The DNI (Direct Normal Irradiance) is the resource utilized by solar concentrators, in CSP and CPV applications. However, it is not often measured due to practical reasons, and is derived instead from measurements of global irradiance or satellite-based models. This paper describes a study of different estimation procedures for the assessment of the DNI, using experimental data with a time scale of 1 min, taken at two different latitudes in Italy. The analyzed approaches include measuring techniques and models. Accurate estimations of DNI are obtained as derived from measurements of global horizontal irradiance and diffuse horizontal irradiance if the offset in pyranometers are properly corrected and the zenith angle is not too high. The measured and calculated values of DNI are used to estimate the hourly, daily, monthly and seasonal irradiation. The results show that the different estimation methods can lead to quite different conclusions when comparing short-term and long-term solar radiation availability. (C) 2015 Elsevier Ltd. All rights reserved.