Power optimiser are becoming increasingly popular in residential photovoltaic installations. Besides monitoring and safety advantages the power optimise can increase production under partial shading conditions due to the individual maximum power point tracking performed independently for each panel. This study quantifies the production gains with power optimisers through both empirical and theoretical city-wide simulations. Empirical measurements on adjacent string and optimiser PV systems showed that shading losses from a tree decreased from 17% to 13 %. A study of 1100 single family houses in an urban suburb showed that 2/3 of the houses would have a gain with optimisers above 20 kWh/kWp and year.