Solar irradiance levels have been estimated from 850 to 1610 using historical data and statistical analysis. The new estimates show that solar radiation increased slightly from the Maunder Minimum to the Medieval Maximum and then to the Modern Maximum. The changes in solar radiation were found to be different from previous estimates, especially in terms of the magnitude and spectral distribution of the changes. The new estimates suggest that solar radiation changes were comparable in the ultraviolet spectrum but somewhat larger in the visible and near-infrared wavelengths. Additionally, the amplitude and phasing of the 11-year solar activity cycle were also different in the new estimates compared to previous recommendations.