Solar Radiation Data Manual for Buildings

Table of Contents

Technical Background

Interpreting the Data Tables

For each of the 239 stations, a data page contains a description of the station location; presents average solar radiation and illuminance values for a horizontal window and vertical windows facing north, east, south, and west; and gives average climatic conditions. Standard International (SI) units are used for illuminance; otherwise, English units are used. To convert values to other units, use the conversion factor table on the inside back cover.

Station Description

Information at the top of each page describes the station.

Solar Radiation Data for Buildings

For the period 1961-1990, tables provide solar radiation data in units of Btu/ft^2/day for five surfaces: a horizontal window and vertical window facing north, east, south, and west. An estimate of the uncertainity (+- %) of the solar radiation data is given in the table headings. The manual includes the solar radiation transmitted through the window into the living space.

Incident solar radiation.For the five windows, monthly and yearly averages of incident solar radiation are given in terms of global radiation, clear-day global radiation, and diffuse radiation.

Global radiation is the total radiation received by the window and is the sum of the direct beam radiation component, sky radiation, and radiation reflected from ground in front of the surface. Clear-day global radiation represents the global radiation obtainable under clear skies.

The diffuse radiation data in the tables do not include the direct beam radiation component. Diffuse radiation is the sum of sky radiation and radiation reflected from the ground in front of the surface. The ground-reflected radiation was calculated using a ground reflectivity or albedo of 0.2, a nominal value for green vegetation and some soil types. Values in the tables may be adjusted for other albedo values by using equation 4 from the Appendix.

Standard deviations and minimum and maximum monthly and yearly values of global horizontal radiation are provided to show the variability of the solar resource at a station. These quatities pertain to monthly and yearly values, rather than to single days.

Click to expand Incident Global Solar Radiation figure.

Transmitted solar radiation.The solar radiation transmitted into a living space is less than the radiation that strikes the outside of a window because of losses associated with radiation reflected off of and absorbed by the glass. The manual presents data for windows with conventional, single-strength clear double glazing and a glass thickness of 0.125 in. (3.18 mm)

Tables contain values for unshaded and shaded windows. Unshaded values are for windows with no external shading. Shaded values are for windows with no external shading. Shaded values are for windows shaded by a roof overhang. The roof overhang and window geometry are shown at the top of each data page. The overhang width and the vertical distance from the window to the overhang are given in dimensionless units for a window height of 1.0.

For south-facing windows, the geometry balances the need for maximum heat gain during the heating season without creating unreasonable heat gain during the cooling season. The same shading geometry is used for all vertical windows for a station. The shading geometry is not applicable for the horizontal surface; consequently, shaded transmitted solar radiation values for a horizontal surface are not included.

As described in the Appendix, the shading geometry is generally a function of the station latitude but consideration is also given to heating and cooling requirements. For example, Hawaii, Guam, and Puerto Rico have zero heating degree days; consequently, their shading geometry provides complete shading of south-facing windows at non throughout the year. Alaskan stations, with no summer cooling loads, have shading geometries that do not shade south-facing windows at noon throughout the year.

Click to expand shading geometry and sun positions for 40 degrees N latitude

For south-facing windows, the shading geometries provide guidance for the appropriate dimensions of roof overhangs. However, situations may require a different geometry, depending on the balance between heating and cooling loads for the particular building and factors such as required window sizes and building practices. For east- and west-facing windows, overhangs are not particularly effective in preventing unwanted heat gain. Additional shading strategies such as vertical louvers may be needed.

Climatic Conditions

The tables containing average climatic condition information list monthly and yearly values for the following parameters.

Degree days indicate heating and cooling requirements of buildings. They are defined as the difference between the average temperature for the day and a base temperature. If the average for the day (calculated by averaging the maximum and minimum temperature for the day) is less than the base value, then the difference is designated as heating degree days. If the average is greater than the base value, the difference is designated as cooling degree days.

The clearness index (Kt) is the station's global horizontal solar radiation divided by its extraterrestrial horizontal radiation. Because clouds decrease the amount of solar radiation reaching the earth, stations in cloudy regions will have lower values for Kt than stations in regions will fewer clouds.


The illuminance tables contain diurnal profiles of the average illuminance incident of five surface; a horizontal window and vertical windows facing north, east, south, and west. The illuminance profiles are given for 4 months of the year (March, June, September, and December) and consist of two data values; separated by a slash, for each of the following hours: 9 a.m., 11 a.m., 1 p.m., 3 p.m., and 5 p.m. The value before the slash is the average illuminance for mostly clear conditions (total cloud cover less than 50%), and the value after the slash is the average illuminance for mostly cloudy conditions (total cloud cover equal to or greater than 50%).

The last line in the illuminance tables indicates the percentage of time during the hour that the station location was mostly clear (M. Clr.). These values, along with the illuminance values, can be used to determine the average hourly illuminance:

The illuminance data are given in units of kilolux-hours (klux-hr) and represent the illuminance received during the preceeding hour. For example, data for 3 p.m. include the illuminance received from 2 p.m. to 3 p.m. The hours for March and December are local standard time. The hours for June and September are either local standard time or daylight saving time, depending on whether the staion observes daylight saving time. Arizona, Hawaii, Indiana (except for Evansville), Puerto Rico, and Guam do not observe daylight saving time; consequently, the hours for June and September for these stations are local standard time. For all other station, the hours for June and September are daylight saving time. An estimate of the uncertainty (+_%) of the illuminance data is given in the table headings.

Electronic Data

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