In an urban environment, dual vertical axis wind turbines (VAWTs) are expected to demonstrate more advantage than horizontal axis wind turbine. For VAWT, solidity is one of the most important characteristics, while it has not been well studied. In this study, to investigate the effect of rotor solidity and other characteristics on the power output of counter-rotating dual VAWTs at different rotor centre spacings in a low turbulence intensity condition, twelve wind tunnel experiments for dual VAWTs and six wind tunnel experiments for an isolated VAWT were conducted. The results show that the effect of the dual VAWTs configuration on the VAWT power output is small compared with isolated VAWT under low wind speeds in an urban environment, but the effect exhibits a slight improvement with an increase in rotor solidity. As rotor solidity decreases, the sensitivity of the effect on rotor centre spacing increases. The effect of skewed flow on the power output of dual VAWTs are also well discussed. Furthermore, dual-VAWT systems are studied under real urban environment of a number of major cities such as Guangzhou, London, and Copenhagen. Potentials in different heights are compared with different configurations based on the city planning of those major cities. •An experimental test on various dual-rotor system was conducted in a wind tunnel.•The relationship between solidity and the dual rotor system is analyzed.•The performance of the dual rotor system under skewed flow is discussed.•The potential of dual rotor system on building are studied in various major cities.