Small wind turbines have the potential to provide a significant amount of useful electricity; particularly in urban areas where it is necessary to use self-supporting monopole towers. Their take-up can be increased by reducing tower costs. The numerical optimisation technique called differential evolution (DE) was used to design a minimal mass self-supporting tower for a 5 kW wind turbine, whilst retaining the required strength and stability. The main problem in the optimisation was the limited availability of appropriate simple equations for buckling analysis of the chosen octagonal geometry as required for design certification to the appropriate international standards. Performing linear buckling analysis (which is unsuitable for global optimisation) on towers designed to meet the available buckling equations showed that the buckling strength was significantly overestimated for low wall thicknesses. A correction factor was formulated and applied to the existing buckling equations to remove this inconsistency. DE was then used to design a tower that was 7% lighter and 20% more resistant to buckling than the current reference design.