This paper discusses the identification of the environmental consequences of marginal electricity supplies in consequential life cycle assessments (LCA). According to the methodology, environmental characteristics can be examined by identifying affected activities, i.e. often the marginal technology. The present ‘state-of the-art’ method is to identify the long-term change in power plant capacity, known as the long-term marginal technology, and assume that the marginal supply will be fully produced at such capacity. However, the marginal change in capacity will have to operate as an integrated part of the total energy system. Consequently, it does not necessarily represent the marginal change in electricity supply, which is likely to involve a mixture of different production technologies. Especially when planning future sustainable energy systems involving combined heat and power (CHP) and fluctuating renewable energy sources, such issue becomes very important.

This paper identifies a business-as-usual (BAU) 2030 projection of the Danish energy system. With a high share of both CHP and wind power, such system can be regarded a front-runner in the development of future sustainable energy systems in general. A strict distinction is made between, on the one hand, marginal capacities, i.e. the long-term change in power plant capacities, and on the other, marginal supply, i.e. the changes in production given the combination of power plants and their individual marginal production costs. Detailed energy system analysis (ESA) simulation is used to identify the affected technologies, considering the fact that the marginal technology will change from one hour to another, depending on the size of electricity demand compared to, among others, wind power and CHP productions. On the basis of such input, a long-term yearly average marginal (YAM) technology is identified and the environmental impacts are calculated using data from ecoinvent.

The results show how the marginal electricity production is not based solely on the marginal change in capacity but can be characterised as a complex set of affected electricity and heat supply technologies. A long-term YAM technology is identified for the Danish BAU2030 system in the case of three different long-term marginal changes in capacity, namely coal, natural gas or wind power.

Four analyses and examples of YAMs have been used in order to present examples of the cause–effect chain between a change in demand for electricity and the installation of new capacity. In order to keep open the possibilities for further analysis of what can be considered the marginal technology, the results of four different situations are provided. We suggest that the technology mix with the installation of natural gas or coal power plant is applied as the marginal capacity.

The environmental consequences of marginal changes in electricity supply cannot always be represented solely by long-term change in power plant capacity, known as the long-term marginal technology. The marginal change in capacity will have to operate as an integrated part of the total energy system and, consequently, in most energy systems, one will have to identify the long-term YAM technology in order to make an accurate evaluation of the environmental consequences.

This paper recommends a combination of LCA and ESA as a methodology for identifying a complex set of marginal technologies. The paper also establishes values for Danish marginal electricity production as a yearly average (YAM) that can be used in future LCA studies involving Danish electricity.