Ceramic-based　transparent　dielectric　materials　are　regarded　as　the　best　candidates　for　advanced　energy　storage　and　conversion　materials　because　of　their　outstanding　optical　and　electric　properties.　Nevertheless,　due　to　the　presence　of　low　density,　low　band　gap　energy　and　large　grain　size,　it　is　difficult　to　simultaneously　obtain　high　energy　storage　density　and　high　optical　transmittance　in　lead-free　based　ceramics,　which　limiting　their　further　development　of　practical　applications.　In　this　work,　the　relaxor　ferroelectric　ceramics　of　x　mol%　Er3+-doped　0.91　K0.5Na0.5NbO3-0.09BamSrnTiO3　(xEr-SrmBan)　were　constructed　via　A-site　non-stoichiometric　defect　engineering.　It　is　worth　noting　that　excessive　addition　of　Sr　and　Ba,　especially　Sr,　can　significantly　refine　the　grain　size　and　domain　size　on　account　of　vacancy-related　defect　pinning.　Finally,　high　energy　storage　density　(W　=　6.39　J/cm3,　Wrec　=　3.42　J/cm3)　together　with　high　optical　transmittance　(∼72%　at　900　nm)　can　be　achieved　simultaneously　in　0.25Er-Sr1Ba0.5　due　to　the　exist　of　dense　structure,　ultrafine　grain　size　(<100　nm),　small-sized　nano-microdomains　(i.e.,　PNRs,　<　50　nm)　and　large　bandgap　energy　(∼3.10　eV).　Furthermore,　a　fast　discharge　time　of　42　ns　can　also　be　realized.　The　above　results　indicate　that　the　present　study　helps　to　promote　the　development　of　advanced　transparent　energy　storage　and　conversion　materials.　©　2022　Elsevier　B.V.