Improving the energy efficiency in the real estate sector plays a decisive key factor in the German energy and climate policy. In order to meet the EU-targets of reducing the energy consumption as well as CO2 emissions energy performance certificates (EPCs) have been introduced to prove the energy efficiency of commercial and residential properties. The aim of the present work is to determine the role of EPCs in the rental decision-making process by investigating their influence on tenants' preferences in the housing market. In this study, we apply a choice-based conjoint (CBC) analysis to investigate the relative importance of different housing characteristics including the apartments' energy efficiency represented by the buildings' EPC. The results suggest an ongoing change in the perception of energy efficiency in the housing market, as the attribute received the third highest importance score after monthly rent and residential area. Moreover, the analysis of different respondent segments provides new insights into the importance of EPCs that strongly depended on the awareness and conscious consideration of the provided information. Thus, the results emphasize a novel perspective on EPCs in the housing market and point out crucial factors for its success as a marketing tool.