Inventory · Scope 1 + 2 · CHP
Scope 1 + 2 CHP / Cogeneration Calculator | GHG Protocol Efficiency Method
Allocate combined heat and power (CHP / cogeneration) emissions between heat and electricity using the GHG Protocol efficiency method. Computes Scope 1 fuel-combustion emissions, the heat/power allocation split, derived per-unit factors for each output stream, and any imported grid electricity or heat under Scope 2 — with the displaced-generation comparison reported separately, never netted, per GHG Protocol CHP guidance.
GHG Protocol efficiency method — allocates a CHP plant’s total combustion emissions between its heat and electricity outputs in proportion to the fuel each stream would have required if produced separately, at assumed standalone-production efficiencies.
E_H = E_T × [ (H / e_H) / ( H / e_H + P / e_P ) ]
E_P = E_T × [ (P / e_P) / ( H / e_H + P / e_P ) ]
EF_heat = E_H / H · EF_elec = E_P / P
Where E_T = total CHP fuel-combustion emissions, H = useful heat output, P = electricity output, e_H = assumed efficiency of producing that heat separately, e_P = assumed efficiency of producing that electricity separately.
Default reference efficiencies (GHG Protocol US defaults):
Electricity e_P = 0.35 (35%) · Heat e_H = 0.80 (80%).
Both are stated on a Net CV (LHV) basis.
Displaced / exported generation: emissions from own-generated electricity, heat, or steam sold to another entity are not deducted from Scope 1 and are not booked as a negative Scope 2. They are reported separately as optional information — the “efficiency benefit vs separate production” line.
Reference: GHG Protocol — Allocation of GHG Emissions from a Combined Heat and Power (CHP) Plant, WRI/WBCSD, September 2006, v1.0; GHG Protocol Corporate Standard; GHG Protocol Scope 2 Guidance; DEFRA 2026 conversion factors (AR5 GWP-100).
Assumed efficiencies of producing electricity and heat separately — read live from MasterBrain, net-CV basis.
Enter the CHP fuel input and its electricity + heat outputs to calculate
Results appear instantly — combined Scope 1 + 2, the efficiency-method allocation with per-unit factors for electricity and heat, and the efficiency benefit vs separate production appear after calculation.
Results are estimates for organisational greenhouse-gas accounting. Combustion is reported as Scope 1; imported electricity and heat as Scope 2. Emissions are allocated between the cogeneration unit’s electricity and heat outputs using the GHG Protocol efficiency method, with assumed separate-production efficiencies you can edit (defaults from the GHG Protocol CHP guidance, net-CV basis). The “efficiency benefit vs separate production” is an avoided-emissions comparison shown separately — it is not part of the GHG inventory and must not be netted against Scope 1. Conversion factors are DEFRA 2026 (AR5 GWP-100) via MasterBrain. For assured disclosure obtain third-party verification under ISO 14064-3. Full methodology notes.
A combined heat and power unit burns one stream of fuel and produces two useful outputs — and the entire difficulty of CHP carbon accounting is deciding how much of that single fuel bill each output should carry.
Get the split wrong and every downstream number — your heat factor, your power factor, your product carbon footprint — inherits the error.
The GHG Protocol efficiency method splits CHP emissions by the fuel each output would have needed if produced separately: heat gets H ÷ e_H, electricity gets P ÷ e_P, each as a share of the total. With default efficiencies of 80% heat and 35% power, electricity carries a disproportionately large share.
The CHP fuel figure is a Scope 1 combustion quantity — size it with the Scope 1 Stationary Combustion Calculator. Any grid electricity you import to top up on-site generation is a Scope 2 figure — calculate it with the Scope 2 Electricity Calculator. All three share the same MasterBrain data layer, so factor versions stay aligned across your inventory.
What is combined heat and power, and why does it complicate Scope 1+2?
A combined heat and power (CHP, or cogeneration) plant generates electricity and captures the waste heat from that generation for useful purposes — space heating, process steam, hot water. Because it recovers heat that a power-only plant throws away, a CHP unit can reach total fuel-utilisation efficiencies of 70–85%, against roughly 35–40% for grid-scale thermal generation. That recovery is the real reason to run CHP. It is not, at modern grid carbon intensities, a Scope 2 electricity-carbon play — a point the results section returns to.
The accounting problem is structural. One fuel input produces two useful outputs, and the emissions from that fuel have to be divided between them. The fuel itself is unambiguous: on-site combustion is a direct emission, so it lands in Scope 1. What is not unambiguous is how much of that Scope 1 total belongs to the heat your boilers would otherwise have produced, versus the electricity you would otherwise have imported. That division — the allocation — is the entire subject of this calculator.
A CHP unit is not “just a boiler”. A boiler has one output (heat) and one number. A CHP unit has two outputs sharing one fuel bill, so you need a rule for splitting the bill. The GHG Protocol’s efficiency method is that rule: each output pays for the fuel it would have burned on its own.
Two further wrinkles sit on top. First, most CHP sites also import some grid electricity to cover demand peaks the unit cannot meet — that top-up import is ordinary Scope 2. Second, some sites export surplus electricity or heat to the grid or a neighbour, and the treatment of that export is the single most misunderstood part of CHP accounting. Both are handled below.
The efficiency method: how CHP emissions split between heat and power
The efficiency method allocates the plant’s total combustion emissions in proportion to the fuel each output stream would have consumed if produced separately. Heat that would have come from an 80%-efficient boiler is charged the fuel an 80%-efficient boiler would have burned; electricity that would have come from a 35%-efficient power station is charged the fuel that station would have burned. The two notional fuel demands set the split.
The allocation formula
E_P = E_T × [ (P / e_P) ÷ ( H / e_H + P / e_P ) ]
EF_heat = E_H / H EF_elec = E_P / P
E_T is the plant’s total fuel-combustion emissions; H and P are the useful heat and electricity outputs; e_H and e_P are the assumed standalone efficiencies. The two terms H/e_H and P/e_P are the notional separate-production fuel demands. Their ratio is the allocation; multiply each fraction by E_T to get the emissions assigned to heat (E_H) and to power (E_P). Dividing each back by its own output gives the per-unit factors the calculator reports — the numbers you carry into a product footprint or a tenant sub-meter.
The default reference efficiencies are the GHG Protocol’s recommended US values: e_P = 0.35 for electricity and e_H = 0.80 for heat, both on a Net CV basis. The asymmetry matters. Because separate electricity production is far less efficient than separate heat production, each unit of electricity carries a much larger notional fuel demand — and therefore a much larger share of the emissions — than each unit of heat. This is why CHP electricity, allocated honestly, is not cheap carbon.
You could just report one combined CHP emissions number. But the moment you need a carbon factor for the heat alone — to bill a tenant, to footprint a product, to compare against a gas boiler — you need the split. The efficiency method gives a defensible, auditable factor for each output instead of one undifferentiated lump.
A note on GCV/NCV basis — the part auditors check
DEFRA fuel factors are published on a Gross CV (HHV) basis; the GHG Protocol reference efficiencies are Net CV (LHV). That looks like a mismatch, and it is the question a careful reviewer will raise. The resolution is that the allocation split is basis-invariant.
Total emissions E_T cancel out of the f_heat and f_elec fractions — the split depends only on H, P, e_H, and e_P. Because H and P are basis-neutral energy outputs and both efficiencies are on the same (NCV) basis, the allocated tonnes and the two derived per-unit factors are correct regardless of whether the fuel factor is GCV or NCV, provided the fuel quantity and the fuel factor share a basis — which the unit selector enforces (pick kWh GCV, get the GCV factor; pick kWh NCV, get the NCV factor). Basis only affects the energy-balance sanity check and the avoided-emissions counterfactual, both of which compare against actual fuel input. The engine flags any basis mismatch and records the basis in the audit trail.
In practice: state your fuel input on a declared basis (UK natural gas defaults to GCV, factor 0.18231 kg CO₂e/kWh under DEFRA 2026), note that the efficiencies are NCV per the GHG Protocol, and add one sentence that the split is unaffected by the GCV/NCV choice. That sentence is what makes the page bullet-proof to an auditor.
Allocation methods compared
The GHG Protocol describes more than one allocation method. This calculator implements the efficiency method, which is the most widely applicable and the one the Protocol recommends as a default. The two it is most often confused with:
Efficiency method implemented
Allocates by the fuel each output would have needed if produced separately, at assumed standalone efficiencies (heat 80%, power 35%). Reflects that separate electricity generation is far less efficient than separate heat, so it loads more emissions onto power. The Protocol’s recommended default; works for any fuel and any CHP configuration.
Energy-content method
Allocates by raw useful-energy output — a kWh of heat and a kWh of electricity each carry the same emissions weight. Simpler, but it ignores that the two outputs are not thermodynamically equivalent, and it under-charges electricity. A distinct GHG Protocol method, not a synonym for the efficiency method despite the similar name. Not implemented here.
EU CHP Directive reference values deferred
Regulation 2015/2402 publishes harmonised EU reference efficiencies for primary-energy-savings calculations. A valid basis under EU energy regulation, but a different framework from the GHG Protocol corporate-inventory method. Deferred pending verification of the regulatory reference values — not offered as a preset.
If your reporting obligation specifically requires the energy-content method or the EU reference values, this calculator is not the right tool for that submission. For GHG Protocol corporate-inventory and most voluntary-disclosure purposes, the efficiency method is the defensible default.
Choosing your reference efficiencies
The reference efficiencies e_H and e_P are the only judgement calls in the method, and they drive the entire split. The calculator defaults to the GHG Protocol US values, but you should override them when you have a defensible local benchmark — for example, the efficiency of the specific boiler the CHP heat displaces, or a published grid-generation efficiency for your region.
| Reference | Heat e_H | Power e_P | Basis / use |
|---|---|---|---|
| GHG Protocol US default | 0.80 | 0.35 | Net CV · calculator default, broadly applicable |
| Modern condensing gas boiler (heat benchmark) | 0.85–0.92 | — | Net CV · use when displacing a known efficient boiler |
| CCGT grid generation (power benchmark) | — | 0.50–0.55 | Net CV · use where gas combined-cycle sets the margin |
| Older OCGT / coal margin (power benchmark) | — | 0.33–0.38 | Net CV · conservative power-displacement assumption |
If you override an efficiency, keep it on a Net CV basis to match the method. Mixing a Gross CV efficiency with the NCV defaults silently distorts the split. The calculator assumes NCV efficiencies throughout; the figures above are all stated NCV.
Document whichever values you use. A validator’s first question about a CHP allocation is “where did your reference efficiencies come from?” — the calculator records them in the audit trail, and your inventory documentation should cite the same source.
Worked example — UK gas-fired CHP, one reporting year
This reproduces the full efficiency-method arithmetic for a typical UK gas CHP installation. The values are reproducible in the calculator above and reconcile to the engine’s verification set.
How to read your results
The result panel reports three things: your Scope 1 CHP combustion total, the heat/power allocation split with the two derived per-unit factors, and any Scope 2 imported electricity or heat. Read them in that order. The Scope 1 total is the anchor; the split tells you how to attribute it; the Scope 2 line captures whatever the CHP unit could not supply.
The result most people get wrong is the displaced-generation line, so address it head-on. Exported or self-consumed power that displaces grid electricity is reported as a separate, optional comparison — never deducted from Scope 1, never booked as a negative Scope 2. The GHG Protocol is explicit: emissions from own-generated electricity, heat, or steam sold to another entity are not netted from direct Scope 1 emissions and may be reported as optional information. The calculator follows that rule exactly.
The intuitive move — “my CHP avoided grid electricity, so subtract that from my emissions” — produces a non-compliant inventory. Avoided emissions live outside the inventory boundary. Report them separately if you wish, but your Scope 1 and Scope 2 totals stand on their own.
And the figure itself is frequently uncomfortable. At the live 2026 UK grid factor of 0.131 kg CO₂e/kWh, gas-CHP allocated electricity (around 0.35 kg/kWh in the worked example) is dirtier than grid electricity. The “efficiency benefit” line comes out as a net additional figure, not a saving. This is honest and defensible: as the grid decarbonises, the carbon case for gas CHP electricity weakens, while the case built on heat recovery, energy resilience, and operating cost holds. Report the comparison as it falls, and let the heat-recovery argument carry the unit — not a grid-carbon saving that no longer exists at UK intensities.
The comparison below shows where the carbon actually sits: the CHP fuel split against what separate heat and power production would have emitted.
Bars scaled to the worked-example figures. The allocated electricity share (1,128.7 t) against the grid would-have-emitted figure (419.1 t = 3,200,000 kWh × 0.131) is the visual form of the “net additional” result — CHP electricity above, grid counterfactual below.
For internal use, export the allocation as CSV from the result panel and load the two per-unit factors into your sustainability dashboard. Those factors — not the combined total — are what feed a downstream product carbon footprint or a tenant heat bill.
Common CHP accounting mistakes
The five errors that most often turn up in CHP inventories under review. Each one distorts either the scope totals or the per-unit factors, and several are silent — the number looks plausible until someone checks the method.
CHP under reporting frameworks
The allocation feeds several disclosure regimes, each with its own emphasis. The underlying numbers are the same; what changes is how they are presented and which line is mandatory.
Under the GHG Protocol Corporate Standard, the CHP fuel is Scope 1 and any grid top-up is Scope 2 — the displacement comparison is optional information only. For UK SECR reporting, the CHP combustion sits in the Scope 1 total and the per-unit factors support the intensity ratio; an ISO 50001 energy-management system, where you run one, is the natural home for the efficiency benchmarks you cite. Larger CHP installations above the capacity threshold fall under the EU Emissions Trading System, which has its own monitoring-and-reporting rules for the combustion emissions — those run in parallel to, not instead of, the corporate inventory.
For EU corporate sustainability reporting, the allocated factors flow into the CSRD / ESRS E1 energy and emissions datapoints — particularly where a site reports self-generated versus purchased energy separately. Keep the allocation method documented in the inventory management plan; ESRS E1 expects the methodology behind a reported factor to be transparent, and “GHG Protocol efficiency method, e_H 0.80 / e_P 0.35, NCV” is exactly the kind of disclosure a reviewer wants to see.
Frequently Asked Questions
No. They are two distinct GHG Protocol allocation methods. The efficiency method allocates by the fuel each output would have needed if produced separately, using assumed standalone efficiencies — so it loads more emissions onto electricity, which is harder to produce separately. The energy-content method allocates by raw useful-energy output, treating a kWh of heat and a kWh of electricity as equivalent. This calculator implements the efficiency method, the Protocol’s recommended default.
No. The GHG Protocol is explicit that emissions from own-generated electricity, heat, or steam — whether self-consumed or sold — are not deducted or netted from direct Scope 1 emissions. They may be reported separately as optional information. The calculator reports the displacement as a standalone comparison line, outside the Scope 1 and Scope 2 totals. Netting it in would produce a non-compliant inventory.
Because the UK grid has decarbonised faster than gas CHP. At the 2026 UK grid factor of about 0.131 kg CO₂e/kWh, allocated gas-CHP electricity — around 0.35 kg/kWh under the efficiency method — is higher-carbon than grid import. This is a real and increasingly common result, not a calculation error. The modern case for gas CHP rests on heat recovery, energy resilience, and operating cost, not on grid-electricity carbon savings. The calculator reports the comparison honestly, including when it comes out as a net additional figure.
No — the split is basis-invariant. Total emissions cancel out of the heat/power fractions, which depend only on the two outputs and the two reference efficiencies. As long as your fuel quantity and fuel factor are on the same basis (the unit selector enforces this), the allocated tonnes and the two derived per-unit factors are correct regardless of whether the fuel factor is Gross CV or Net CV. Basis only affects the energy-balance sanity check and the avoided-emissions counterfactual, both of which compare against actual fuel input.
The calculator defaults to the GHG Protocol US values — heat 0.80, power 0.35, both Net CV. These are broadly defensible. Override them when you have a documented local benchmark: the efficiency of the specific boiler your CHP heat displaces, or a published grid-generation efficiency for your region. Whatever you choose, cite the source and keep both efficiencies on the same calorific-value basis. A validator’s first question about a CHP allocation is where the reference efficiencies came from.
Yes. The region selector covers US grid factors by state and eGRID subregion alongside the UK DEFRA factors. The worked example on this page is UK-based for clarity, but the method is identical — only the fuel and grid factors change with the region. Select your region before calculating so the imported-electricity and displacement figures use the correct grid intensity.