System for simultaneously calibrating two or more solar radiation measuring pyranometers (1a, 1b), comprising an electronic data processor configured for generating mean calibration constants of the pyranometers by carrying out the steps of: installing a reference pyrheliometer with a known calibration constant RDIR placed proximal to the pyranometers (1a, 1b) to be calibrated, and acquiring measurements of direct normal irradiance; recording the average values VDIR × CZA, VDIF ( i) with i = 1,..., NDIF and VGBL ( j) with j = 1,..., NGBL at a same and synchronized averaging rate for a plurality of sampling instants, where NDIF is the total number of i-th pyranometers measuring diffuse horizontal irradiance, NGBL is the total number of j-th pyranometers measuring global horizontal irradiance, and VDIR is the output of the reference pyrheliometer, CZA is the cosine of the solar zenith angle, VDIF(i) is the output of the i-th pyranometer, VGBL(j) is the output of the j-th pyranometer, for each of the sampling instants; fitting a linear function in the form Y = M × X + B to the data points defined by X i,j,k = VDIF ( i) k / VGBL ( j) k and Y i,j,k = VDIR × CZA k / VGBL ( j) k with k = 1,..., NREC, by calculating values of slope M and intercept B, where NREC is the total number of valid average value records; calculating calibration constants of the i-th pyranometer by RDIF i,j = - RDIR / M, and of the j-th pyranometer by RGBL j,i = RDIR / B; calculating a mean calibration constant for the i-th pyranometer measuring diffuse irradiance by RDIF ̅ i = 1 NGBL ∑ j = 1 NGBL RDIF i, j, and a mean calibration constant for the j-th pyranometer measuring global irradiance by RGBL ̅ j = 1 NDIF ∑ i = 1 NDIF RGBL j, i. installing a reference pyrheliometer with a known calibration constant RDIR placed proximal to the pyranometers (1a, 1b) to be calibrated, and acquiring measurements of direct normal irradiance; recording the average values VDIR × CZA, VDIF ( i) with i = 1,..., NDIF and VGBL ( j) with j = 1,..., NGBL at a same and synchronized averaging rate for a plurality of sampling instants, where NDIF is the total number of i-th pyranometers measuring diffuse horizontal irradiance, NGBL is the total number of j-th pyranometers measuring global horizontal irradiance, and VDIR is the output of the reference pyrheliometer, CZA is the cosine of the solar zenith angle, VDIF(i) is the output of the i-th pyranometer, VGBL(j) is the output of the j-th pyranometer, for each of the sampling instants; fitting a linear function in the form Y = M × X + B to the data points defined by X i,j,k = VDIF ( i) k / VGBL ( j) k and Y i,j,k = VDIR × CZA k / VGBL ( j) k with k = 1,..., NREC, by calculating values of slope M and intercept B, where NREC is the total number of valid average value records; calculating calibration constants of the i-th pyranometer by RDIF i,j = - RDIR / M, and of the j-th pyranometer by RGBL j,i = RDIR / B; of the i-th pyranometer by RDIF i,j = - RDIR / M, and of the j-th pyranometer by RGBL j,i = RDIR / B; calculating a mean calibration constant for the i-th pyranometer measuring diffuse irradiance by RDIF ̅ i = 1 NGBL ∑ j = 1 NGBL RDIF i, j, and a mean calibration constant for the j-th pyranometer measuring global irradiance by RGBL ̅ j = 1 NDIF ∑ i = 1 NDIF RGBL j, i.