Cell switch off (CSO) is a technique in the mobile networks sphere that has been proposed and implemented widely to achieve reduced energy consumption of base station (BS) deployments. The energy consumption of BSs has increasingly become an interesting research area and rightly so, given the advancement in modern networking technology and the increase in network resources demand. The aim of this work is to examine CSO closely by evaluating it over a real urban area in Europe and using real data from the mobile network deployment of a national European mobile network operator. Our aim is to quantify the energy impact of applying CSO techniques to real BS network deployments. Through a detailed modelling of the BS power consumption, we show that a total of 18 MW can be saved over the studied urban area in 24 hours within the purview of the defined quality of service constraints.