In a cluster of equal wind turbines the capacity factor, i.e., the ratio of average to maximum power, is less than the capacity factor of an isolated wind turbine, because of wake effects. In this paper the decrease of the capacity factor is calculated with the aid of a simple model of the atmosferic boundary layer and the wake of a cluster of wind turbines. The theory is compared with measurements of a model cluster in a wind tunnel. Numerical examples, applying to The Netherlands, indicate that clusters of the order of 10 km2 may yield an average power density of 3 W m-2 with little reduction in the capacity factor, but the (space and time)-averaged power density of large areas, say 1000 km2, may not exceed 0.3 W m-2.