Archimedes screw generators (ASGs) are beginning to be widely adopted at low-head hydro sites in Europe due to their high efficiency, competitive costs, and low environmental impact. ASGs are particularly appropriate for low-head sites. Power is transferred from a water flow to an Archimedes screw by the distribution of static pressure produced by the water volumes between the flights of the screw. Two theoretical models based on quasi-static pressure analysis are developed to predict the performance of ASGs. The first model uses idealized geometry, while the second incorporates the geometric properties of a rotating Archimedes screw, including slope, pitch, and inner and outer diameter. The second model was also formulated to simulate the performance of Archimedes screws operating across a full range of fill levels from empty to overfull. Both models predict that if all friction losses and entry and exit effects are neglected, the Archimedes screw can convert all potential energy in a water flow into mechanical power. Including gap leakage effects in the simulation decreases efficiency. A new leakage model is used because the current standard leakage model is only appropriate for screws in a full condition. It was confirmed that the maximum efficiency of an ASG will occur when the screw is operating near full; however, reasonable efficiencies are maintained if the screw is either overfilled or underfilled. The model predictions were consistent with the measured performance of a laboratory-scale Archimedes screw at low rotational speeds.