The development of offshore wind power plants (OWPPs) can last between seven and ten years, depending on the scale of the plant and on the situation of the supply chain involved. Project stages are usually overlapping and iterations implicating different teams are sometimes necessary to find good solutions. The approaches for the cost-benefit analyses necessary to support the decisions that have to be taken along the project can differ significantly. These design teams have the challenge of providing a technically and economically feasible solution to the subsystem they are designing, bearing in mind the contracts to be signed afterwards with their suppliers. And they should do it without losing the overall perspective of the OWPP and respecting the conditions fixed in the contracts, because changes introduced afterwards usually lead to significant increases in the costs. On the other hand, these techno-economic analyses are also interesting in relation to engineering research. A researcher focused on a technical improvement for a certain subsystem of the OWPP should counterbalance its technical or economical benefit with the increase in cost necessary to include that innovation in the project. And this analysis should be done maintaining the overall perspective, because partial approaches can overestimate the benefits derived from technical improvements, and besides, precious time can be lost considering technical alternatives that are not realistic due to economic, logistic or maintenance determinants. However, to fulfil a realistic cost-benefit analysis of an offshore windfarm is a real challenge. The difficulties come from the technical complexity of the asset, from the correlation among technical, economical and environmental variables, and from the lack of data due to the incipient stage of the technology. Ideally, these assessments should be developed using a methodology which has a scope as wide as possible, which has a simple formulation that enables the evaluation of a sufficient number of scenarios, and which is opened to the consideration of new topologies proposed by the research community. In this context, this thesis provides a new methodology to evaluate techno-economically offshore wind power plant electric topologies. The proposal is aimed at engineering researchers and project planning engineers. The methodology presented is based on the analysis of three related issues: the already commissioned offshore wind farms (OWFs), the medium-term technological trends and the previous evaluation approaches published during the last decade. The formulation of the methodology is presented in detail, the relevant sources of information are cited, the scope is clarified and, finally, the methodology is applied to a case study.