Since the 1980s, the Brazilian government has implemented many policies and programmes addressing energy efficiency. The electricity consumption in residential and commercial, service and public buildings in Brazil is approximately 50% of the total electricity consumed in the country; 70% of electricity consumption in public buildings is due to the lighting and air-conditioning systems. This paper presents a case study in a Brazilian Navy building, using DIALux software to study energy efficiency and to make an energy diagnosis regarding lighting to propose improvements so as to obtain the Brazilian partial National Energy Conservation Label (partial ENCE). Three different scenarios were simulated, and the results showed that it is possible to obtain about 2475 kWh/(m2/year) in energy savings, by replacing the current lighting system of fluorescent lamps with light-emitting diode (LED) lighting. Thus, it was possible to observe, through the results of the simulation and the energy diagnosis, that the replacement of the lighting technology, associated with layout modification, provides satisfactory results and meets the minimum desirable requirements of the Brazilian technical standard ABNT NBR ISO/CIE 8995-1, 2013.