The fuel efficiency of a modern hybrid electric vehicle (HEV) powertrains has reached levels where the impact of the 12V auxiliary electrical system on Fuel Economy becomes increasingly pronounced. While improving the energy consumption of an individual component may result in minor improvements, the optimization of many consumers across a complete vehicle system can result in meaningful Fuel Economy gains. Traditional methods using chassis dynamometer testing alone to quantify the impact of a specific 12V consumer can lead to issues where signal state changes are too small for accurate detection. This presents difficulties in accurately predicting the influence of a 12V load on Fuel Economy of next-generation electrified vehicles under development. This paper describes a newly developed method where dynamometer test results are combined with computer simulation analyses to create a practical technique for assessing the impact of small changes in aux load energy consumption. The process combines the best features of empirical testing with model-based system engineering, and accurately estimates the effects of small changes in total average on-cycle auxiliary load power. It supports timely and resource-efficient estimates of the Fuel Economy impact of auxiliary load components and control strategies. An overview of the effects of 12V auxiliary load power on the Fuel Economy or a modern HEV is presented for different drive cycles and the estimation process is described. Lessons learned from dynamometer testing of small auxiliary loads are discussed and the application of the process to the development of a fuel-efficient low-voltage system is described.