An energy system is a system primarily designed to supply energy-services to end-users.^[1]: 941 The intent behind energy systems is to minimise energy losses to a negligible level, as well as to ensure the efficient use of energy.^[2] The IPCC Fifth Assessment Report defines an energy system as "all components related to the production, conversion, delivery, and use of energy".^[3]: 1261

The first two definitions allow for demand-side measures, including daylighting, retrofitted building insulation, and passive solar building design, as well as socio-economic factors, such as aspects of energy demand management and remote work, while the third does not. Neither does the third account for the informal economy in traditional biomass that is significant in many developing countries.^[4]

The analysis of energy systems thus spans the disciplines of engineering and economics.^[5]: 1 Merging ideas from both areas to form a coherent description, particularly where macroeconomic dynamics are involved, is challenging.^[6][7]

The concept of an energy system is evolving as new regulations, technologies, and practices enter into service – for example, emissions trading, the development of smart grids, and the greater use of energy demand management, respectively.

From a structural perspective, an energy system is like any system and is made up of a set of interacting component parts, located within an environment.^[8] These components derive from ideas found in engineering and economics. Taking a process view, an energy system "consists of an integrated set of technical and economic activities operating within a complex societal framework".^[5]: 423 The identification of the components and behaviors of an energy system depends on the circumstances, the purpose of the analysis, and the questions under investigation. The concept of an energy system is therefore an abstraction which usually precedes some form of computer-based investigation, such as the construction and use of a suitable energy model.^[9]

Viewed in engineering terms, an energy system lends itself to representation as a flow network: the vertices map to engineering components like power stations and pipelines and the edges map to the interfaces between these components. This approach allows collections of similar or adjacent components to be aggregated and treated as one to simplify the model. Once described thus, flow network algorithms, such as minimum cost flow, may be applied.^[10] The components themselves can be treated as simple dynamical systems in their own right.^[1]

The concept of an energy-service is central, particularly when defining the purpose of an energy system:

It is important to realize that the use of energy is no end in itself but is always directed to satisfy human needs and desires. Energy services are the ends for which the energy system provides the means.^[1]: 941

Energy-services can be defined as amenities that are either furnished through energy consumption or could have been thus supplied.^[18]: 2 More explicitly:

Demand should, where possible, be defined in terms of energy-service provision, as characterized by an appropriate intensity^{[lower-alpha 2]} – for example, air temperature in the case of space-heating or lux levels for illuminance. This approach facilitates a much greater set of potential responses to the question of supply, including the use of energetically-passive techniques – for instance, retrofitted insulation and daylighting.^[19]: 156

A consideration of energy-services per capita and how such services contribute to human welfare and individual quality of life is paramount to the debate on sustainable energy. People living in poor regions with low levels of energy-services consumption would clearly benefit from greater consumption, but the same is not generally true for those with high levels of consumption.^[20]

The notion of energy-services has given rise to energy-service companies (ESCo) who contract to provide energy-services to a client for an extended period. The ESCo is then free to choose the best means to do so, including investments in the thermal performance and HVAC equipment of the buildings in question.^[21]

ISO 13600, ISO 13601, and ISO 13602 form a set of international standards covering technical energy systems (TES).^{[22][23][24][25]} Although withdrawn prior to 2016, these documents provide useful definitions and a framework for formalizing such systems. The standards depict an energy system broken down into supply and demand sectors, linked by the flow of tradable energy commodities (or energywares). Each sector has a set of inputs and outputs, some intentional and some harmful byproducts. Sectors may be further divided into subsectors, each fulfilling a dedicated purpose. The demand sector is ultimately present to supply energyware-based services to consumers (see energy-services).

Energy system design includes the redesigning of energy systems to ensure sustainability of the system and its dependents and for meeting requirements of the Paris Agreement for climate change mitigation. Researchers are designing energy systems models and transformational pathways for renewable energy transitions towards 100% renewable energy, often in the form of peer-reviewed text documents created once by small teams of scientists and published in a journal.

Considerations include the system's intermittency management, air pollution, various risks (such as for human safety, environmental risks, cost risks and feasibility risks), stability for prevention of power outages (including grid dependence or grid-design), resource requirements (including water and rare minerals and recyclability of components), technology/development requirements, costs, feasibility, other affected systems (such as land-use that affects food systems), carbon emissions, available energy quantity and transition-concerning factors (including costs, labor-related issues and speed of deployment).^{[26][27][28][29][30]}

Energy system design can also consider energy consumption, such as in terms of absolute energy demand,^[31] waste and consumption reduction (e.g. via reduced energy-use, increased efficiency and flexible timing), process efficiency enhancement and waste heat recovery.^[32] A study noted significant potential for a type of energy systems modelling to "move beyond single disciplinary approaches towards a sophisticated integrated perspective".^[33]

• Control volume – a concept from mechanics and thermodynamics
• Electric power system – a network of electrical components used to generate, transfer, and use electric power
• Energy development – the effort to provide societies with sufficient energy under the reduced social and environmental impact
• Energy modeling – the process of building computer models of energy systems
• Energy industry – the supply-side of the energy sector
• Mathematical model – the representation of a system using mathematics and often solved using computers
• Object-oriented programming – a computer programming paradigm suited to the representation of energy systems as networks
• Network science – the study of complex networks
• Open energy system databases – database projects which collect, clean, and republish energy-related datasets
• Open energy system models – a review of energy system models that are also open source
• Sankey diagram – used to show energy flows through a system