It was used data from 30 locations of the Rio Grande do Sul State, Brazil, collected from 1960 to 2007, with the objective to determine the sample size (number of years) to estimate the average daily month insolation . The average and variance was calculated for each of the 360 time series (12 months x 30 locations) and the aleatory and normality data were tested. Then it was verified the homogeneity of variance among months in each locality and among places in each month and it was determined the sample size to estimate the average monthly insolation daily in each month and locality. The sample size (number of years) to estimate the average monthly insolation daily is dependent on the month and locality. One concluded that 44 years of data are enough to predict the average monthly insolation daily, with an estimation error equal to ±0.5 hours days-1, with a degree confidence of 95%.
sunshine; time series; error of estimation
