Energy trilemma is one of the most pressing concerns on agendas of many governments and international organizations. In this paper, the authors propose a novel quantitative tool to aid designing policies for energy systems planning and creating research and developments programs aimed at achieving specific policy goals. The proposed modified Markowitz theory can be used to build the efficient plane considering sustainability, security and affordability of the energy system composed of a portfolio of competing technologies. The novel and original contribution of this paper is the extension of the Markowitz theory to include the measure of portfolio's sustainability and proposing a new tool to tackle the energy trilemma. The proposed tool was used to compare policy packages to support possible steady green growth of Singapore economy - a densely populated city state with no natural resources, dynamic economy requiring secured supply of energy, and ambitions of being a leader in sustainable development. It was found that efforts aimed at simultaneous improving performance and lowering costs of novel technologies is more desirable, contrary to policy focusing on either alone. Fuel cells and solar photovoltaic panels were found to be important pieces of an efficient power generation portfolio. (C) 2017 Elsevier Ltd. All rights reserved.