In recent years the fuel efficiency of modern hybrid electric vehicle (HEV) powertrains has progressed to a point where low voltage auxiliary electrical system loads have a pronounced impact on fuel economy (FE). While improving the energy consumption of an individual component may result in minor improvements, the collective optimization of such loads across a complete vehicle system can result in meaningful FE gains. Traditional methods using chassis dynamometer testing alone to quantify the impact of a specific auxiliary load can lead to issues where signal state changes are too small for accurate detection. This presents difficulties in accurately predicting the influence of such loads on FE 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 auxiliary load energy consumption. The process combines the best features of empirical testing with model-based system engineering and accurately estimates the effect of small changes in total average oncycle auxiliary load power. This approach supports timely and resource-efficient estimates of the FE impact of auxiliary load components and control strategies. An overview of the effects of auxiliary load power on the FE of a modern HEV is presented for different drive cycles and the estimation process is presented.