Estimates for lifetime costs of wave energy arrays are difficult to obtain due to the uncertainty surrounding weather windows and failure rates for wave energy converters (WECs). An operations & maintenance (O&M) simulation tool has been used to assess the sensitivity of WEC failure rates on the profitability of a wave farm, with the Pelamis P2 device used as a case study. Two offshore wave energy sites in the UK have been characterised and presented in terms of accessibility for marine operations and power generation. It was found that a wave farm at one site would incur higher costs due to vessels having to wait longer for suitable weather windows. This was balanced by higher generated revenue, showing how the tool can be used to support strategic planning and site selection. The results identify the sensitivity to failure rate estimations for different components, helping target future work to reduce uncertainties and costs. The results highlight the need for WEC developers to collaborate closely with component manufacturers in order to design the best device possible for the challenging marine environment. Collaboration enables more realistic failure rate estimates to be obtained, leading to better understanding of the operational costs for commercial wave energy farms.