To　optimize　the　benefits　of　deploying　the　battery　energy　storage　system　(BESS),　this　paper　considers　the　problem　of　optimizing　the　battery　capacity　based　on　smart　meter　data.　An　energy　management　system　is　implemented　in　the　office　building　of　an　automation　technology　company.　The　smart　meters　monitor　the　energy　consumption　of　the　building,　and　the　data　are　transmitted　to　the　cloud　server　by　wireless　communication.　Based　on　the　smart　meter　data,　we　formulate　an　optimization　problem　to　determine　the　battery　capacity.　The　optimization　problem　considers　the　time-of-use　(TOU)　price,　the　investment　cost　and　the　maintenance　cost.　The　solution　for　the　optimization　problem　is　provided　based　on　particle　swarm　optimization　(PSO).　A　case　study　is　provided　to　prove　the　effectiveness　of　the　optimization.　©　2019　IEEE.