Ireland has a target of meeting 40% of its electricity requirement with renewable energy by 2020. It is estimated that Ireland has a practical wind energy resource of 613 TWh, which is about 21 times the total electricity requirement in Ireland at the end of 2008. Therefore wind energy is worth pursuing, however many technical challenges remain as it progresses offshore, one of which is the foundation and this will be designed and optimised in this paper. Four different wind turbine (WT) foundations will be designed and optimised in this paper, which include the small-scale onshore, large-scale onshore, offshore monopile and offshore gravity-based foundation. According to the European Wind Energy Association, the foundation makes up on average 6.5% of the capital cost for onshore projects and 34% of that of offshore projects. This justifies the need for optimisations to be performed on all WT foundations to make wind energy more cost-competitive with conventional forms of thermal electricity generation. The primary driver in foundation size is wind loading, however it is also more desirable to locate these structures in areas of high wind resources to maximise the annual energy yield and consequently return on investment. Therefore a specific indicator has been applied for this paper to measure performance of each foundation in terms of its mass compared to the annual energy yield of the WT. This indicator is the energy payback time (EPT) and it is minimised in order to develop the most cost-effective and optimal foundation. (C) 2012 Elsevier Ltd. All rights reserved.