An availability index, Generation Ratio Availability (GRA), is proposed to evaluate the electrical system of offshore wind farms (OWF). The GRA is the probability that at least a certain percent of wind power could be transferred to the grid system through the concerned electrical system. The GRA does not depend on the load demand and has weaker correlation with the wind speed in comparison with other reliability indices, such as loss-of-load probability, expected energy not supplied, etc. On the basis of the particular characteristics of the OWFs, the algorithm to compute the GRA is proposed. Comprehensive studies have been conducted to investigate the influence of the network design, component parameters, and wind-speed regimes on the GRA. The analysis presented in this paper is useful for both future wind farm planning and existing OWF evaluation.