Although measurements of the solar radiance provide the most accurate information of solar radiation, these measurements can be difficult to obtain. A simpler method of measuring sunshine hours is to use a card system whereby sunlight is focused into a recording card. If the sunshine is greater than about 200W/m2 a mark is burned onto the recording chart. The number of sunshine hours is determined as the number of hours during which the sun is shining. When the number of sunshine hours is combined with a knowledge of local atmospheric conditions, sunshine data can be used to estimate the available monthly average solar radiation according to the equation:
a and b are constants determined by the location and depend on atmospheric conditions
HO is the average solar radiation for that month on a clear day (which can be calculated)
n is the measured number of sunshine hours data
N is the number of sunshine hours calculated as shown in the page Calculation of Solar Insolation.
The calculations for HO and N are shown in later pages. The constant a varies around 0.25, with values just below 0.2 used for some locations and values up to 0.4 used in others. The constant b takes values in the range of 0.4 to 0.6.
For Australia, studies by Telecom Australia (now Telstra) indicate that values of a = 0.24 and b = 0.48 are accurate to within 10%.
Values for several of the locations in India have been tabulated by G.D. Rai and are given below.