Portugal has made great progress in implementing renewable energy systems (RES) to use its endogenous renewable resources. As the cost of renewable energy generation is decreasing, mainly for photovoltaic energy, a significant increase in its production is expected, in particular at the local and domestic levels. Yet, much investment and development is still needed to fulfil the goals for renewable energy generation defined by the Portuguese governme. Portugal has made great progress in implementing renewable energy systems (RES) to use its endogenous renewable resources. As the cost of renewable energy generation is decreasing, mainly for photovoltaic energy, a significant increase in its production is expected, in particular at the local and domestic levels. Yet, much investment and development is still needed to fulfil the goals for renewable energy generation defined by the Portuguese government and the European Union, in order to decarbonize energy generation and reduce energy dependence. Besides limitations in the installed capacity, the full potential of existing and future RES is not fulfilled, mainly due to imbalances in supply and demand, resulting from the varying climatic conditions and limited energy storage capacity. Although some investment was made in large scale Energy Storage Systems (ESS), especially pumped hydro, distributed energy storage (DES), in particular for stationary domestic storage appliances, have received little attention from decision makers. When properly defined, designed and implemented, they can contribute to increase the efficiency of existing and future RES and the capacity factor. Thus, in this work the questions regarding the implementation of DES are analysed. The main criteria that have to be considered when selecting the proper storage technology for DES are defined, taking into account information and data from current legislation and/or strategic plans and goals, and the technical and scientific literature, in order to support. Decentralized renewable energy generationDistributed energy storageTechnology selectionPortugalModern societies require abundant quantities of energy to perform normal daily activities, such as cooking or transportation, to ensure human comfort and wellness, and to provide the products and services used by people and companies. As the standards of living are improving, in particular in the developing world, the energy consumption is increasing and will continue to rise in the near future (BP, 2019). Currently, most of the energy needs is generated from fossil sources, in particular oil, coal, and natural gas, a state of affairs that is clearly unsustainable and has resulted in significant environmental problems, in particular climate change due to increasing emissions of greenhouse gases, and in economic and even social impacts due to price variability and supply security.Therefore, it is consensual that it is necessary to develop and implement other energy sources, intrinsically renewable, and with lower environmental impacts (Dincer, 2000). Hence, governments and international organizations are developing and implementing policies and/or strategies to promote the generation of more renewable energy, contributing to reduce the dependence on fossil fuels and to decarbonize the economy, thus also contributing to fulfil the goals of the Paris Agreement (UNCC, 2016).In. Portugal does not possess significant fossil energy resources to fulfil the needs of citizens and companies. This state of affairs has resulted in significant dependence on imported energy (Pordata, 2017), especially in the transportation sector, where the dependence on fossil fuels is almost complete. Thus, it is imperative to counteract this dependence that has significant economic and supply security impacts. To generate the energy needed to support the economic development and the expansion of electrical network to supply the electricity necessary to increase the standards of living, there is an ongoing strategy to develop the internal energy resources, focusing in electricity generation since the second half of the twentieth century (Martins et al., 2018). Due to its climate conditions and topography, there is significant potential in hydroelectricity and wind power in the Northern Portugal, and PV power nationwide (DGEG, 2019).On an initial phase, a focus was given to explore hydro power resources (Nunes, 2018). But, by the beginning of the 21st century most of the hydro resources were exploited, as almost all the sites and rivers adequate to construct and implement large hydro are already used. Although retrofitting existing hydroelectric power plants is being implemented and considered to increase the generation of hydroelectricity, the overall increase in the renewab. As stated before, to increase the efficiency of existing and future RES it is necessary to store the renewable energy produced in excess using proper energy storage technologies. Many possibilities exist based on different physical and/or chemical phenomena or methods (Huggin, 2010; IRENA, 2017), including: mechanic (pumped hydroelectric currently dominant in Portugal); electric (super capacitors), electrochemical (batteries of different types, including Li-ion, flow batteries, and lead acid), thermal energy (PCM - phase change materials), Chemical (hydrogen, power to gas, biofuels), among others. A full description of the available energy storage technologies, costs and optimal utilization conditions, among other factors, is outside the scope of this work and can be found in the literature (Ferreira et al., 2013; IRENA, 2017; Mostert et al., 2018; Mongird et al., 2019).The different technologies vary in their range of applications, requirements and suitability to various renewable energy generation systems or energy applications. Currently, when selecting the most adequate energy storage system the challenge is how to take into account, not only the economic aspects but also the environmental and social ones. In other words, it is necessary to consider the full sustainability impacts of decisions to be made. This also must be taken into account when considering policy definition and i.
