Functional Unit and System Boundary: Making Comparisons Fair
What this domain tests
This domain tests two of the most mark-sensitive choices in LCA: the quantified function used to scale all flows, and the boundary that decides which unit processes are inside the study.
Core rules and concepts
- The functional unit is the quantified performance of the system used as the reference. The reference flow is the amount of product needed to deliver that function.
- The functional unit must be measurable and linked to the function, not merely to the product mass.
- System boundary defines unit processes, inputs, outputs, geography, time horizon, and whether capital goods and infrastructure are included.
- Cut-off criteria may exclude negligible mass, energy, or environmental significance, but exclusions must be transparent and must not remove controversial flows.
- Multifunctional processes require subdivision, system expansion, or allocation; the choice must follow the goal.
Worked example
Two exterior paints are compared. Paint A covers 10 m2 per litre and lasts 5 years; Paint B covers 8 m2 per litre and lasts 8 years. The functional unit is "protect 100 m2 of facade for 16 years". Paint A needs two application cycles, so its reference flow is 20 litres. Paint B needs two cycles at 12.5 litres each, or 25 litres. The comparison must include both production and repainting burdens. A per-litre comparison would favour A only because it ignores the shorter service life and the extra application cycle.
Traps that cost marks
- Using "1 kg of product" when the function is coverage, cooling, or transport.
- Ignoring product lifetime when comparing durability.
- Setting a boundary that excludes a phase likely to dominate the result.
- Using cut-off to hide a small but toxic or controversial flow.
- Applying allocation before considering subdivision or system expansion.