Solar reflectance can vary with the spectral and angular distributions of incident sunlight, which in turn depend on surface orientation, solar position and atmospheric conditions. A widely used solar reflectance metric based on the ASTM Standard E891 beam-normal solar spectral irradiance underestimates the solar heat gain of a spectrally selective "cool colored" surface because this irradiance contains a greater fraction of near-infrared light than typically found in ordinary (unconcentrated) global sunlight. At mainland US latitudes, this metric RE891BN can underestimate the annual peak solar heat gain of a typical roof or pavement (slope 5:12 [23°]) by as much as 89 W m−2, and underestimate its peak surface temperature by up to 5 K. Using RE891BN to characterize roofs in a building energy simulation can exaggerate the economic value N of annual cool roof net energy savings by as much as 23%.

We define clear sky air mass one global horizontal ("AM1GH") solar reflectance Rg,0, a simple and easily measured property that more accurately predicts solar heat gain. Rg,0 predicts the annual peak solar heat gain of a roof or pavement to within 2 W m−2, and overestimates N by no more than 3%. Rg,0 is well suited to rating the solar reflectances of roofs, pavements and walls. We show in Part II that Rg,0 can be easily and accurately measured with a pyranometer, a solar spectrophotometer or version 6 of the Solar Spectrum Reflectometer.