The European Union aims to ensure that investment in energy efficiency measures is cost-effective. Thus, the minimum energy performance requirements of buildings must follow the so-called cost-optimal levels. It is known that the impact of a specific measure on the energy performance is affected by others measures when implemented simultaneously, influencing its profitability. for this reason, the profitability of a given package of measures cannot result from the simple sum of potential benefits of each measure. consequently, to define a cost-optimal solution it is needed to run a great amount of combinations, implying an expensive computational effort. In order to help with the selection of the energy systems, this work proposes a simplified method for selecting heating and domestic hot water systems as a function of the following variables: initial investment, maintenance cost, energy needs and cost, and efficiency of energy systems. The proposed method is user-friendly and can assist various stakeholders: policy makers, energy experts, suppliers of products and services and building owners.
cost-optimal, energy retrofit, buildings, sustainability
 European Commission, Commission Delegated Regulation (EU) No 244/2012 of 16 January 2012 supplementing Directive 2010/31/EU of the European Parliament and of the Council on the energy performance of buildings by establishing a comparative methodology framework for calculating. Official Journal of the European Union, L 81/18-36, 2012.
 EN 15459–Energy performance of buildings—Economic evaluation method for energy systems in buildings. European Committee of Standardisation, Bruxels, 2007.
 Instituto Nacional de Estatística (INE) and DGEG. Inquérito ao Consumo de Energia no Sector Doméstico 2010, 2010.
 Serra, C., Simões, N., Tadeu, S. & Tadeu, A., Definition of Reference Buildings for Energy Performance Calculation—Portuguese Case, Energy for Sustainability 2013, Coimbra, Portugal, 2013.
 Tadeu, S., Tadeu, A., Simões, N., Gonçalves, M. & Prado, R., A sensitivity analysis of a cost optimality study on the energy retrofit of a single-family reference building in Portugal. Energy Efficiency, 11(6), pp. 1411–1432, 2018. https://doi.org/10.1007/s12053-018-9645-5
 Ministério do Ambiente, Ordenamento do Território e Energia. Despacho n.o 15793-E/2013. Diário da Repúplica, (14), pp. 14–25, 2013.
 ISO EN 13790—Energy performance of buildings—Calculations of energy use for space heating and cooling. 2008.
 Diário da República, no 15793-I/20013 (extrato), 2.a série — N.o 234 — 3 de dezembro de 2013. 2013.
 European Commission, EU Energy, Transport and GHG Emissions: Trends to 2050, Reference Scenario 2013. 2013.
 Energy Services Regulatory Authority. Reference Prices in the Liberalized Market of Electricity and Natural Gas in Continental Portugal (in Portuguese), 2017.