CHP and Cogeneration Allocation
A combined heat and power plant burns one stream of fuel and produces two useful products. The emissions are real and known to the kilogram — but how much belongs to the electricity and how much to the heat is not a measurement. It is a choice of allocation method.
Pick a different method and the carbon intensity of the same megawatt-hour can almost double.
CHP allocation splits a cogeneration plant's single fuel emissions between its electricity and heat outputs. The three accepted methods — energy, efficiency and work-potential — redistribute the same total differently, so the method must be disclosed.
Cogeneration — combined heat and power, or CHP — captures heat that a power-only plant would reject to the atmosphere, raising total fuel-to-useful-energy efficiency to 75–90%. That efficiency gain is the reason CHP exists. But it creates an accounting problem the moment two parties, or two reporting boundaries, share the output: the plant’s combustion emissions must be divided between the electricity and the heat. There is no physically correct division — only methods, each internally consistent and each producing different per-product intensities. This page sets out the three accepted methods, the formulas, a fully worked three-method comparison, and the rules that govern which to use and how to disclose it.
The Allocation Problem
A CHP unit consumes one fuel input and delivers two energy products: electricity and useful heat (typically steam or hot water). The combustion emissions are unambiguous — they follow directly from the fuel burned and its emission factor. What is ambiguous is the share of those emissions that should attach to each output.
The total emissions are fixed; only their distribution between the two products varies. This is a classic co-product allocation problem — the same structure that ISO 14040/14044 addresses for multi-output processes in life-cycle assessment. Allocation never changes the plant’s total footprint. It changes the intensity stamped on each megawatt-hour of electricity and each kilowatt-hour of heat that leaves the plant.
Whichever method is used, the electricity emissions plus the heat emissions equal the plant’s total combustion emissions, exactly. A method is a way of dividing a fixed quantity, not of estimating an unknown one. This is the single most important property to verify on any CHP allocation: the two product shares must sum to 100% of the fuel emissions, with nothing created or lost in the split.
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Why Allocation Is Required, Not Optional
Allocation becomes necessary the moment the two outputs cross a reporting boundary differently. Three common situations force it:
- Exported electricity or heat. A site that operates a CHP unit and sells surplus electricity to the grid, or steam to a neighbouring facility, must attach an emission intensity to the exported product. That intensity depends entirely on the allocation method.
- Purchased heat or steam. A facility that buys heat from a third-party CHP plant reports it as Scope 2 and needs the supplier’s heat emission factor — which the supplier derived through allocation.
- Product carbon footprints. Any product footprint drawing on CHP-supplied energy needs a per-unit energy intensity, which only allocation can provide.
Where a CHP unit serves a single facility entirely on-site, with no export and no third-party purchase, allocation is not strictly required for the corporate inventory — the whole fuel emission sits in Scope 1 regardless of how it splits internally. Allocation still matters for internal energy management and for any product-level footprint, but the corporate Scope 1 total is unaffected. The need for allocation tracks the crossing of a boundary, not the existence of two outputs.
Scope Treatment of CHP Outputs
Before allocating, the reporter must place each energy flow in the correct scope. CHP touches Scope 1 and Scope 2 differently depending on ownership and direction of flow.
| Flow | Who reports it | Scope | Allocation needed? |
|---|---|---|---|
| Fuel combusted in an owned/controlled CHP unit | CHP operator | Scope 1 | Only to split intensities for export / product footprints |
| Electricity self-consumed on-site | CHP operator | Scope 1 (already in fuel) | No double-count — it is part of the Scope 1 fuel emission |
| Electricity exported to grid | Grid/buyer (operator may report avoided) | Buyer’s Scope 2 | Yes — export intensity from allocation |
| Heat/steam sold to another facility | Buyer | Buyer’s Scope 2 | Yes — heat intensity from allocation |
| Heat/steam purchased from third-party CHP | Purchasing facility | Scope 2 | Uses supplier’s allocated heat factor |
The Scope 2 treatment of purchased heat and steam mirrors purchased electricity under the GHG Protocol Scope 2 Guidance, and connects directly to the district heating and cooling methodology where the supplier is a network rather than a single plant. The boundary rule from the GHG Protocol Corporate Standard determines which organisation books the Scope 1 fuel and which books the Scope 2 purchase.
A frequent double-count: a site books its CHP fuel as Scope 1, then also books the electricity its CHP generated and used on-site as Scope 2. The electricity’s emissions are already captured in the Scope 1 fuel figure — adding a Scope 2 line for self-generated, self-consumed power counts the same combustion twice. Only electricity purchased from outside, or heat purchased from a third party, belongs in Scope 2.
The Three Allocation Methods
Three allocation methods are recognised in GHG accounting and CHP standards. They differ in what they treat as the basis for division: raw energy content, the fuel each output would separately have required, or the thermodynamic work potential of each output.
Energy Energy (proportional) method
Allocates emissions in proportion to the energy content of each output. Simplest and most transparent; treats a kilowatt-hour of electricity and a kilowatt-hour of heat as equivalent, which understates electricity’s value and overstates heat’s.
Efficiency Efficiency method
Allocates by the fuel each output would have needed if produced separately — power in a reference power plant, heat in a reference boiler. The GHG Protocol and the EU CHP framework favour this; it rewards the cogeneration efficiency gain across both products.
Exergy Work-potential (exergy) method
Allocates by thermodynamic work potential — electricity at full value, heat discounted by a Carnot quality factor reflecting its temperature. The most physically rigorous; loads most emissions onto electricity, the higher-quality product.
The Calculation, Step by Step
Every method shares the same first step — compute the total fuel emissions — then differs only in the allocation fraction applied. The total is the fuel input multiplied by its emission factor.
Energy method allocation
Efficiency method allocation
Work-potential (exergy) method allocation
Electricity carries a quality factor of 1.0 — it is pure work. Heat carries a quality factor below 1.0 that falls as its temperature approaches ambient, because low-grade heat can do little thermodynamic work. This is why the exergy method assigns the most emissions to electricity: it is the higher-quality product.
Worked Example — One Plant, Three Methods
Hardcoded audit record at the snapshot date of this methodology revision. The same gas-fired CHP unit is allocated three ways so the divergence is visible. Reference efficiencies and the heat supply temperature are illustrative — substitute your jurisdiction’s reference values (the EU harmonised reference efficiencies, or a measured reference boiler) and your actual steam conditions.
The electricity intensity ranges from 0.22870 to 0.38247 kg CO₂e/kWh — a 67% swing — for the identical plant burning the identical fuel. The heat intensity moves inversely. This is the practical stakes of method choice, and why disclosure is mandatory.
| Method | Elec emissions (tCO₂e) | Heat emissions (tCO₂e) | Elec intensity | Heat intensity |
|---|---|---|---|---|
| Energy | 800.45 | 1,029.15 | 0.22870 | 0.22870 |
| Efficiency | 1,045.49 | 784.11 | 0.29871 | 0.17425 |
| Exergy | 1,338.65 | 490.95 | 0.38247 | 0.10910 |
Compare the allocated electricity intensities against buying grid electricity at the live UK location-based factor of 0.131 kg CO₂e/kWh [GreenCalculus grid.gbr.electricity.location_based · DEFRA 2026 'UK electricity'!E25] (DEFRA 2025): even the exergy method’s 0.38247 is close to grid, and the heat is far below a standalone gas boiler (which at 90% efficiency on the same fuel would emit roughly 0.203 kg CO₂e/kWh of heat). The cogeneration gain is real under any allocation — the method only decides which product is credited with how much of it.
Choosing a Method
No single method is mandated across all frameworks, but each context has a defensible default.
| Context | Preferred method | Rationale |
|---|---|---|
| GHG Protocol corporate inventory / purchased heat | Efficiency | Rewards the cogeneration gain across both products; the GHG Protocol allocation guidance default |
| EU regulatory / CHP qualification | Efficiency | Aligns with the EU harmonised reference efficiency framework for high-efficiency cogeneration |
| ISO 14040/14044 LCA / product footprint | Exergy | ISO allocation hierarchy prefers a physical relationship; exergy reflects true thermodynamic value |
| Simple disclosure / first inventory | Energy | Most transparent and reproducible; acceptable where it is disclosed and applied consistently |
Verifiers care less about which of the three methods is used than about two properties: that the method is disclosed explicitly, and that it is applied consistently across years and across both products. Switching methods between reporting periods — or, worse, applying one method to the electricity and another to the heat from the same plant — breaks comparability and is a validation finding. Pick the defensible default for your framework, document the reference values, and hold the method steady.
Edge Cases and Error Traps
| Error | What happens | How to avoid |
|---|---|---|
| Double-counting self-consumed electricity | CHP fuel booked as Scope 1 and self-generated electricity also booked as Scope 2 — the same combustion counted twice. | Self-consumed CHP power is already in the Scope 1 fuel. Only purchased electricity or third-party heat is Scope 2. |
| Mixing CV bases | Fuel input on gross CV but efficiencies computed on net CV (or vice versa) — efficiencies and allocation fractions silently wrong. | State the calorific-value basis once and hold it across fuel input, efficiencies, and the fuel factor. UK DEFRA factors are gross CV. |
| Applying different methods to the two outputs | Electricity allocated by efficiency, heat by energy — the two shares no longer sum to 100% of fuel emissions. | One method allocates both products simultaneously. Verify the sum-to-total check after every allocation. |
| Counting only useful heat that is actually used | Including rejected or vented heat as “useful” output inflates the heat denominator and mis-allocates emissions toward heat. | Only metered, delivered useful heat counts as Q_heat. Rejected heat is a loss, not an output. |
| Stale or wrong reference efficiencies (efficiency method) | Using outdated or mismatched reference efficiencies shifts the allocation fraction without any change in the physical plant. | Use the current jurisdiction reference values (EU harmonised reference efficiencies, or a documented reference boiler/plant) and cite them. |
| Switching methods between years | A method change moves reported product intensities and breaks year-on-year comparability — can look like a real reduction that did not occur. | Hold the method constant. If a change is unavoidable, restate prior years and disclose the change. |
Governance and Verification
CHP allocation is audited on four points:
- Sum-to-total. The verifier confirms electricity emissions plus heat emissions equal the plant’s total combustion emissions, exactly — the conservation property of any valid allocation.
- Method disclosure. The allocation method is stated explicitly in the inventory methodology, along with any reference efficiencies or heat temperatures used.
- Consistency. The same method is applied to both outputs and across reporting years; any change is restated and disclosed.
- Scope placement. Self-consumed power sits in Scope 1 fuel; exported and purchased flows sit in the correct Scope 2 boundary per the Corporate Standard and Scope 2 Guidance.
The governing references are the GHG Protocol Corporate Standard and Scope 2 Guidance for inventory placement, ISO 14040/14044 for the co-product allocation hierarchy in product footprints, and DEFRA (or the relevant national dataset) for the fuel emission factor.
What the Calculator Handles vs What You Decide
An automated engine handles the allocation arithmetic and the three-method comparison once the physical inputs and method are fixed. The upstream judgments remain yours.
The calculator handles
Total fuel emissions, all three allocation methods in parallel, per-product emissions and intensities, the sum-to-total verification, and a side-by-side method comparison with the displaced-grid and standalone-boiler counterfactuals.
You decide before using it
The calorific-value basis, the allocation method for the inventory, the reference efficiencies (efficiency method) or heat supply temperature (exergy method), which heat is genuinely useful and delivered, and the Scope 1/Scope 2 boundary for every flow.
Ready to allocate your CHP emissions between heat and power? Open the CHP / Cogeneration Calculator →
Frequently Asked Questions
No. Allocation only divides a fixed quantity of combustion emissions between electricity and heat. Whichever method is used, the electricity emissions plus the heat emissions equal the plant’s total fuel emissions exactly. The method changes the carbon intensity stamped on each product — which matters for exported energy, purchased heat, and product footprints — but never the plant’s total footprint. Verifying that the two product shares sum to 100% of the fuel emissions is the core integrity check on any allocation.
The GHG Protocol allocation guidance favours the efficiency method for CHP, because it credits the cogeneration efficiency gain across both products rather than treating a kilowatt-hour of electricity and a kilowatt-hour of heat as equivalent. The EU high-efficiency cogeneration framework also works on a reference-efficiency basis. For product life-cycle assessment under ISO 14040/14044, the exergy (work-potential) method better fits the standard’s preference for a physical allocation relationship. No method is universally mandated, so the choice must be disclosed and applied consistently.
No — not if you consume it on-site. The emissions from electricity your own CHP unit generates and you use yourself are already captured in the Scope 1 fuel combustion figure. Adding a Scope 2 line for it would count the same combustion twice. Scope 2 captures electricity purchased from outside your boundary, or heat and steam purchased from a third-party CHP plant. Electricity you export to the grid leaves your boundary and is reported by the buyer.
From the supplier, who derives it by allocating their plant’s fuel emissions to the heat output. Request the allocated heat emission factor and the method behind it — energy, efficiency, or exergy — so you can confirm it is consistent and disclosed. If the supplier is a heat network rather than a single plant, the district heating and cooling methodology applies. Purchased heat and steam are reported in your Scope 2, mirroring purchased electricity under the GHG Protocol Scope 2 Guidance.
Because exergy allocates by thermodynamic work potential, and electricity is pure work — quality factor 1.0 — while heat is discounted by a Carnot factor that falls as its temperature approaches ambient. Lower-grade heat can do little useful work, so it carries a small share of the emissions, leaving the high-quality electricity with most of them. This makes the exergy method the most physically rigorous and the one that gives heat the lowest intensity, which is why it is preferred for product-level LCA but tends to make CHP electricity look closer to grid power.
No. A single allocation method divides the fuel emissions between both products simultaneously; the two product fractions are designed to sum to 100%. Allocating electricity one way and heat another breaks that conservation property — the two shares will no longer reconcile to the plant’s total emissions, and the result is a validation finding. Choose one method, apply it to both outputs, and run the sum-to-total check to confirm the split is closed.