Capex pathway — marginal abatement cost
Pawston House: rank every applicable retrofit measure by £ per tonne abated, then read off what it takes to push the building under the CRREM curve
What the curve says
Controls, metering and rooftop solar sit at the cheap end of the curve — setpoint tuning, BMS optimisation and PV abate ~40 tCO₂e for under £172,000. Heat-pump and deep-fabric measures are the expensive tail: solid-wall insulation costs ~£32,738/tCO₂e and barely moves the needle. Sequence cheap first, then decide how far up the tail the stranding date justifies going.
Marginal abatement cost curve (MACC)
Each bar is one measure. Width = de-rated tCO₂e/yr it abates; height = standalone cost-effectiveness (£/tCO₂e). Bars are ordered cheapest first, so the area under the curve up to any capex point is the abatement that budget buys.
Resulting intensity vs CRREM pathway
Baseline strands in 2029. The full pathway drops intensity to 10.5 kgCO₂e/m², below the CRREM curve immediately — the building re-strands only in 2040, once the curve's own decline overtakes the flat post-retrofit line. That is ~11 years of alignment headroom bought by the retrofit.
Working to a budget
Cap the spend at £200,000 and the planner funds the 8 cheapest-per-tonne measures — abating 44 tCO₂e and pulling intensity to 22.5 kgCO₂e/m². Pass ?budget_cap_gbp= to the API to re-rank under any cap.
Ranked measures (cheapest abatement first)
| Measure | £/tCO₂e | Abated | Capex | Life | Overlap |
|---|---|---|---|---|---|
| Heating/cooling setpoint tuningControls | 893 | 6.7 t | £6,000 | 8y | — |
| Energy dashboard & analyticsMetering | 2,976 | 2.5 t | £8,000 | 8y | — |
| Smart / sub-meteringMetering | 3,472 | 2.6 t | £14,000 | 12y | de-rated ×1 |
| BMS optimisationControls | 3,571 | 8.6 t | £48,000 | 10y | de-rated ×1 |
| Draught-proofing & airtightnessFabric | 3,720 | 4.6 t | £20,000 | 15y | — |
| Pipework & valve insulationHVAC | 3,720 | 1.5 t | £10,000 | 20y | de-rated ×1 |
| Solar PV (rooftop)Generation | 4,092 | 12.9 t | £66,000 | 25y | — |
| Variable-speed drives on pumps & fansHVAC | 4,167 | 4.8 t | £28,000 | 12y | — |
| Power-factor correctionElectrical | 4,464 | 1.8 t | £12,000 | 15y | — |
| Voltage optimisationElectrical | 5,952 | 1.9 t | £24,000 | 15y | de-rated ×1 |
| Demand-controlled ventilation (CO₂ sensing)Ventilation | 6,548 | 2.1 t | £44,000 | 15y | de-rated ×2 |
| Lighting & daylight controlsLighting | 6,696 | 3.4 t | £36,000 | 12y | — |
| Solar-control window filmFabric | 7,441 | 1.6 t | £20,000 | 12y | — |
| Roof / loft insulationFabric | 7,937 | 4.8 t | £64,000 | 40y | — |
| LED relightingLighting | 8,185 | 4.2 t | £88,000 | 15y | de-rated ×1 |
| Air-source heat pumpHeat pump | 12,175 | 6.3 t | £360,000 | 20y | de-rated ×1 |
| Solid-wall insulationFabric | 32,738 | 0.7 t | £440,000 | 40y | de-rated ×3 |
Engine: hemera/services/capex_pathway.py — filters the 30-measure UK CRE library by building type, age and EPC, de-rates overlapping savings, ranks by £/tCO₂e, and overlays the CRREM curve to find the de-stranding year. Live route: GET /api/assets/{id}/capex-pathway. Numbers on this page are a real engine run against Pawston House.