Co-rotating twin-screw extruders are generally operated in starve-fed mode, where large proportions of the screw are partially filled. The conveying characteristics of the screw elements that are fully filled have considerable influence on the overall degree of filling along the screws. The fully filled regions also contribute significantly to the total power requirement and melting and mixing behavior of the machinery. Modeling the transport phenomena in a fully intermeshing co-rotating twin-screw extruder is key to predicting its conveying characteristics and power requirements. In this work, we analyzed the conveying characteristics and power consumption of double-flighted fully intermeshing co-rotating twin-screw extruders, including the effects of screw and nip clearances. First, we transformed the problem under consideration into a dimensionless representation by applying the Buckingham PI Theorem, and identified the characteristic influencing and target parameters. We then analyzed the fundamental relationships between flow rate, pressure gradient and power requirement, and identified suitable conveying and power parameters for a Newtonian fluid. Based on these findings, a parametrically driven numerical study was conducted: For each modeling setup the flow field was predicted by means of a full three-dimensional FEM analysis to gain fundamental insights into the conveying characteristics and power requirements of commonly used conveying elements in co-rotating twin-screw extruders. Since the results include the effects of screw and nip clearances, they also allow the effect of wear on the screw characteristics to be assessed. Additionally, by combining the results of the conveying and power parameters, the viscous dissipation rate and consequently the increase in melt temperature can be estimated.