LPG Combustion Emissions: Methodology and Emission Factors

Step-by-step method for converting metered LPG consumption into a defensible Scope 1 inventory across all three meter bases (litres, kilograms, kWh), handling propane/butane composition uncertainty, separating tank-to-wheel from well-to-tank emissions, and comparing LPG against grid natural gas on a like-for-like energy basis. Aligned to the GHG Protocol Corporate Standard, UK DEFRA 2026 GHG Conversion Factors, and IPCC AR6.

A methodology page is the execution layer — it takes the gas definitions from the glossary, the factor publication from the standards page, and the raw factor dataset from the data layer, and tells you exactly how to translate metered LPG consumption into a defensible Scope 1 (and optional Scope 3 Cat 3a) inventory. Unlike diesel, where the dominant error is variant misselection, LPG’s dominant error is meter-basis confusion — the same delivery can be measured in litres (volume), kilograms (mass), or kWh (energy), and the conversions between them are not interchangeable. Jump to the worked example or open the Scope 1 Combustion Calculator.

Why LPG Combustion Matters for GHG Reporting

LPG (liquefied petroleum gas — a propane-butane blend) is the largest off-grid Scope 1 fuel in the UK and across most of Europe. It powers rural heating, commercial kitchens, care homes and hospitals beyond the gas grid, leisure-park cottages, food processing plants, glasshouse horticulture, and a substantial share of forklift fleets in distribution centres. For organisations whose physical footprint lies outside the natural-gas distribution network, LPG typically sits in the top three Scope 1 line items by emissions volume — sometimes the single largest.

That ubiquity in stationary heat applications makes LPG a routine target for Scope 1 verification. CSRD ESRS E1-6 requires gross Scope 1 disclosure with a breakdown by fuel type, separating mobile from stationary combustion. SBTi corporate net-zero targets set under the Absolute Contraction Approach require year-on-year LPG reduction trajectories — often the deepest decarbonisation lever for off-grid sites considering electrification or biomass conversion. The GHG Protocol Corporate Standard defines the inventory boundary inside which LPG sits. ISO 14064-1 verifiers will sample LPG transactions at the delivery-note level for traceability to the metered source.

Three things make LPG harder to get right than diesel: (1) the same fuel arrives in three different meter bases (litres, kilograms, kWh) depending on the supply route, (2) the propane/butane composition varies seasonally and regionally, with density swinging ~13.7% between pure propane and pure butane, and (3) bulk-tank, cylinder, and autogas activity data come from completely different sources (delivery weighbridge, cylinder manifest, pump receipt) with different audit-trail patterns. This methodology page addresses all three.

The Three Gases — What LPG Actually Emits

LPG combustion produces three greenhouse gases: carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O). CO₂ dominates by mass — even more so than for diesel, because LPG burns more cleanly with less unburnt hydrocarbon slip. CH₄ is a trace combustion product; N₂O forms in trace quantities from nitrogen in combustion air at high temperatures. The DEFRA published factor aggregates all three into a single CO₂-equivalent value, but understanding the underlying split matters for audit reconciliation and for engineering-mode AR6 reconstruction.

The CO₂-dominant character of LPG (99.86%) explains why the AR5→AR6 GWP basis change has almost no effect on this fuel — the trace gases that benefit from the higher AR6 GWPs contribute too little to the total to move the headline number. This is a structural feature of LPG, not a coincidence; clean-burning gaseous fuels show the same pattern.

Propane / Butane Composition — and Why Volume↔Mass Conversion Is the Hardest Step

“LPG” is not a single chemical — it is a propane-butane blend whose composition varies by season and region. UK winter LPG is typically near-pure propane (boiling point −42°C, vaporises in cold weather), while UK summer and commercial-grade LPG often runs 70/30 propane/butane or higher butane content. Butane has a higher boiling point (−0.5°C), more energy per litre, and higher density. DEFRA publishes a single blended factor (1.55713 kg CO₂e/L) representing the UK pump-LPG average composition; pure-component factors are also available where the supply is documented.

3.1 Why density variation is the killer for volume↔mass conversion

The factor difference between pure propane (1.54358) and pure butane (1.74533) is +13.07% — small enough that a misclassified blend factor produces a manageable error in per-litre reporting. But the density difference between pure propane (~0.508 kg/L) and pure butane (~0.580 kg/L) is +14.2%, and that difference compounds with the factor when converting between mass and volume. The DEFRA standard blend density of 0.52975 kg/L assumes a UK-typical mostly-propane composition; using it on a butane-heavy supply (hot-climate or summer formulation) understates the volume that a tonne of LPG actually occupies, with a chained downstream error.

The practical implication: if you have a choice, report LPG by mass, not by volume. Mass-based reporting (delivery weighbridge, cylinder kg, bulk-tank inventory in tonnes) avoids the density assumption entirely — a kilogram of LPG is a kilogram regardless of composition. Volume-based reporting (autogas pump, tank dip in litres) inherits the density assumption and propagates composition uncertainty into the inventory. For most stationary LPG use (bulk delivery to a tank), the delivery note already gives kilograms — use them.

3.2 When the blended factor is wrong

The DEFRA blended factor is correct for UK pump LPG and for most undocumented bulk supply. It is the wrong choice in three specific cases. First, a documented pure-propane supply (winter-grade, industrial spec) should use the propane factor (1.54358) — the difference is small (−0.87%) but the methodology choice should be visible. Second, a documented pure-butane supply or near-pure butane (some chemical-industry feedstocks, hot-climate cylinder gas) should use the butane factor (1.74533) — the +12.09% difference is material enough that misclassification is a quantified disclosure risk. Third, refrigerant-grade R-290 (pure propane used as a working fluid) is not a combustion source and should not be in the combustion inventory at all — it sits in the refrigerants section under fugitive Scope 1.

Scope Boundary — TTW vs WTT

The combustion factors above are tank-to-wheel (TTW) Scope 1 figures. They cover the molecules released by burning LPG inside owned or controlled equipment. The full life-cycle carbon footprint of LPG includes upstream extraction, refining, and distribution: the well-to-tank (WTT) share, which sits in Scope 3 Category 3a (fuel- and energy-related activities not included in Scope 1 or 2).

The LPG WTT-to-TTW ratio of ~11.9% is meaningfully smaller than diesel’s ~24%. Two reasons: LPG is a refinery by-product (lower attribution of refining energy to the LPG fraction than to road diesel), and the distribution chain is shorter (fewer transfer steps from refinery to user, no retail forecourt aggregation). For an organisation switching from oil heating to LPG and adding WTT to the disclosure, the headline Scope 1 reduction is supplemented by a smaller Scope 3 Cat 3a addition than the equivalent diesel switch would produce.

The Calculation — Step by Step

The core LPG combustion equation is identical regardless of meter basis. The variables differ; the structure does not.

5.1 Match the factor to the meter — do not pre-convert

The single most important methodology rule for LPG: use the factor that matches your meter unit; do not convert the activity data first. If your delivery note shows 9.2 tonnes, multiply by the per-tonne factor (2,939.36 kg CO₂e/t). If your pump receipt shows 250 litres, multiply by the per-litre factor (1.55713). Pre-converting tonnes to litres before multiplying inserts a density assumption (0.52975 kg/L) that may not match your supply composition, propagating ~10–14% composition uncertainty into the inventory unnecessarily. The four DEFRA factors are internally consistent — they describe the same molecules at different unit slices — but only when each is applied to its own native unit.

5.2 Regulatory mode — DEFRA pre-aggregated

The simplest path. Take the metered quantity in its native unit, multiply by the matching DEFRA factor, divide by 1,000 to convert kilograms to tonnes. Document the meter basis and the DEFRA publication year in the methodology statement. This path is sufficient for UK SECR, GHG Protocol Corporate Standard compliance, and most Scope 1 disclosure obligations. The GWP basis is AR5 by virtue of using the DEFRA factor; no further GWP work is needed.

5.3 Engineering mode — gas-by-gas at AR6

Compute CO₂, CH₄, and N₂O separately using the implied raw masses from the §2 component table, then apply AR6 GWP-100 values (CH₄ fossil = 29.8, N₂O = 273). Sum the three CO₂e contributions for the engineering-mode total. For LPG, the AR5→AR6 delta is approximately +0.0073% — even smaller than the +0.04% delta on diesel — because CO₂ accounts for 99.86% of the LPG factor and CO₂ has no GWP attached. The natural gas methodology page covers the full AR5↔AR6 reconstruction logic in detail; for LPG the same logic applies with even smaller magnitudes.

5.4 GCV vs NCV — the 6.9% inversion risk

For any LPG inventory reported per kWh, the calorific basis must be declared explicitly. UK convention is gross calorific value (GCV, also called higher heating value); international energy reporting frequently uses net calorific value (NCV, also called lower heating value). The two factors differ by ~6.9% for LPG (0.21450 vs 0.23032 kg CO₂e/kWh). Using the wrong factor against an energy total inverts the disclosure direction — silent confusion produces a clean-looking but materially wrong number. The defence: capture the calorific basis at activity-data ingest, not at calculation. Energy bills from UK suppliers are GCV by default; meter readings from CHP plant should be checked at the meter spec sheet.

Stationary-Dominant — Heating, Cooking, and Process Heat

LPG is overwhelmingly a stationary combustion fuel. Unlike diesel — where mobile combustion (vehicle fleets, off-road plant) is the headline use case — LPG sits primarily in fixed installations: bulk tanks feeding boilers, cylinder banks supplying commercial kitchens, on-site process heat for food production and horticulture. Mobile LPG combustion is real but secondary, dominated by forklift fleets and a small autogas vehicle population. The activity-data sourcing pattern follows the application.

Cylinder, Bulk Tank, and Autogas — Three Activity-Data Patterns

The three LPG supply routes generate completely different activity-data trails, with different audit-quality characteristics. Understanding which route applies to your inventory determines what evidence you need to retain and which factor to apply.

7.1 Bulk tank — the highest-quality LPG activity data

Bulk LPG arrives by tanker delivery to an on-site storage tank, typically 1–10 tonnes capacity for commercial sites. Each delivery generates a weighbridge-certified delivery note in kilograms. The activity data is the sum of delivery notes for the reporting year, optionally reconciled against opening and closing tank-level readings. Audit quality is high: each kilogram traceable to a date, a tanker, and a delivery driver signature. Tank dip readings provide a sanity check but are not strictly required for bulk reporting where delivery notes are reliable. Use the per-tonne factor (2,939.36 kg CO₂e/t) directly — no density assumption required, no volume conversion needed. This is the cleanest LPG inventory pattern.

7.2 Cylinder — labelled mass and exchange manifests

Cylinder LPG comes in standard sizes — 47 kg (industrial / heating), 19 kg (forklift), 13 kg (commercial cooking), 6 kg (patio). Each cylinder is labelled with a net product mass; the supplier’s exchange manifest records the count and date of cylinders delivered against returned empties. The activity data is (cylinder size in kg × count) summed over the reporting year. Audit quality is good but slightly lower than bulk because the cylinder turnover at site introduces a timing question — was a cylinder delivered in December actually consumed in December or in January? The pragmatic answer is to use delivery-date as the consumption date for annual reporting, with a beginning/end-of-year stock check for material accounts. Use the per-tonne factor.

7.3 Autogas — pump dispense in litres

Autogas (LPG dispensed at fuel-station pumps for vehicles) is metered in litres. Activity data is the pump receipt or fuel card aggregation. Audit quality is good for the meter reading itself (calibrated dispense pumps), but volume-based reporting inherits the density assumption discussed in §3.1 — composition uncertainty propagates into the inventory. Autogas is the only LPG application where per-litre factor use is unavoidable; it is also the smallest application volumetrically, so the methodology consequence is bounded. Use 1.55713 kg CO₂e/L. Where ambient temperature deviates significantly from the 15°C UK pump reference (rare in UK climate; common abroad), temperature correction is required for material inventories.

LPG vs Natural Gas — When to Switch

Many off-grid sites face a strategic question: is LPG the right long-term choice, or should the site connect to the natural gas grid (where geographically feasible) or transition straight to electrification or heat pumps? The carbon-per-kWh comparison between LPG and natural gas is a first-order input to that decision. Both are gaseous fossil fuels with similar combustion chemistry; the difference at the point of use is consistent and meaningful.

Two methodology cautions when comparing LPG and natural gas. First, the comparison must be on the same calorific basis — both factors above are GCV; mixing GCV and NCV across the two fuels inverts the comparison by ~6%. Second, the cleanest decision frame is per-kWh-of-useful-heat, not per-kWh-of-fuel — boiler efficiency on LPG vs natural gas typically differs by 2–4 percentage points (LPG burners run slightly more efficiently at small commercial scale) which narrows the headline +17.66% by roughly that margin. For strategic decarbonisation modelling, the per-kWh-of-useful-heat comparison is the right number to put in the business case.

Data Quality, Uncertainty, and the Activity-Data Hierarchy

The LPG emission factor is the easy part of a Scope 1 LPG inventory — DEFRA publishes it, the MasterBrain stores it, the calculator multiplies. The hard part is the activity data: how many kilograms (or litres or kWh) did the organisation actually combust? The hierarchy of data quality, from best to worst:

1. Bulk delivery notes in kilograms (Tier 1 best practice) — Weighbridge-certified delivery records summed for the reporting year. Each kilogram traceable to a date, tanker, and supplier signature. Use the per-tonne factor directly. Audit-grade and the only acceptable basis for reasonable assurance on material accounts.
2. Cylinder exchange manifest with labelled mass — Cylinder count × labelled kg-per-cylinder. Slightly weaker than bulk because of timing uncertainty (delivery date vs consumption date) but acceptable for annual reporting with stock-check reconciliation.
3. Tank dip with delivery reconciliation — Periodic tank dip readings bracketing delivery records, with consumption derived as (opening + delivered − closing). Acceptable; requires temperature-corrected dip readings and a documented reconciliation cadence.
4. Autogas pump receipts in litres — Pump dispense aggregated by fuel card or receipt. Acceptable for the pump-meter reading; inherits density-assumption uncertainty (§3.1) for any conversion to mass.
5. kWh from energy bills — Acceptable for CHP and heat-network use. Confirm GCV vs NCV basis at ingest; a wrong-basis factor introduces ±6.9% systematic bias.
6. Spend-based proxy — Expenditure ÷ assumed price per litre or per kg. Screening grade only; not acceptable for material categories at audit.

Three uncertainty sources matter for material LPG inventories:

• Composition variation. Pump and bulk LPG composition varies seasonally; the DEFRA blended factor is the regulatory safe harbour for this uncertainty band. Documented pure-component supply (winter propane spec, butane chemical feed) should switch to the pure factor and disclose the methodology choice.
• Density and temperature correction for volume-based reporting. LPG density varies ~10–14% between pure propane and pure butane, plus a smaller temperature dependence. The DEFRA 0.52975 kg/L is the 15°C reference for the standard UK blend; volume-based reporting should be avoided where mass-based data is available.
• Audit-trail retention. ISO 14064-1 verifiers will sample LPG transactions at the line level. Retain delivery notes, cylinder manifests, tank dip records, supplier composition declarations, and pump receipts for the inventory year plus the longer of (a) the regulator’s retention rule and (b) the planned re-validation cycle. SBTi 5-year re-validation cycles set a practical floor at 5 years.

Standards Alignment

Seven standards govern LPG combustion accounting. They nest rather than compete: each layer supplies a different input to the same audit-grade output.

Worked Example: A Rural Care Home on Bulk LPG Heating

A complete worked example, computed end-to-end with all arithmetic shown. Stationary-dominant by design — this is the most common LPG application. Hardcoded values throughout — this is an audit record at the snapshot date of this methodology revision.

What the Calculator Handles vs What You Decide

The Scope 1 Combustion Calculator automates the mechanical steps. This methodology page covers the upstream decisions the calculator cannot make for you.

Ready to compute? Open the Scope 1 Combustion Calculator →

Error Traps with Calculable Magnitudes

Each error below produces a specific, quantifiable distortion. Magnitudes shown for CareCo’s 9.2-tonne / 17,367 L LPG base — large enough to assess validation and disclosure risk for a typical off-grid commercial site.

Methodology Metadata — for GHG Inventory Documentation

Copy verbatim into your GHG inventory methodology statement for ISO 14064-1 transparency compliance. Adjust the variant selection, meter basis, and reporting mode lines to match your inventory choices.

Building a multi-fuel inventory. This method covers one fuel. For the conventions that have to hold across all of them before a site total is defensible — one calorific-value basis, one GWP basis, three gases on every fuel, biogenic CO₂ held as a memo item — see Stationary Combustion.

Frequently Asked Questions

Calculator

Apply this methodology with audit-grade output, dual-mode (regulatory + engineering) display, automatic WTT companion calculation, unit-matched factor selection, and full source traceability: Open the Scope 1 Combustion Calculator →

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