2.74 -4.00 -5.13 -5.08 -3.23 -2.10 -1.38 -0.47 

PSP_C 0.51 0.66 0.61 0.90 0.90 -1.46 -3.27 0.80 1.70 1.17 0.45 0.57 

PSP_V_U -0.04 0.01 -1.11 -2.65 -4.98 -7.46 -8.38 -8.22 -5.47 -3.32 -1.93 -0.69 

PSP_U 0.21 0.42 -0.42 -1.45 -3.35 -8.29 -10.31 -5.99 -3.01 -1.35 -0.83 0.18 

TSP700 0.67 0.30 0.54 0.75 0.83 1.253 -0.01 1.20 1.49 1.50 0.86 0.57 

TSP1 -0.21 -0.31 -0.27 0.42 1.34 2.72 -0.41 3.79 4.59 4.60 3.66 2.16 

GLO_SPN1 ---- ---- ---- ---- ---- ---- ---- -6.04 -4.04 -1.70 -0.67 -0.98 

RSR_UNCOR --- -1.58 -2.76 -3.14 -2.59 -2.56 -3.59 -2.5 -2.27 -0.99 -1.55 -0.81 

RSR_CORR --- -1.13 -2.42 -2.54 -1.42 -0.48 -0.78 0.08 -0.32 0.53 -0.30 0.34 

SP_LITE 0.79 0.57 0.82 1.04 1.18 1.18 1.39 0.74 0.98 0.92 0.02 0.81 

LI200_TOT 1.81 1.65 1.31 0.69 0.52 -0.35 -2.01 -0.21 0.89 1.63 1.70 1.92 

5. SUMMARY

At the NREL SRRL location, all pyranometer data based 

on a single calibration factor (sometimes in combination 

with corrections for cosine response and infrared offsets) 

agree to within 5% for zenith angles in the range of 30° to 

60° on a minute by minute basis. Various cosine 

responses for the radiometers, in conjunction with a single 

responsivity (at 45° zenith angle ±0.2°) may lead to lack 

of agreement, exceeding 20% at Z=70°, and more than 

20% at Z=80°. Correction and ventilation techniques 

applied to the data or instruments may not always 

improve the situation, but may add additional sources of 

uncertainty. The ±5% range of agreement observed for 

zenith angles less than 60° is typical of the historically 

quoted uncertainty in sub-hourly pyranometer data over 

the past 30 years. Averaging may cancel some of the 

random variability between instruments, but differences 

exceeding 5%, especially in winter, are possible among 

this family of instruments. Pyrheliometers differ by up to 

±2% on a 1 minute and averaged data basis.±2% 

differences observed among thermopile pyrheliometers is 

typical of often quoted uncertainties in direct beam data. 

6.

 

ACKNOWLEDGMENTS

The physical measurement capability and data quality at 

the NREL SRRL is maintained by A. Andreas, P. Gotseff, 

B. Kay, I. Reda, T. Stoffel, and S. Wilcox. This work was 

supported by the U.S. Department of Energy under 

Contract No. DE-AC36-99GO10337 with the National 

Renewable Energy Laboratory. 

7. REFERENCES

1. Michalsky, J.J.; Harrison, L.C.; and Berkheiser, 

W.E., “Cosine Response of some Radiometric and 

Photometric Sensors”, Solar Energy Vol. 54 No. 6, 

pp. 397-402, 1995. 

2. Myers, D. R.; Stoffel, T. L.; Reda, I.; Wilcox, S. M.; 

Andreas, A. M., "Recent Progress in Reducing the 

Uncertainty in and Improving Pyranometer 

Calibrations", ASME Journal of Solar Energy 

Engineering (124:1), pp. 44-50, 2002.

3. Reda, I.; Hickey, J.; Long, C.; Myers, D.; Stoffel, T.; 

Wilcox, S.; Michalsky, J.J.; Dutton, E.G.; and 

Nelson, D., “Using a Blackbody to Calculate Net 

Longwave Responsivity of Shortwave Solar

Pyranometers to Correct for Their Thermal Offset 

Error during Outdoor Calibration Using the 

Component Sum Method”, Journal of Atmospheric 

and Oceanic Technology, Vol 22, pp. 1531-1540, 

2005. 

4. Gulbrandsen, A., "On the Use of Pyranometers in the 

Study of Spectral Solar Radiation and Atmospheric 

Aerosols”, Journal of Applied Meteorology, Vol 17, 

pp. 899-904, 1978.
8 

5. Zhou, C.; Michalsky, J.; Harrison, L., “Comparison 

of Irradiance Measurements Made With the Multifilter Rotating Shadowband Radiometer and First 

Class Thermopile Radiometers”, Solar Energy, Vol. 

55; No. 6, pp. 487-491, 1995.

6. Vignola, F., “Removing Systematic Errors from 

Rotating Shadowband Pyranometer Data”, paper

A090.pdf in Proceedings of Solar 2006, July 9-13 

Denver, CO. (CD-ROM), American Solar Energy 

Society, Boulder, CO. 

7. WMO (1996), International Pyrheliometer 

Comparison VIII,

8. Reda, I.; Stoffel, T. L.; Myers, D. R., “Calibration of 

a Solar Absolute Cavity Radiometer with 

Traceability to the World Radiometric Reference”, 

NREL/TP-463-20619, Golden CO: National 

Renewable Energy Laboratory, 1996. 

World Meteorological 

Organization, Geneva, Switzerland. 

9. Maxwell, E.; Wilcox, S.; Rymes, M. (1993), Users 

Manual for SERI_QC Software: Assessing the 

Quality of Solar Radiation Data, NREL Report No. 

TP-463-5608, Golden, CO: National Renewable 

Energy Laboratory. 

10. Wilcox, S. M.; Myers, D. R. (2008), Evaluation of 

Radiometers in Full-Time Use at the National 

Renewable Energy Laboratory Solar Radiation Research 

Laboratory, pp. 57; NREL Report No. TP-550-44627, 

Golden, CO: National Renewable Energy Laboratory. 

 
F1147-E(10/2008) 

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March 2009

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4. TITLE AND SUBTITLE

Relative Accuracy of 1-Minute and Daily Total Solar Radiation Data 

for 12 Global and 4 Direct Beam Solar Radiometers

5a. CONTRACT NUMBER

DE-AC36-08-GO28308

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6. AUTHOR(S)

D.R. Myers and S.M. Wilcox

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NREL/CP-550-45374

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PVD93110

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14. ABSTRACT (Maximum 200 Words)

This report evaluates the relative performance of 12 global and four direct beam solar radiometers deployed at a 

single site over a 12-month period. Test radiometer irradiances were compared with a reference irradiance consisting 

of either an absolute cavity radiometer (during calibrations) or a low uncertainty thermopile pyrheliometer (during the 

evaluation period) for pyrheliometers; and for pyranometers a reference global irradiance computed from the 

reference pyrheliometer and diffuse irradiance from a shaded pyranometer.

15. SUBJECT TERMS

NREL; National Renewable Energy Laboratory; Solar Radiation Laboratory; SRRL; radiometer; pyrheliometer; 

pyranometer

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