This paper evaluates the Solar Chimney Power Plant performance subject to adverse solar radiance conditions. For that, numerical simulations are performed to estimate the optimal ratio of the turbine pressure drop to the total pressure potential (x-factor) as an independent control variable. The results show that x-factor values remain around 0.8 for periods with sufficient heat gain from sun or ground. Otherwise, x-factor values have a tendency to drop to zero. In winter, due to lower system heat gains, this trend is more accentuated. (C) 2013 Elsevier Ltd. All rights reserved.