The amount of solar energy that reaches the earth in one hour is sufficient to supply the world's energy needs for one year. Harvesting this energy efficiently is a huge challenge. In most countries including India, the number of observing stations is inadequate. Therefore, it is essential that some reliable mathematical models be developed to estimate the solar radiation for places where measurements are not carried out and for places where measurement records are not available. In this paper, Angstrom-Prescott model parameters are estimated for seven different sites in India, and a correlation is developed for India, which is found to be a good fit. Also a correlation is developed for predicting the solar radiation using only sunshine hour data. 1. Introduction Solar radiation which we receive as heat and light can be converted to useful thermal energy or can be used for production of electricity by using solar photovoltaic cells or solar thermal power. Availability of reliable solar radiation data is vital for the success of solar energy installations in different sites of the country. The kind of solar radiation data required depends on application and user. For example, monthly or daily averaged data are required for climatologic studies or to conduct feasibility studies for solar energy systems. Data for hourly (or shorter) periods are needed to simulate the performance of solar devices or during collector testing and other activities [1]. Knowledge of the local solar radiation is essential for the proper design of building energy systems, solar energy systems, and a good evaluation of thermal environment within buildings [2–7]. Systematic measurements of diffuse solar energy and the global (total) irradiation incident on a horizontal surface are usually undertaken by a national agency, which is the national meteorological office in many countries. There is a relative abundance of sunshine duration data, and therefore it is a common practice to correlate the solar radiation with sunshine duration measurements. 2. Materials and Methods In practical studies it is logical to consider that the solar radiation is directly proportional to the sunshine duration. The formulation of the proportionality can be derived from the measurements of the variables through scatter diagrams and most often by the application of statistical regression methods. The original models in solar energy expressed this relationship between solar radiation and the sunshine duration as linear. Such a pioneering relationship was presented by Angstr？m in 1924 [8]. 2.1. Angstrom
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