An astonishing number of bat fatalities (2,371 belonging to 15 species) were recovered in a provincial surveillance program at wind farms in Cádiz, at the southern tip of Spain, in 2005–2016. We carefully analysed a subset of this sample intending to estimate the true fatality rate in the year 2011, the year for which we had the richest data set (582 fatalities at 38 wind farms). To estimate the true fatality rate, we conducted search-efficiency and scavenger-removal trials in nine wind farms involving 122 turbines and 289 observed fatalities and calculated the searchable surface. An annual fatality rate of 41.1 dead bats per turbine (26.4 per MW) was estimated in the study area using a self-developed estimator due to the singularities of the surveillance program. Our estimator results are lower than those obtained using the Generalized Mortality Estimator (GenEst) developed by U.S. Geological Survey. Estimates of 37,689 or 33,370 fatalities were made according to the province's number of turbines or MW in 2011. Some areas have registered the highest fatality rates ever recorded for bats in wind turbines anywhere globally, even more so because we have been conservative. There is an urgent need for efficient preventive and mitigation measures at wind farms in risky landscapes and with a high rate of bat fatalities, but trials are also needed to know how well the post-operational monitoring is done and how far it is from reality. Clearly, current official post-construction surveillance programs are inefficient for searching bats and therefore mask a high fatality rate, mainly of sedentary bats.