Which retrofit steps abate carbon at negative cost? Reconstructing a consultancy annuity calculation and a sequential marginal abatement cost curve for an anonymised nonresidential building

Energy consultancy reports usually present cumulative renovation variants with a payback period, an internal rate of return and a mean annual saving. These figures describe each cumulative package against the baseline, but they do not show what each additional step costs per tonne of carbon dioxide avoided. This paper converts the economic results of an anonymised nonresidential energy report into a sequential marginal abatement cost curve. The report's capital costs, internal rates of return and dynamic payback periods are first reconstructed exactly. The reconstruction shows that the report applies a 30-year annuity at 2% to the energy-related cost share, levelises fuel costs with a 4% price escalation, and assumes a single replacement of technical components after 15 years with a 3.5% cost escalation. On this basis, replacing openings abates carbon at approximately −602 EUR per tonne, ventilation with heat recovery at −416 EUR/t and heating optimisation at −100 EUR/t, whereas wall and roof insulation cost approximately +842 and +500 EUR/t. All five cumulative variants remain at negative average cost against the baseline. The sign of the step costs depends strongly on the assumed fuel-price escalation: without escalation, only the first step is cost-saving. The ventilation step also raises the nitrogen-oxide inventory by about 35% because the added electricity carries a much higher NOx factor. The marginal view thus reveals cross-subsidies hidden in cumulative indicators. Results are model-based and do not establish procurement prices, funding eligibility or measured savings.