Beyond the constraints of closed systems, open systems can gain or lose energy in association with matter transfer (both of these process are illustrated by fueling an auto, a system which gains in energy thereby, without addition of either work or heat).

Energy transfer can be considered for the special case of systems which are closed to transfers of matter.

The energy tower is a device for producing electrical power. Energy towers spray water on hot air at the top of the tower, making the cooled air fall through the tower and drive a turbine at the tower's bottom.

Energy technology is an interdisciplinary engineering science having to do with the efficient, safe, environmentally friendly, and economical extraction, conversion, transportation, storage, and use of energy, targeted towards yielding high efficiency whilst skirting side effects on humans, nature, and the environment.

An energy system is a system primarily designed to supply energy-services to end-users.

Energy subsidies are measures that keep prices for customers below market levels, or for suppliers above market levels, or reduce costs for customers and suppliers.

Energy storage as a service (ESaaS) allows a facility to benefit from the advantages of an energy storage system by entering into a service agreement without purchasing the system.

A means of storing energy for use at a later time, usually to meet peak energy requirements.

An energy slave is that quantity of energy (ability to do work) which, when used to construct and drive non-human infrastructure (machines, roads, power grids, fuel, draft animals, wind-driven pumps, etc.) replaces a unit of human labor (actual work).

A business that provides a broad range of energy solutions including designs and implementation of energy savings projects, retrofitting, energy conservation, energy infrastructure outsourcing, power generation and energy supply, and risk management.

Energy security is the association between national security and the availability of natural resources for energy consumption.

Energy Savings Performance Contracts (ESPCs), also known as Energy Performance Contracts, are an alternative financing mechanism authorized by the United States Congress designed to accelerate investment in cost effective energy conservation measures in existing Federal buildings.

The ratio of the amount of usable energy (the exergy) delivered from a particular energy resource to the amount of exergy used to obtain that energy resource.

Energy recovery ventilation is the energy recovery process in residential and commercial HVAC systems that exchanges the energy contained in normally exhausted air of a building or conditioned space, using it to treat (precondition) the incoming outdoor ventilation air.

Energy recovery includes any technique or method of minimizing the input of energy to an overall system by the exchange of energy from one sub-system of the overall system with another.

Energy quality is a measure of the ease at which a form of energy can be converted to useful work or to another form of energy.

Energy poverty is lack of access to modern energy services.

Energy policy is the manner in which a given entity has decided to address issues of energy development including energy conversion, distribution and use.

During large building construction the framework of pilings can be filled with heat exchanger material prior to concrete being poured. The concrete and surrounding structure becomes the heat (or cooling) sink.

The energy payback time (EPBT) describes the time span a PV system needs to operate in order to generate the same amount of energy that was used for its manufacture and installation.