The objective of this study is to determine a method of quantifying the energy consumption in friction stir welding (FSW). Qualitatively, it has long been known that FSW uses less energy than fusion welding processes because the average FSW weld temperature does not exceed the solidus temperature. However, tools and data to quantitatively determine the energy consumption in FSW have been missing. The power consumption as a function of time was measured during FSW of 5.2-mm-deep welds in 6061-T6 and 7075-T6 aluminum alloys on a 3-axis CNC mill. The energy consumption is divided into four parts: idle energy and standby energy related to the machine being used as well as plunge energy and FSW energy related to creating the joint. Equations for calculating each of the energy components and the total energy consumption are presented. The concept of specific weld energy is presented as an intrinsic material property that can be used to estimate the FSW power if the weld cross-section and weld speed are known. A method of estimating the weld cross-section based on the FSW tool geometry is presented. It is found that for these two aluminum alloys the specific weld energy decreases significantly with increased weld speed, however, it can be treated as independent of spindle rotation rate. The FSW process/machine is identified as low tare and it is acknowledged that the strategies to reduce total energy consumption may be different than those used for metal cutting.