The effect of an upstream flat deflector on the power output of two counter-rotating straight-bladed vertical-axis wind turbines (VAWTs) was investigated experimentally. Although many studies on the deflector effect have been done for a drag-based Savonius VAWT, there is little information on the performance of a lift-based VAWT with a deflector. If a flat vertical plate is normal to the free-stream wind direction, the local wind speed outside the near-wake region of the plate is larger than the free-stream velocity. Thus, when each of the counter-rotating turbines is positioned outside the near-wake region, its power output increases significantly. The power output increase with the deflector installed is dependent on the width and height of the deflector and the turbine position relative to the deflector. (c) 2013 Elsevier Ltd. All rights reserved.