1. Purpose of the Functional Unit
The functional unit (FU) is the quantified measure of the function or service that the product provides. It serves as the common reference against which all material, energy, and emission flows are calculated. This ensures that results are meaningful and comparable across different products or systems. A well-defined FU reflects what the product does, not merely what it is.
2. Key Considerations When Defining the FU
The client must ensure that the FU is:
Relevant to the product’s function: It must capture the actual service provided, not just the physical product.
Comparable: All compared products must share the same FU to ensure consistency.
Measurable and practical: Data collection should be feasible for the chosen FU.
Clearly bounded: Time horizon, service life, maintenance, and use conditions must be defined.
Neutral and representative: The FU must not favor one alternative or technology by its definition.
Stable over time: It should represent typical performance during the period of use.
3. Examples of Functional Units
For a smartphone, one could use “1 smartphone produced” if the goal is to assess manufacturing impacts per unit, or “1 kg of smartphone produced” if comparing models of different weights.
For building insulation, a suitable FU would be “providing thermal resistance of 1.5 m²K/W over 50 years per square meter,” which incorporates both performance and durability.
For lighting systems, the FU could be “annual lighting of a 10 m² workspace at 30 lux,” focusing on the service delivered rather than the number of bulbs.
For fuels or energy carriers, “1 MJ of delivered energy” is the most relevant FU, since the functional output is energy provision.
For a toaster, one could define the FU as “toasting two slices of bread,” corresponding to the actual service provided.
4. Declared Units and Multiple FUs
A single product may have several possible FUs, depending on what the study aims to evaluate. In construction-related LCAs and EPDs, a distinction is often made between a functional unit (service-based) and a declared unit (product-based). For instance, “1 m² of insulation board with 50 mm thickness” can serve as a declared unit when the functional performance (thermal insulation) is not directly modeled.
When comparing products or alternatives, maintaining an identical FU is essential. The FU ensures that all results are normalized to the same reference — making the comparison fair, transparent, and scientifically valid.