This guide seeks to provide information about the IEC’s methodology and raise awareness amongst transformer specifiers about the TCO approach. The IEC’s specification and equations are all published in IEC Technical Specification (TS) IEC TS 60076-20 (Ed. 1.0) 2017 Annex A. In addition to this document, U4E has prepared an Excel spreadsheet tool1 which puts the IEC equations into a user-friendly spreadsheet, to facilitate the derivation of the calculation of the loss evaluation factors.