Introduction  This article aims to study the optimal planned capacity setting of large-scale offshore wind farm.

  Method  A series of numerical experiments were conducted on a single planned site with a capacity of 1 GW using three models with different unit capacities and three offshore wind farm wake models, combined with an offshore wind resource atlas, to gradually expand it to 2 GW. Power generation, wake effects and marginal utility of the whole site were analyzed.

  Result  The results show that employing the wind turbine with larger single-unit capacity, the higher gaining of the power generation and slower growth of wake loss appear during the capacity expansion. Meanwhile, the larger effective expansion range which taking into account the safety of the wind turbine and the marginal utility exists. However, evaluation of effective expansion range may be affected by wake model selection.

  Conclusion  This article shows that the optimal planned capacity of a single offshore wind farm can be slightly higher than the existing benchmark when multiple constraints, such as utilization of sea area, wind turbine safety and economical efficiency are met. Individual site needs to be subdivided scientifically and the optimal planned capacity should be reasonably set in the future planning of offshore wind farm bases with capacities in the tens of millions of kilowatts.