The location of step-up offshore substations for offshore wind power plants forces power transformers to be placed on a platform. This offshore platform means a new cost (avoided in a conventional onshore substation) which grows with the transformer weight and footprint surface. As a result, the weight and footprint surface of transformers are new parameters that must be considered in the optimization process of power transformers for offshore wind power plants. The use of compact transformers should lead to the reconsideration of the platform design criteria as compact transformers can reduce significantly the investment cost [1]. The total cost of the transformer, platform and electrical losses have to be evaluated as a whole in order to find the optimal solution. Up to now, no special value has been given to the most compact solution, but the results show that compact solutions have an important impact into the overall economic evaluation.