A　modular　efficient　single-source　surface　integral　equation　(SS-SIE)　formulation　is　proposed　to　solve　the　electro-magnetic　problems　induced　by　arbitrarily　penetrable　and　perfect　electric　conductor　(PEC)　objects.　In　the　proposed　formulation,　each　penetrable　object　is　first　replaced　by　the　background　medium　independently,　and　an　equivalent　model　with　only　the　surface　electric　current　density　on　the　boundary　is　derived.　Then,　the　equivalent　model　for　the　composite　object　is　obtained　through　combining　all　the　modular　equivalent　models　together.　The　proposed　formulation　is　flexible,　easy　to　implement,　intrinsically　supports　non-conformal　meshes,　and　does　not　require　additional　boundary　conditions　to　couple　the　shared　boundaries.　Numerical　results　validate　its　robustness　and　accuracy.　©　2021　IEEE.