A single method to estimate the daily global solar radiation from monthly data

• New Ångström–Prescott coefficients locally calibrated in daily and monthly time bases
• Permutability of calibrated Ångström–Prescott coefficients
• Performance of the permutability is carried out against other sunshine models.
• A–P equation can estimate global radiation using daily/monthly data interchangeably.
• Results highlight the importance of the local calibration.

The performance of Ångström–Prescott (A–P) linear equation to estimate global solar radiation on a horizontal surface at daily and monthly mean daily time bases over Spain is studied. The pretty good performance obtained for both time bases ensures a fast first estimation of the global solar radiation. Here, new sets of A–P coefficient values over Spain for the selected stations are provided. Moreover, the impact of both time bases on the A–P performance is analysed concluding that the A–P obtained coefficients can be permutated between daily and monthly mean daily time bases. Therefore, the monthly mean daily (daily) calibrated A–P coefficients can be directly applied for estimating the global solar radiation using daily (monthly mean daily) data. In order to test the validity of the A–P model coefficients' permutability for estimating global solar radiation over Spain, the performance of the permutated A–P model has been compared with that of other sunshine hour-based models (polynomic, exponential, logarithmic, regional and global A–P calibration and latitudinal dependent models). According to the results, the permutated A–P linear expression can be selected to estimate the solar radiation instead of the remainder models. The obtained results also remark the importance of the local calibration on the accuracy of global solar radiation estimation instead of using a single pair of coefficients for the whole domain.