As rooftops in cities are mostly underused, they have a large potential for decentralised electricity production. In that context, photovoltaic (PV) panels have proven to be an effective solution. Meanwhile, the market of small wind turbines is increasing, and some building owners have already installed one or more units on their roof. While the economic comparison between PV panels and wind turbines has already largely been addressed, in general, the space constraint of a rooftop has never been taken into account. In this work, the authors propose a methodology to compare the energy production and the return on investment both for rooftop-mounted PV panels and wind turbines. The comparison is made for relatively tall buildings (≥60 m) with good wind conditions (≥5 m/s annual mean wind speed). Using a brute-force approach, this study presents the results of the methodology applied to a case study: the Brussels Region. On tall rooftops, considering the space already taken by other installations and assuming an average wind speed of 5 m/s, small building-mounted wind turbines are shown to produce more energy than PV panels. Nevertheless, their return on investment is always lower than that of the PV panels.