A crucial step to designing and building more efficient vehicles is modeling powertrain energy consumption. While accurate modeling is indeed key to effective and efficient design, a fundamental understanding of the powertrain and auxiliary systems that contribute to the energy consumption of a vehicle is equally as important. This paper presents a methodology that has been packaged into a tool, called VTool (short for Vehicle Tool), which can be used to estimate the energy consumption of a vehicle powertrain. The method is intrinsically designed to foster understanding of the vehicle powertrain as it relates to energy consumption and losses while still providing reasonably accurate results.This paper briefly explains the methodology of VTool and demonstrates the capability of VTool as a design tool by presenting 4 example exercises. The first example is a parametric study of accessory load that examines the impact of accessory load on overall energy consumption by varying the accessory load from 0 kW to 4 kW. This study concludes that accessory load has a more pronounce effect during less aggressive, lower-speed driving. It also demonstrates that small changes in accessory load (such as turning on headlights) have a relatively negligible effect on overall fuel consumption, while larger changes (such as using A/C or an electric heater) do significantly impact overall energy consumption. Similarly, the next parametric study examines the impact of regenerative (or regen) braking on overall energy consumption by varying the regen brake fraction from 0 to 1. This study shows that regen braking can have a significant impact on overall energy consumption, saving up to about 800 Wh/mi of fuel energy for a Series Hybrid Electric Vehicle (HEV).Next, an example battery sizing exercise illustrates how VTool can be used to explore the effects of battery size on an Electric Vehicle (EV) or Plug-in Hybrid Electric Vehicle (PHEV). This exercise shows that energy consumption increases only 3% per 100 kg of battery added, which means that the addition of more battery energy is well worth the additional mass. In the final example, VTool is used to compare the losses between an EV, a Series HEV and a Parallel HEV in charge sustaining mode. This endeavor illustrates that an EV consumes significantly less energy than either HEVs and shows that the two HEV powertrains were relatively similar in overall energy consumption even though the cycle-average engine efficiency has the potential to be higher for a Series HEV.