CAPEX/OPEX and EN 15804 modules: the two axes of cost + €/t CO2e marginal abatement cost

Classical accounting splits cost into CAPEX and OPEX; life cycle thinking spreads cost across stages (A1-A3 through D). Each alone is incomplete: the CAPEX/OPEX split tells you the timing but not the position in the life cycle; the module split tells you the position but not the financial nature. clca's LCC module classifies every cost item on both axes at once — and this dual axis is the precondition for cost and carbon talking in the same table.

Axis 1: EN 15804 module — Axis 2: cost nature

The first axis is the EN 15804 module structure inherited by EN 16627: A1-A3, A4-A5, B1-B7, C1-C4, D. The second axis is cost nature: CAPEX (investment), OPEX (operational), EoL (end of life) and revenue. clca derives the second axis automatically from the module — A1-A5 items default to CAPEX, B1-B7 to OPEX, C1-C4 to EoL, D to revenue — with a per-item override. A B4 replacement, for instance, sits technically in the use stage but is budgeted like an investment decision; you can flag that item as CAPEX without touching its module axis.

• A1-A3 materials + manufacturing → CAPEX (default)
• A4-A5 transport + installation → CAPEX (default)
• B2 maintenance, B6 energy, B7 water → OPEX
• B4 replacement → OPEX by default; can be overridden to CAPEX per item
• C1-C4 deconstruction, transport, processing, disposal → EoL
• D recovery value → revenue; never netted into totals

Replacement schedule and linear residual value

Replacement count derives automatically from service life: N = ceil(T/SL) − 1. Each replacement is discounted from the end of its year; unused life at the end of the period is credited back as a linear residual value. This pair keeps the comparison between a cheap short-lived product and an expensive long-lived one honest — precisely the spot where LCC is most often gamed.

Marginal abatement cost — €/t CO2e

With two scenarios (say standard concrete vs 30% GGBS concrete), MAC is computed as MAC = ΔNPC / ΔGWP — the extra cost divided by tonnes of CO₂eq avoided. A positive MAC means "abating this carbon costs you this many € per tonne"; a negative MAC is a win-win: the scenario is both cheaper and lower-carbon. clca detects win-win cases automatically and flags them separately, because the numerical magnitude of a negative MAC is misleading for ranking on its own.

The cost-carbon quadrant plot

The quadrant plot places each scenario on the ΔNPC (x-axis) vs ΔGWP (y-axis) plane. The bottom-left quadrant (cheaper, less carbon) is the win-win zone; the top-right (more expensive, more carbon) is indefensible. The real decision zone is the bottom-right: more expensive but lower-carbon — here the MAC is judged against your internal carbon price. If your company's internal carbon price is 100 €/t, a scenario with a MAC of 60 €/t is rational; one at 250 €/t is not.

One-way sensitivity: tornado and switching values

An LCC result is highly sensitive to a handful of parameters. clca's tornado analysis perturbs the discount rate, reference service life (RSL), energy price escalation and the top cost items one at a time, showing the NPC impact as bar length. A switching value is computed per driver: "if the discount rate rises above 6.2%, scenario B becomes more expensive than scenario A". The switching value is the plainest tool for showing which assumption actually carries your decision.