nut are solar radiation data? 

Solar radiation data provide information 

on how much of the sun's energy strikes a 

surface at a location on earth during a particular time period. The data give values of 

energy per unit of area. By showing naturally occurring changes in the amount of solar 

radiation over the course of days, months, 

and years, these data determine the 

amount of solar radiation for a location. 

The units of measurement are expressed 

as kilowatt-hours per square meter 

(kWh / m2), megajoules per square meter 

(Mj/m'), langleys (L), or British thermal 

units per square foot (Btu/ft'). 

Today, the primary source of solar radiation data for the United States comes from 

measurements made by the National 

Weather Service at 26 SOLMET (SOlar 

METeorological) stations from 1952 to 

1975. In addition, mathematical models 

estimated data for 222 ERSATZ (synthetid 

stations where no solar radiation measurements were made. Because the equipment 

did not always accurately measure the 

solar radiation and the models used. were 

limited in their application, the data do not 

always correlate well with 

more recent field measurements. To provide better data, 

we developed a National 

Solar Radiation Data Base. 

This data base covers 30 years 

(1961-1990) and comes from 

information recorded by 

more accura te instruments 

and from better models. In 

1992, this new data base will 

be available for 250 sites. 

4 
" SOlMET • ERSATZ 

From 1952 to 1975 solar radiation was measured 

at 26 SOLMET stations ( .. ) and modeled for 222 

ERSATZ stations ( • ). Most of these stations will 

be included in the new National Solar Radiation 

Data Base. 

Guantanamo Bay, Cuba 

Koror Island. Pacilic 

Kwajalein ISland. Pacific 

San Juan. Puerto Rico 

Wake Island. Pacilic 

5 
do we need solar radiation data? 

The earth receives a vast amount of energy 

from the sun in the fonn of solar radiation. 

If we converted to usable energy just 0.2% 

of the solar radiation that falls on our na· 

tion, we would meet the energy demand of 

the entire United States. A variety of solar 

energy technologies are being developed to 

harness the sun's energy including: 

• solar electric (photovoltaid for converting sunlight directly into electricity; 

• solar heat (thennaD for heating water for 

industrial and household uses; 

• solar thermal electric for producing 

steam to run turbines that generate 

electricity; 

• solar fuellechnologies for converting 

biomass (plants, crops, and trees) into 

fuels and by-products; 

• passive solar for lighting and heating 

buildings; and 

• solar detoxification for destroying hazardous waste with concentrated sunlight. 

6 

"The more accurately rue knolU the 

solar resource, the better we call 

optimize the system. Therefore, 

accurate soffir radiatio1J data are 

011 important factor ill solar 

system design." 

o.wld F. u.nkuc:d 

~ NationIII..abonIIOrte. 

The economics of these technologies 

depend on the equipment and operating 

costs, the percentage of the soL.1r radiation 

that can be converted into the desired 

energy product, and the amount of solar 

radiation available. Users of these technologies need high-quality solar radiation 

data. If the actual solar radiation for a location is less than indicated by available data, 

the perfonnance and the economic goals 

for the system will not be met. On Iheother 

hand, if the actual solar energy at a location 

is greater than indicated by the data, the 

perfonnance and economic projections 

may be 100 conservative and prevent a 

viable technology from being used. 

To minimize energy consumption, heating and air conditioning engineers also 

use solar radiation data 10 select building 

configurations, orientations, and air conditioning syslems. Because energy costs 

are a significant expense in building ownership, an energy-efficient design can significantly reduce the life-cycle cost of a 

building. 

The amount of solar radiation received 

changes throughout the day and year 

due to weather patterns and the changing 

position of the sun. and solar radiation 

data reflects this variability. By knowing 

the variability, we can size storage systems 

so they can provide energy at night and 

during cloudy periods. For technologies 
) 

Because of absorption and scattering by the atmosphere, the spectral distribution of solar radiation 

outside the ahnosphere differs significant1y from 

thai on earth. Also, the spectral dislribut.ion on 

earth changes throughout the day and year and 

is influenced by location, climate, and atmospheric 

conditions. Consequently, the percentage of energy 

that is composed of UV, visible, or neal'-infrared 

radialion, or portions thereof, 3.150 varies by 

location, time of day, and year. 

with no energy storage, we can eva ll1ate 

load matching by comparing the profile of 

the available solar radiation throughout the 

day with the profil~ of the energy required 

by the load. Solar radiation data also help 

determine the best geographic loca tions for 

solar energy technologies. Other factors 

being equal, a site receiving more solar 

radiation will be more economical. 

For certain technologies, we also need to 

know the spectral, or wavelength, distribution of the solar radiation. For example, 

photovoltaic devices respond primarily to 

wavelengths in the visible and near-infrared region of the spectrum, while solar 

detoxifi cation uses energy from the 

ultraviolet (UV) region. Location, climate, 

and atmospheric conditions influence the 

spectral distribution of solar radia tion. 

.. • .!.. " . J_ )-' 

- u_ '~ 

Th is remote wate .... leve l-monitoring station uses photovoliaics for charging 

storage batteries that supply electric power. Solar radiation data provide 

information for determining the size of the photovoltaic and battery system 

needed to supply remote stations like this with reliable elecb'ic service. 

7 
nu~ influences the amount of solar radiation? 

The amount of solar radiation reaching the 

earth's surface varies greatly because of 

changing atmospheric conditions and the 

changing position of the sun, both during 

the day and throughout the year. Clouds 

are the predominant atmospheric condition 

that detennines the amount of solar radiation that reaches the earth. Consequently, 

regions of the nation with cloudy climates 

receive less solar radiation than the cloudfree desert climates of the southwestern 

United States. For any given location, the 

solar radiation reaching the earth's surface 

decreases with increasing cloud cover. 

Local geographical features, such as 

mountains, oceans, and large lakes, influence the formation of douds; therefore, 

the amount of solar radiation received for 

these areas may be different from that 

received by adjacent land areas. For example, m01.Ultains may receive less solar 

radiation than adjacent foothills and plains 

located a short distance away. Winds blowing against mountains force some of the air 

to rise, and douds fonn from the moisture 

in the air as it cools. Coastlines may also 

receive a different amount of solar radiation than areas further inland. Where the 

changes in geography are less pronounced, 

such as in the Great Plains, the amount of 

solar radiation varies less. 

8 
Many atmospheric scientists 

believe that the eruption of 

MOllnt Pinatubo in June 1991 

will have worldwide effects 

during the next few years. 

This was one of the largest 

volcanic eruptions of the 20th 

century. Volcanic ash and 

sulfur dioxide spewed high 

above the Philippines and 

into the stratosphere; the 

resulting dust cloud spread 

around the earth's equator 

and toward higher latitudes. 

The increased dust diminishes 

the solar radiation received 

at the earth's surface. Peak 

effects will occur in 1992, 

but colder winters and 

cooler summers may ensue 

until near the middle of 

this decade. Long-term 

measurement of solar 

radiation at numerous sites 

permits naturally occurring 

events such as this to be 

evaluated with respect to 

their impact on the solar 

resource and the climate. 
Clouds are the predominant atmospheric condition that detennines the amount 

of solar radiation reaching the earth. 

10 

The amount of solar radiation also varies 

depending on the time of day and the 

season. In general, more solar radiation is 

present during midday than during either 

the early morning or late afternoon. At midday, the sun is positioned high in the sky 

and the path of the sun's rays through the 

earth's atmosphere is shortened. Consequently, less solar radiation is scattered or 

absorbed, and more solar radiation reaches 

the earth's surface. In the northern hemisphere, south-facing collectors also receive 

more solar radiation during midday because the sun's rays are nearly perpendicular to the collector surface. Tracking 

collectors can increase the amount of solar 

radiation received by tracking the sun and 

keeping its rays perpendicular to the collector throughout the day. In the northern 

hemisphere, we also expect more solar 

radiation during the summer than during 

the winter because there are more daylight 

hours. This is more pronounced at higher 

latitudes. 

Both man-made and naturally occurring 

events can limit the amount of solar radiation at the earth's surface. Urban air pollution, smoke from forest fires, and airborne 

ash resulting from volcanic activity reduce 

the solar resource by increasing the scattering and absorption of solar radiation. This 

has a larger impact on radiation coming in 

a direct line from the sun (direct beam) 

than on the total (global) solar radiation. 

Some of the direct beam radiation is scattered toward earth and is called diffuse (sky) 

radia tion (global = direct + diffuse):Consequently, concentrators that use only direct 

beam solar radiation are more adversely 

affected than collectors that use global solar 

radiation. On a day with severely polluted 

air (smog alert), the direct beam solar radiation can be reduced by 40% whereas the 

global solar radiation is reduced by 15% to 
25%. A large volcanic 

eruption may 

decrease, over a 

large portion of the earth, 

the direct beam solar radiation 

by 20% and 

the global solar radiation by 

nearly 10% for 

6 months to 2 years. As the volcanic ash 

falls out of the atmosphere, the effect is 

diminished, but compl

ete removal of the 

ash may take se

veral years. 

11 
parts of solar radiation are measured? 

The total or global solar radiation striking a 

collector has two components: (1) direct 

beam radiation, and (2) diffuse radiation. 

Additionally, radiation reflected by the 

surface in front of a collector contributes to 

the solar radiation received. But unless the 

collector is tilted at a steep angle from the 

horizontal and the ground is highly reflective (e.g., snow), this contribution is small. 

As the name implies, direct beam radiation comes in a direct line from the sun. 

For sunny days with clear skies, most of 

the solar radiation is direct beam radiation. 

On overcast days, the sun is ob~ by 

the clouds and the direct beam radiation 

is zero. 

Diffuse radiation is scattered out of the 

direct beam by molecules, aerosols, and 

douds. Because it comes from all regions 

of the sky, it is also referred to as sky radiation. The portion of total solar radiation 

that is diffuse is about 10% to 20% for clear 

skies and up to 100% for cloudy skies. 

Some of the solar radiation entering the earth's abnosphere is absorbed and scattered. Direct beam 

radiation comes in a direct line from the sun. Diffuse radiation is scattered out of the direct beam by 

molecules, aerosols, and clouds. The sum of the direct beam, diffuse, and ground-reflected radiation 

arriving at the surface is caUed total or global solar radiation. 

12 
The type of data needed and the funds 

available help determine the number and 

kinds of instruments used at a site to 

measure solar radiation. A complete solar 

radiation monitoring station has instrumentation for measuring three quantities: 

(1) total or global radiation on a horizontal 

surface, (2) diffuse radiation on a horizontal 

surface, and (3) direct beam radiation. 

Measuring all three quantities provides 

sufficient information for understanding 

the solar resource and for rigorous quality 

assessment of the data. Any two of the 

measured quantities can be used to calculate a range of acceptable values for the 

third. Many monitoring stations also have 

equipment for measuring solar radiation on 

tilted and tracking surfaces and for measuring meteorological parameters such as ambient temperature, relative humidity, and 

wind speed and direction. 

A station with a lower level of funding 

may only measure two quantities; the third 

is calculated. For example, the direct beam 

component can be derived by subtracting 

the diffuse radiation from the global 

radiation and applying trigonometTic 

relationships to account for the position 

of the sun. The trade-off for this approach is 

that the calculated direct beam data are less 

accurate than if the direct beam data were 

measured . . 

HistOrically, many stations have 

measured only the global radiation on a 

horizontal surface. This necessitates calculating both the diffuse and direct beam 

solar radiation, which results in less 

accurate values for these two quantities 

than if they were measured. 

In the absence of any solar radiation 

measurements, we employ models using 

meteorological data such as cloudiness 

and minutes of sunshine to estimate solar 

radiation. Although much less accurate, 

this is often the only option we have for 

locations where solar radiation is not 

measured. Cloudiness data are based on 

observations by a trained meteorologist 

who looks at the sky and estimates the 

amount of cloud cover in tenths. A dear 

sky rates a cloud cover value of 0 tenths, 

and an overcast sky rates a cloud cover 

value of 10 tenths. Minutes of sunshine are 

recorded by an instrument that measures 

the time during the day when the sun is 

not obscured by clouds. 

To investigate the spectral distribution 

of solar radiation, an instrument called a 

spectroradiometer measures the solar 

radiation intensity at discrete wavelengths. 

Spectroradiometers are complex and 

relatively expensive instruments, and 

their operation and maintenance require 

Significant effort. Consequently, spectroradiometers are not routinely used for 

long-tenn data collection. Rather, they 

help establish data bases that have sufficient information to validate models that 

predict the spectral distribution based 

on meteorological data and the position 

of the sun. 

13 


ow do we use solar radiation data? 

Solar energy technologies rely on solar 

radiation to provide energy for producing 

electricity, heating water, destroying toxic 

wastes, and lighting and heating buildings. 

Common to these technologies is that the 

end-use product is, for the most part, a 

direct function of the amount of solar radiation received and the conversion efficiency. 

Windows can significantly affect the heating and cooling loads of buildings. 

Engineers and architects can use solar radiation data 10 evaluate the effects 

that windows will have on the energy consumption of a building and hence 

delennine the size of heating and air conditioning equipment needed. 

16 

That is, if the amount of solar radiation 

is increased, then the end-use product 

increases also. This is also true for solar 

fuel production, in which crops are grown 

and then converted into fuels and byproducts. Although dependent on the 

soil type and rainfa ll, crops also depend on 

the amount of solar radiation received. 

To determine the performance and economics of sola r conversion technologies, 

designers and engineers use solar radiation 

data to estimate how much solar energy is 

available for a site. Depending on the particular technology, the solar collector might 

be a photovoltaic array, a concentrating 

parabolic trough, a domestic hot water 

collector, a window, a skylight, or a canopy 

of foliage. Desig ners and engineers use 

hand calculations or computer simulations 

to estimate the solar radiation striking a 

collector. 

Hand calculations are appropriate when 

using solar radiation data that represent 

an average for an extended period. For 

example, designers of remote photovoitaic 

powered systems for charging batteries use 

average daily solar radiation for the month 

to determine the size of the photovoltaic 

array. The criteria for this application is not 

the amount of solar radiation for a given 

hour or day but whether or not the average 

daily solar radiation for the month is sufficient to prevent the batteries from becoming d ischarged over several days. 

The month used in the design process 

depends on the relative amount of solar 

radia tion available compared to the energy 

required by the load. For a system in which 

the load is constant throughout the year, 

solar radiation data for December or 

January are usually used for the northern 

hemisphere. 
Computer simulations are an effective 

tool when an hour-by-hour performance 

analysis is needed. Utility engineers may 

want to know if the output of a solar . 

electric power plant could reliably and 

economically help meet their daytime 

electric demand. (One of the potential 

benefits of a solar electric power plant is 

that its output may coincide with the utility 

peak electric demand for summertime air 

conditioning loads.) By using the hourly 

solar radiation data for its location, the 

utility can run computer programs that 

show how much energy could be 

produced on an hour-by-hour basis 

throughout the year by the solar electric 

power plant. 

Some solar energy conversion technologies require a threshold value of solar 

radiation before certain operations can 

begin or be sustained. As an example, a 

central receiver solar thermal electric 

power plant may require direct normal 

solar radiation values above 450 W 1m2 to 

produce stearn for the turbine generator. 

Consequently, to evaluate a site's potential 

for solar thermal electric production, a 

designer examines the solar radiation data 

to determine the times of day when the 

solar radiation exceeds the threshold value. 

Heating and air conditioning engineers 

use solar tadiation'data to optimize building designs for energy efficiency. For 

example, window orientation and size can 

affect the heating and cooling of the building. South-facing windows transmit solar 

energy in the winter that is beneficial in 

reducing heating requirements. But in the 

swnmer, solar energy transmitted through 

windows (primarily those that face east or 

west), must be offset by increased operation 

of the air conditioning system. By having 

access to solar radiation data for their location, engineers and architects can evaluate 

the effects of window orientation and size 

o 4 8 12 16 20 24 

lime of day (hour) 

Computer simulation using solar radiation data shows how the output of two 

photovoltaic power systems could be added to the utility's generation to help 

meet peak electric demand in the summer. The fixed-tilt array faces south and 

is tilted from the horizontal at an angle equal 10 the site's latitude. The tracking 

array uses motors and gear drives to point the array at the sun throughout the 

day. Depending on location, the photovoltaic system with the 2-axis tracking 

array receives annually 25% to 40'7~ more global solar radiation than the fixedtilt photovoltaic system and provides more power for longer periods. This 

must be weighed against the higher initial cost and maintenance required 

for the tracker. 

"Because the solar load is the largest 

compO/lent for building exterior 

surfaces, and because windows are 

the most sensitive to the solar load, 

solar radiation data are essential for 

the accurate and energtJ efficient 

desigtl of buildings and their air 

conditioning systems." 

Jack F. Roberts, P.E. 

American Society of Heating, Refrigerating 

and Air-Conditioning Engineers 

17 
Concentrator collectors (top) use direct beam solar radiation; flat-plate collectors 

(bottom) use direct beam radiation, diffuse (sky) radiation, and ground-reflected 

radiation. 

18 

on the energy constunption of the building 

and determine the size of the heating and 

air conditioning equipment needed. They 

can use this information, combined with 

desired levels of natural lighting and the 

building aesthetics, to fonnulate the final 

building design. 

Except for concentrator systems, solar 

radiation data cannot be used without first 

accounting for the orientation of the solar 

collector. Concentrators track the sun and 

focus only direct beam radiation, but flatplate collectors receive a combination of 

direct beam radiation, diffuse (sky) radiation, and radiation reflected from the ground 

in front of the collector. Depending on the 

direction the collector is facing and its tilt 

from the horizontal, flat-plate collectors 

receive different amounts of direct beam 

radiation, diffuse radiation, and groundreflected radiation. Designers employ 

equations to calculate the total or global 

radiation on a flat-plate collector. The 

equations use values of the direct beam 

radiation, the diffuse radiation on a 

horizontal surface, and the orientation 

of the collector. 

To maximize the amount of solar radiation received during the year, flat-plate 

collectors in the northern hemisphere face 

south and tilt from the horizontal at an 

angle approximately equal to the site's 

latitude. The annual energy production 

is not very sensitive to the tilt angle as long 

as it is within plus or minus 15° of the 

latitude. As a general rule, to optimize the 

perfonnance in the winter, the collector can 

be tilted 15° greater than the latitude. To 

optimize performance in the summer, the 

collector can be tilted 15° less than the 

latitude. Solar radiation data combined 

with computer simulations can define 

these relationships more precisely. 

In the initial design stage, d esigners of 

cells used in photovoltaic mod ules can 

use spectral solar radiation da ta bases and 
models to optimize the ceUs for maximum 

energy production. Because the spectral 

content of solar radiation changes throughout the day and season, photovoltaic ceUs 

are tailored for a specific range of solar 

radiation wavelengths that will produce 

the most energy. Different photovoltaic 

materials have different peak responses; 

performance models using spectral solar 

radiation data bases can compare two or 

more photovoltaic materials operating 

under a range of seasons and climates. 

This results in optimizing the design early 

and eliminates the expense and time that 

would otherwise be needed for preliminary field testing. 

"For sizing stand-alone PV systems, we calclilate the 

Ilwnber of PV modliles required to keep the batteries 

charged by lIsing the average daily solar radiation incident 

on the collector for the month of the year with the smallest 

ratio of solar radiation to electric load demand." 

RIchard N. Chapman 

SandIa NaUonal Laboratories 

I Wor1<sheet It2 I Determine Design Current and Array Tilt 

I ~ ~I ~ I 

~,. ~ " r".., ~ ~ ~ f"9,... f"_" (t.~y) I (=J ~ ( .... J ~ (t.~ I ~ 

II!:+: 

SeIed the largest Design Current and Corresponding Peak &.11rom each Lalilude and Enter Below 

I ~"."~ (hr~ J I = (aJT11$J I 1""1 (1Ys:.:1'l 

"" ~ ~ (ampSJ 

4.44 10.2 5.21 8.6 

Now Selecl the SmaUesl Oesigll Current aJ'ld Corresponding Peak Sun 

Note: ~ ~ lIfI'ay is desired. use \racking data'rom Appendix A. 00 not mIX \l'ac:Img and lilced array data on !he $afTIII sheet 

!>9 ~JI "'" ~ ~ (~J 

5.74 7.9 

£.J ~ """", 5.74 

, 

~ .... , 

7.9 

SO' 

19 

Stand-alone PV system 

worksheet fordetennining 

the best collector tilt angle 

and the total charging 

current required from 

the PV modules. (As per 

Stand-A fane PllOtovoftaic 

Syste",s: A Handbook of 

Recommended Desigtl 

Practices,SAND87-1023, 

Albuquerque, NM: Sandia 

National Laboratories, 

March 1990.) 
ne1'e can you obtain solar radiation data? 

The National Weather Service of the 

National Oceanic and Atmospheric 

Administration (NOAA) operates monitoring stations in the United States to collect 

and disseminate information about solar 

radiation. This information is available on 

computer readable magnetic tape from 

NOAA's National Climatic Data Center 

(NCOC), Federal Building, Asheville, NC 

28801 (704) 259-0682. 

Most of NOAA's solar radiation data 

sets are from 26 SOLMET stations and 

222 ERSATZ stations and consist of hourly 

values of solar radiation and meteorological data from 1952 to 1975. For theSOLMET 

stations, instruments measured the global 

horizontal solar radiation and researchers 

modeled the direct beam solar radia tion 

data. For the ERSATZ stations, although no 

solar radiation measurements were made, 

researchers modeled global horizontal 

radiation based on observed meteorological data such as cloudiness and minutes of 

sunshine; the ERSAlZ data do not include 

direct beam radiation. Because all the 

ERSATZ data are modeled, these data are 

less accurate than the SOLMET data. 

NOAA also has available more recent 

data for the periods 1977 to 1980 and 1988 

to the present. The data include hourly 

values of measured global horizontal solar 

radiation for 38 stations, measured direct 

beam solar radiation for 32 stations, and 

measured diffuse horizontal radiation for 

nine stations. 

Two of NOAA's data sets are of particular interest to designers and engineers: the 

typical meteorological year (TMY) data set 

and the weather year for energy calculations 

(WYEC) data set. For these, researchers 

extracted infonnation from SOLMET / 

ERSATZ data to make data sets of hourly 

values spanning one year. For the ERSATZ 

20 

TMY data, researchers 

included values of direct 

beam radiation with 

modeled values of global 

horizontal radiation. These data sets represent typical values occurring from 1952 to 

1975, and not the minimum or maximum 

values. For example, a cloudy year in this 

period may have had an annual solar radiation value 10% below theTMYvalue, and a 

very cloudy month in this period may have 

had a solar radiation value 40% percent 

below its TMY value. A difference between 

TMY and WYEC data is that the TMY data 

are weighted toward solar radiation values 

and their hourly distribution, whereas the 

WYEC data are weighted toward average 

monthly values of temperatures and solar 

radiation. Researchers recently revised 

the WYEC data to include estimates of 

direct beam and diffuse solar radiation 
and estimates of illuminance for lighting 

applications. llluminance refers to solar 

radiation in the visible region of the solar 

spectrum to which the human eye 

responds. 

Solar radiation data derived from the 

SOLMET /ERSATZ data sets are also published in tabular fonn by the National 

Technicallnfonnation Service (NTIS), U.s. 

Department of Commerce, 5285 Port Royal 

Road, Springfield, VA 22161. Two of these 

tabular data sets are listed below. 

Illsolatioll Data Manunl and Direct Nonnai 

Sowr Rndiatioll Data Malmal, SERl/ TP-22O3880, Golden, CO: Solar Energy Research 

Institute, July 1990. 

This map shows the globa l 

solar radiation for a south· 

facing collector tilled at an 

angle equal 10 the site 

latitude as an annual daily 

average for different 

locations in the United 

States. one numbers on 

the map represent MJ/m2

; 

multiply by 0.2778 to 

obtain kWhlm1.) 

This manual contains monthly averages 

of global horizontal and direct beam solar 

radiation, ambient temperature, the ratio 

of global horizontal solar radiation on earth 

to that outside the earth's ahnosphere (Kt), 

and hea ting and cooling degree-days. 

This infonnation is presented for all the 

SOLMET /ERSATZ stations. 

Stand-Alone PllOtovo/taic Systems: A 

HandOOok of Recommended Design Prod ices, 

SAND87-7023, Albuquerque, NM: Sandia 

National Laboratories, March 1990. 

The appendix of this handbook contains 

monthly estimates of solar radiation striking collectors. These estimates are calculated for different tilts and sun-tracking 

21 
NOAA's National Oimatic Data Center has solar radiation data ava ilable on 

computer readable magnetic tape. The data sets are for 26 SOLMET stations 

and 222 ERSATZ stations and consist of hourly values of solar radialion and 

meteorological data from 1952 to 1975. 

schemes. The estimates are for a selected set 

of 38 SOLMET (ERSATZ stations and are 

based on theSOLMET (ERSATZ data. 

Maps are available that depict long-term 

average solar radiation data for each month. 

This is a convenient way to show variations 

in the amount of solar radiation and for interpola ting data between sta tions. For the 

United States, these maps were made using 

solar radiation data from the SOLMET / 

ERSATZ data base. The Solar Rndiafioll 

Energy Resource Atlas of tlte United States ItNlS 

published by the Superintendent of Documents, but is out of print. This a tlas is available a t some university and city libraries. 

The University of Lowell compiled an 

intemational solar radiation data base for 

locations outside the United States. This 

data base presents average d aily values 

by month a nd year for global horizontal 

solar radiation. It is ava ilable from the 

University of Lowell Photovoltaic Program, 

1 University Avenue, Lowell, MA 01854 

(508) 934-3377. 

22 

Solar radiation data recorded for 

l-minute intervals are available for four 

locations: Albany, New York; Atlanta, 

Georgia; Davis, California; and San Antonio, 

Texas. The data were recorded over periods 

of 1 year or more by university meteorological research and training stations. Because 

of the time scale used, these da ta are 

primarily of interest to researchers studying transient responses in solar energy technology systems. These data are available 

from the National Renewable Energy 

Labora tory (NREL), 1617 Cole Boulevard, 

Golden, CO 80401. 

A spectral solar radiation data base representing a range of atmospheric and climatic 

conditions is also available from NREL. 

This da ta base includes more than 3()(x) 

spectra measured over a wavelength 

range from 300 to 1100 nanometers at 

2-nanometer increments (1 nanometer is 

one-billionth of a meter) and is the result 

of a cooperative effort between NREL, 

the Electric Power Research Institute, the 

Rorida Solar Energy Center, and the Pacific 

Gas a nd Electric Company. Spectral solar 

radiation was measured at three sites: Cape 

Canaveral, Rorida; San Ramon, California; 

and Denver, Colorado. This data base can 

help determine whether spectraUy selective 

technologies (such as photovoltaics and 

biomass) are optimized for a particular 

location and climate. 

Additionally, other sources of solar radiation data are state and local governments, 

utilities, and universities. Examples include 

the Padfic Gas and Electric Solar Insolation 

Monitoring Program, the University of 

Oregon/Pacific Northwest Solar Radiation 

Data Network, and the Historically Black 

Colleges and Universities Solar Radiation 

Monitoring Network. 
ow accurate do the data need to be? 

The required accuracy of the solar 

radiation data for 

a site depends on 

the application. 

When the cost of the solar conversion 

device is low compared with the overall 

system cost, we can account for uncertainties in the solar radiation data by using 

"engineering judgment" to increase the 

size of the solar collectors. However, as the 

"Utility engineers need solar 

radiation data accurate to within 

±5% to assess the resource, estimate 

the output of a solar system, and 

determine whether the system can 

reliably and economically meet 

daytime demand and energtJ 

requirements. Because there are few 

sites with data of this accuracy, we 

need monitoring stations to collect 

the data at proposed PV sites." 

J.E. Bigger 

Electric Power Research Institute 

solar energy conversion system increases in 

size and cost, this becomes less acceptable, 

and we need more accurate solar radiation 

data to optimize the design and project 

the cost. 

For large-scale applications of solar 

energy conversion technologies, most 

experts agree that solar radiation data 

should be accurate to within 5% so they 

can make reasonable assumptions concerning energy output to evaluate the performance and economics. Unforhmately, 

not much available solar radiation data are 

accurate to within 5%. This is due to the 

measurement uncertainties of the instruments used and the limited number of 

measurement sites. Consequently, designers today have to apply these data more 

conservatively than is ultimately desirable. 

The SOLMET /ERSATZ data are the 

most widely used solar radiation data. On 

an average for all sites, they are accurate to 

within about 10% for average daily values 

on an annual basis. But for average daily 

values on a monthly basis for an individual 

site, they can be in error by 20% or more. 

For interpolating data for sites between 

SOLMEf /ERSATZ stations, microclimate 

differences due to terrain and local weather 

conditions can also increase the uncertainty 

of the data. 

For large-scale applications, like this 6.5-MW photovoltaic system, designers 

prefer solar radiation data that are accurate to within 5% so they can make sound 

assumptions concerning system output, performance, and economics. (Photo 

courtesy of Siemens Solar Industries') 

23 
awwillwemeet our solar radiation data needs? 

One of the goals of the Solar Radiation 

Resource Assessment Project at NREL is to 

provide accurate information about solar 

radiation to minimize the economic risk 

of implementing solar energy conversion 

technologies. The data must accurately 

represent the spatial (geographid, temporal (hourly, daily, and seasonal), and 

spectral (wavelength distribution) variability of the solar radiation resource at 

different locations. 

24 

The new National Solar Radiation Data 

Base (1961-1990) for the United States will 

improve data quality over the existing 

SOLMET /ERSATZ (1952-1975) data base. 

For this new data base, NOAA used better 

equipment for measuring solar radiation 

at more sites and NREL used better modeling techniques for synthetic stations. 

Scheduled for completion in 1992, this new 

data base will include data for 250 sites. 

After completing the data base we will 

produce special purpose products such as 

typical meteorological year (TMy) data 

sets, maps, and data summaries. 

By continuing the long-tenn measurement of solar radiation at numerous sites, 

we can assess changes in climate and add 

new data to existing data bases. We can 

improve the quality of the solar radiation 

data base for the United States by working 

with existing regional solar radiation 

networks and establishing educational 

initiatives so that data are being collected 

at several hundred sites in the United 

States. This large number of measurement 

sites will improve the quality of the solar 

radiation data base, better represent the 

geographic distribution of solar radiation 

in the United States, and provide research 

data to develop techniques to estimate 

solar radiation where there are no measurement stations. 

This type of research involves developing spatial interpolation techniques, such 

as mapping so~ar radiation using cloudcover infonnation from satellites, to 

estima te solar radiation between measurement stations. This cloud-cover mapping 

technique promises high spatial resolution 

for the optimum siting of solar energy conversion technologies and enables estimating solar radiation for countries where 

no solar radiation data base exists. 
NREL is improving the equipment and 

techniques used to measure solar radiation 

and the models and methods used to determine the performance of solar conversion 

technologies. Our recent activities include: 

• angular response characterization and 

uncertainty analysis of solar radiometers, 

• development of improved quality assessment procedures for solar radiation data, 

• calibration of radiometers for industry 

and members of the scientific community, 

• development of both broadband and 

spectral solar irradiance models, and 

• conhibutions to the development of 

solar trackers and spectroradiometers. 

For information about solar radiation 

data, models, and assessments contact 

the NREL Technical Inquiry Service at 

303/231-7303. 

Ooud-cover infonnation. analyzed from photographs 

taken by satellites, has the potential for estimating solar 

radiation at any location on earth. 

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