Functional Units & Comparing Products Fairly

"What's the difference between a functional unit and a declared unit?" and "How do I compare products with different lifespans?" are fundamental questions that determine whether your LCA conclusions are valid. This guide provides practical techniques for fair product comparison.

Functional Unit vs. Declared Unit

Key Differences

When Each Is Used

Functional unit - Required for comparative assertions:

• "Product A is better than Product B"
• Consumer-facing claims
• Internal decision-making between alternatives

Declared unit - Common in B2B and EPDs:

• "Here's the impact of 1 kg of our product"
• Intermediate products (sold for further processing)
• When end-use is unknown or varies widely

FAQ: Functional Unit Decisions

"What's the difference between a functional unit and a declared unit (for EPDs)?"

Functional unit quantifies the function:

• "Drying hands 10,000 times"
• "Providing 1 MJ of thermal energy"
• "Transporting 1 tonne-km of freight"

Enables direct comparison: Which product delivers this function with less impact?

Declared unit quantifies the product:

• "1 kg of steel"
• "1 m³ of concrete"
• "1 m² of glass"

Does NOT enable direct comparison without knowing application rates.

Example:

Two insulation products:

• Product A: 30 kg/m³, thermal conductivity 0.040 W/mK
• Product B: 80 kg/m³, thermal conductivity 0.035 W/mK

Declared unit comparison (1 kg):

• Product A: 2.5 kg CO₂ eq/kg
• Product B: 1.8 kg CO₂ eq/kg
• Conclusion: B seems better (per kg)

Functional unit comparison (R-value 5 for 1 m²):

• Product A needs: 200 mm × 30 kg/m³ = 6 kg → 15 kg CO₂ eq
• Product B needs: 175 mm × 80 kg/m³ = 14 kg → 25.2 kg CO₂ eq
• Conclusion: A is actually better for the function

"How do I compare products with different lifespans?"

The challenge: A cheap product lasting 5 years vs. premium product lasting 20 years.

Approach 1: Fixed service period

Define a service period covering multiple product cycles:

Service period: 20 years

Product A (5-year lifespan):
  - 4 units needed over 20 years
  - Total impact: 4 × (manufacturing + use + EoL)

Product B (20-year lifespan):
  - 1 unit needed over 20 years
  - Total impact: 1 × (manufacturing + use + EoL)

Approach 2: Per-year normalization

Divide total life cycle impact by expected lifespan:

Product A: 100 kg CO₂ eq total ÷ 5 years = 20 kg CO₂ eq/year
Product B: 300 kg CO₂ eq total ÷ 20 years = 15 kg CO₂ eq/year

Approach 3: Discounting (less common)

Apply time-value discount rates to future impacts—controversial in LCA.

Best practice: Use Approach 1 (fixed service period) for most comparisons. It's most transparent and widely accepted.

Lifespan estimation challenges:

"How do I handle different product performance levels?"

The challenge: Products that deliver the function differently.

Example: Light bulbs

Functional unit: "Providing 800 lumens of light for 25,000 hours"

Reference flows:

• Incandescent: 25 bulbs + 1,500 kWh electricity
• CFL: 2.5 bulbs + 325 kWh electricity
• LED: 1 bulb + 225 kWh electricity

The functional unit automatically accounts for efficiency differences.

What about quality differences?

When products aren't functionally equivalent:

Option 1: Adjust the functional unit

• "Providing 800 lumens of warm-white (2700K) light"
• Excludes products that don't meet quality threshold

Option 2: Multiple functional units

• Compare within quality tiers
• "Budget tier" vs. "Premium tier"

Option 3: Include quality as a caveat

• Note differences without adjusting
• "Product A meets basic standards; Product B exceeds them"

Practical Examples

Example 1: Reusable vs. Disposable

Product: Coffee cups

Wrong approach:

• 1 ceramic mug vs. 1 paper cup
• Result: Ceramic has higher impact
• Conclusion: Use paper (wrong!)

Right approach:

• Functional unit: "Serving 1,000 hot beverages"
• Ceramic: 1 mug (survives 1,000+ uses) + washing
• Paper: 1,000 cups + 1,000 lids
• Result: Ceramic wins above ~50-100 uses (varies by study)

Break-even analysis: Calculate how many uses before reusable beats disposable:

n × (disposable impact) = (reusable production) + n × (per-use impact)

Solving for n gives the break-even point

Example 2: Electric vs. Gasoline Vehicle

Product: Passenger cars

Functional unit: "Providing personal transportation for 200,000 km over 10 years"

Key considerations:

• Manufacturing phase (batteries are impact-intensive)
• Use phase (electricity vs. gasoline)
• Regional electricity mix affects results dramatically
• Battery replacement scenarios

Why this is complex:

• A 10-year EV in France (nuclear) beats gasoline easily
• A 10-year EV in Poland (coal) may not
• 200,000 km assumption matters (more distance favors EV)

Example 3: Building Insulation

Product: Wall insulation

Functional unit: "Providing thermal resistance of R-30 to 100 m² of wall for 60 years"

Different products need different thicknesses:

Don't compare by mass—higher-performance products need less material.

Example 4: Packaging

Product: Beverage containers

Functional unit: "Delivering 1,000 liters of beverage to consumer"

Consider:

• Container volume (losses to packaging)
• Weight (transport impacts)
• Recycling rates (by geography)
• Breakage rates
• Reuse potential

Advanced Considerations

Multi-Functional Products

When a product serves multiple functions:

Option 1: Include all functions in FU "Providing communication, photography, and navigation for 3 years" (smartphone)

Option 2: Allocate to main function Focus on primary purpose, note secondary functions

Option 3: System expansion Credit product for avoided separate devices

Services and Digital Products

Challenge: How to quantify "function" for intangibles?

Examples:

• Streaming service: "1 hour of video entertainment delivery"
• Cloud storage: "Storing 1 TB of data for 1 year"
• Software: "Providing word processing capability for 1 user-year"

Include infrastructure (servers, networks) proportionally.

Sensitivity to Lifespan Assumptions

Critical test: How sensitive are results to lifespan assumptions?

If the winner changes based on lifespan assumptions, that's a key finding to report.

Common Mistakes

Mistake 1: Comparing apples to oranges

"Paper bags have X kg CO₂, plastic bags have Y kg CO₂"

Problem: Different sizes, different carrying capacities, different reuse rates.

Fix: "Carrying 50 kg of groceries home per week for 1 year"

Mistake 2: Ignoring use phase

"LED production impacts are higher than incandescent"

Problem: Missing the 90%+ of impacts from electricity use.

Fix: Always include use phase for energy-using products.

Mistake 3: Optimistic lifespan assumptions

"Our product lasts 30 years"

Problem: Based on technical potential, not actual consumer behavior.

Fix: Use realistic lifespans based on market data, warranties, or conservative estimates.

Mistake 4: Single-use comparison

"One steel container vs. one plastic container"

Problem: Steel can be reused hundreds of times; plastic may not.

Fix: Service-based functional unit with realistic reuse rates.

Key Takeaways

1. Functional units enable fair comparison; declared units may not
2. Different lifespans require normalization to a common service period
3. Performance differences must be captured in the functional unit
4. Use phase often dominates for energy-using products
5. Be realistic about lifespans—don't assume best-case scenarios
6. Sensitivity analysis is essential when assumptions are uncertain

Functional Unit Checklist

Before finalizing your functional unit, verify:

☐ Describes a function, not just a product quantity ☐ Includes relevant performance requirements ☐ Specifies a time duration or number of uses ☐ All alternatives can actually deliver this function ☐ Lifespan assumptions are realistic and documented ☐ Use phase impacts are captured appropriately ☐ Sensitivity to key assumptions has been tested

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Next Steps

With functional units clear, the next lesson covers Allocation Methods Decision Guide—choosing between economic, mass, and system expansion approaches.