Currently,　the　air　source　transcritical　carbon　dioxide　heat　pump　is　extensively　used　for　water　heating.　The　refrigerant　charge　plays　a　significant　role　in　optimizing　system　performance.　However,　most　previous　studies　only　focus　on　energy　analysis.　In　this　paper,　an　experiment　was　conducted　and　the　effect　of　refrigerant　charge　was　evaluated　with　energy,　exergy,　economic　and　environmental　analyses.　Results　showed　that　the　optimal　normalized　refrigerant　charge　was　0.214　in　the　comprehensive　analysis.　In　the　most　important　energy　analysis,　the　peak　coefficient　of　performance　was　increased　by　up　to　16.27%　with　optimal　charge　over　other　charge　amounts,　corresponding　to　the　maximum　heating　capacity　and　superheat　of　5.13　°C.　The　maximum　exergy　efficiency　was　34.17%,　and　the　largest　exergy　destruction　was　observed　in　the　evaporator,　accounting　for　44.04–49.22%　of　the　total.　Compared　with　the　direct　electric　heater,　the　annual　operating　cost　was　decreased　by　184.13　thousand　Chinese　yuan　at　the　optimum　charge.　Moreover,　the　payback　period　was　lower　than　1.0　years　and　most　sensitive　to　operating　time　as　shown　by　the　sensitivity　analysis.　The　reduction　of　carbon　dioxide　emissions　reached　the　peak　value　of　378.01　tons　at　the　optimal　charge,　which　was　profitable　in　reducing　the　greenhouse　effect.　Also,　considering　the　various　operating　conditions　and　domestic　hot　water　load,　the　energy,　economic　as　well　as　environmental　benefits　and　the　applicability　of　the　evaluated　optimal　charge　were　presented.　©　2020　Elsevier　Ltd