Defining the functional unit

The functional unit of a product system is a quantified description of the performance requirements that the product system fulfils.

In a comparative study, the functional unit has to be the same for all the compared product systems. After identifying the relevant market segment and the obligatory product properties, the functional unit can be defined and quantified in terms of the obligatory product properties required on the market segment.

The functional unit should as far as possible relate to the functions of the product rather than to the physical product. For example, rather “annual lighting of a work area of 10 square metres with 30 lux” than “bulbs providing 30000 lumen for one year”. In this way, it is ensured that all obligatory properties – as well as the duration of the product performance – are addressed.

When modelling the consequences of a decision, the size of the consequences influence how the size of the functional unit should be defined:

  • For small decisions, where the substitution between alternative products do not affect the overall trends in market volumes, nor the constraints on and production costs of the involved technologies: The precise size of the functional unit will have no importance for the interpretation of the results. Only concern is that it should be as easy as possible for the reader of the study to compare the outcome to something well-known. For this reason, it may be an advantage to set the size of the functional unit equal or close to the annual per capita consumption of the studied product in the studied market segment.
  • For large decisions (decisions that affect the general trend in market volumes, decisions that overcome a constraint which otherwise prevents the use of a specific technology, decisions that affect the production costs of the involved technologies, or decisions that in themselves lead to new technologies being brought into play): Choose a functional unit of the same size as the decision to be supported by the study.

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How to cite this: 
Consequential-LCA (2015). Defining the functional unit. Last updated: 2015-10-27. www.consequential-lca.org.