aB and G are the conventional symbols for the direct (beam) and global irradiation, as recommended by the International Solar Energy Society. IEC standard 61724:1998 recommends the symbols HI and Hd for the daily global and direct irradiation in the plane of the array.
bUnits Whm−2 h−1 and Whm−2 day−1 (or MJm−2 h−1 and MJm−2 day−1) are also used for the hourly and daily irradiation.