Recovering as much braking energy as possible, and then fully reusing it, can significantly improve the vehicle powertrain efficiency, hence reducing the CO2 emissions and fuel consumption. A 48 V mild hybrid system recovers less braking kinetic energy than a HV (High Voltage) hybrid system due to the reduced peak power/current rating. However, the cost of the 48 V mild hybrid system is significantly less than the HV hybrid system which gives the 48 V mild hybrid system a much better cost-benefit ratio. The 48 V mild hybrid system can have several different system layouts (e- machines at different positions, or have numerous e-machines at different position combinations). The aim of this study is to investigate and explain how the system layout affects the powertrain system efficiency and CO2 benefit. Simulation models are used to predict the CO2 of three such configurations. The paper starts with the summary of the 48 V mild hybrid system layouts and an introduction of the possible 48 V mild hybrid system operating modes. Due to the energy flow through different components in each layout, an analysis is presented to compare the powertrain system efficiency of each layout under different operating modes. Then, the suggestions of the most cost effective 48 V mild hybrid systems (based on the different operating mode requirement and vehicle mass) are presented. Finally, several 48 V mild hybrid system layouts and their characteristics are discussed as examples.