Is the solar resource high enough at my location?

Published by firstgreen on

During the planning phase of solar installation, one question about the solar resource will strike in mind. This article will provide the answers to the doubts related to assessing the available solar resources at the site.

 A national or regional solar energy map is available which will show the global horizontal radiation. This will indicate approximately how much solar resource is available in any given location. It represents a measurement of the solar radiation that reaches the earth’s surface. It can be expressed as either Global Horizontal Irradiance or Global Horizontal Irradiation. Global Horizontal Irradiation measures irradiance over a certain time period, such as one year. To convert a measurement of Global Horizontal Irradiance to Global Horizontal Irradiation, the following methodology will be applicable:

Global horizontal irradiation is measured in a variety of units in solar irradiation maps.

Some common conversion factors are as follows:

__________ MJ/ m2/day               ÷             3.6x       =    __________ Wh/m2/day   

__________ W/ m2                        ×             0.024     =    __________    kWh/ m2/day   

__________  kWh/m2/day            ×            365        =    __________     kWh/m2/year             


kWh = kilowatt-hour, m2 = square meter,

MJ = megajoule, W = watt.

Some maps display global horizontal irradiation “at latitude tilt,” meaning that the pyranometer is tilted at the angle of the latitude—this will give a higher number than if it were horizontal. This effect is minimal at lower latitudes, and so latitude tilt maps are more common for areas further from the equator. The global irradiance at latitude tilt (GI) for a site at latitude Ø can be converted to global horizontal irradiance (GHI) as follows:

                              GHI =  GIØ  ÷ cos Ø

The worldwide annual average global horizontal irradiance (GHI) is 170 watts per square meter (W/m2). However, many regions receive much more. Most of Southeast Asia receives an annual average GHI of 180–230 W/m2 (equivalent to 1,600–2,000 kilowatt-hours per square meter per year [kWh/m2/year]. This rough estimation of GHI will be useful later when calculating predicted energy output from the proposed system. The below picture depicts the annual average global horizontal irradiance (GHI).

The available databases for solar radiations are as follows:

  • Ground Data
    1. Indian Meteorological Department (IMD)
    2. Centre for Wind Energy Technology (C-WET)
  • Satellite Data
    1. NASA data
    2. SEC-NREL data
    3. SWERA data
  • TIME SERIES data
    1. Meteonorm
    2. Solar GIS
    3. 3 TIER
    4. ISHRAE

The higher the solar radiation, the higher will be the electricity generation. Hence, assessment of available solar resources is an important step during planning phase.