To test the validity of various sources of data, we have collected output measurements from power plants in India, which have been in operation for at least a period of 6 months. This output can be used to analyse whether the data inputs are accurate or not. For example, the output power generated, minus the losses can give a good estimate of the accuracy of the input radiation data and the estimated generation. Since several source of irradiation data are available, this will be useful is evaluating which source of data is the most accurate.