Understanding the prospective environmental impacts of offshore wind energy development requires monitoring that allows for adequate testing of conditions for comparison of unimpacted vs. impacted states. A critical component when designing impact studies is determining the required sampling needed to statistically measure a difference between before and after states in the system, which is often challenging because there are little observational data available for the system of interest at the proper spatiotemporal scales. Here, we present the survey design with power and effect size analyses that were used to design a before-after gradient survey to assess American lobster impacts from an offshore wind submarine cable installation in coastal United States waters. By leveraging long-term monitoring data collected from a random-stratified sampling design survey, a gradient survey testing for effects on lobster at distance intervals from the cable using similar sampling methods was developed. Generalized linear mixed models were employed to determine the needed sampling frequency to assess varied catch-per-unit-effort impacts. We present the survey design and our findings from the power analyses to serve as an example of methodology for designing before and after impact surveys for offshore wind energy, and how preexisting data may be used to do so.