Appropriate　daylighting　design　can　create　a　comfortable　indoor　lighting　environment,　however,　the　objectives　of　providing　adequate　daylight　and　controlling　solar　gain　often　conflict.　Existing　daylighting　metrics　effectively　evaluate　the　daylighting　performance　of　buildings,　but　no　connection　has　been　established　between　daylighting　and　the　solar　gain　it　brings.　To　address　this　issue,　this　paper　proposes　a　new　metric,　daylighting　efficacy　(DE),　which　is　defined　as　the　ratio　of　the　luminous　flux　obtained　through　daylighting　on　the　whole　horizontal　work　plane　to　the　solar　radiation　received　by　the　daylighting　system　(lm/W).　The　calculation　equation　for　DE　is　established　accordingly,　and　its　application　is　explored　in　a　reference　office　using　the　parametric　design　approach.　Variations　of　the　reference　room＇s　orientation,　glazing　property　and　configuration,　and　window-to-wall　ratio　are　then　examined　to　assess　their　impact　on　DE　with　explanations　provided　for　the　underlying　causes.　Additionally,　three　innovative　daylighting　systems　are　evaluated　using　DE　to　demonstrate　its　suitability　for　assessing　buildings　with　various　daylighting　systems.　Finally,　a　comparison　between　DE　and　Daylight　Factor　(DF)　is　presented　to　highlight　that　DF　cannot　serve　as　a　substitute　for　DE.　The　reasonable　range　for　DE　will　require　determination　through　the　combined　efforts　of　future　researchers.　©　2025　International　Solar　Energy　Society