Our captive cogeneration plant makes steam and power. How does UK CBAM split its emissions?

The technical guide from HM Revenue and Customs (HMRC) splits a combined heat and power (CHP) unit's emissions between heat and power with efficiencies and reference efficiencies. Your product carries the heat share, per terajoule (TJ) of steam used. With no measured or design efficiencies it allows 0.55 for heat and 0.25 for power.

What HMRC asks you to collect

• Fuel into the unit: energy input is the sum of each fuel's quantity multiplied by its net calorific value (NCV), in TJ.
• Net measurable heat produced by the unit, in TJ.
• Net electricity produced, in TJ. Net means the electricity exported from the unit's boundary.
• Total emissions of the unit: each fuel's quantity x NCV x emission factor, plus process emissions from flue gas cleaning, in tonnes of carbon dioxide (CO2).

The four steps after that

1. Average efficiencies: heat efficiency is net heat divided by fuel energy input; electricity efficiency is net electricity divided by fuel energy input.
2. Attribution factor for heat: the heat efficiency divided by the reference efficiency for heat in a stand-alone boiler, over the sum of that term and the electricity efficiency divided by the reference efficiency for power without cogeneration.
3. Emission factor for CHP heat: the unit's total emissions x the attribution factor for heat, divided by net heat produced, in tonnes of CO2 per TJ.
4. Heat-related emissions of your process: that emission factor x the TJ of heat the process consumed.

If you cannot measure the efficiencies

Where measuring them is not technically feasible or would cost unreasonably, the guide says values can come from technical documentation such as design values. If none exist, it says conservative values of 0.55 for heat and 0.25 for electricity can be used. HMRC's draft force of law notice, which the guide points to, words the same fallback as "must be used".

If waste gas is in the CHP fuel mix and its emission factor is higher than natural gas, use the standard natural gas factor instead of the waste gas factor.

The reference efficiencies, and what we could not find

The guide says the fuel-specific reference efficiencies are in section F of the force of law notice. In the draft notice text we read, section F.1 holds the fuel emission factor tables, and we did not find a reference efficiency table there. Ask your verifier or HMRC which values apply before you finalise a number.

Example: one unit, one year, with example reference efficiencies

• Fuel in: 100 TJ of natural gas. Net heat 45 TJ, net electricity 30 TJ
• Heat efficiency 45 / 100 = 0.45. Electricity efficiency 30 / 100 = 0.30
• Unit emissions: 100 TJ x 56.1 tCO2/TJ = 5,610 tCO2, with no flue gas cleaning
• Example reference efficiencies: 0.90 for heat, 0.40 for power (not HMRC's values)
• Heat attribution factor: (0.45 / 0.90) / ((0.45 / 0.90) + (0.30 / 0.40)) = 0.5 / 1.25 = 0.4
• Heat emission factor: 5,610 x 0.4 / 45 = 49.87 tCO2/TJ
• Process uses 20 TJ of the heat: 49.87 x 20 = 997 tCO2 (997.3 using the unrounded factor)

tCO2 means tonnes of carbon dioxide. All volumes and the two reference efficiencies are example values. The 56.1 is the natural gas factor in Table 4 of HMRC's draft notice. Our reading: the power share (5,610 minus 2,244 = 3,366 tCO2) is not attributed to your CBAM goods, because electricity is not captured by UK CBAM.

What Senseible does here

Senseible reads fuel invoices and electricity bills into monthly records. That gives the monthly fuel quantities going into a unit. The standard product reads invoices and bills, not heat or power output; metering is a custom installation on request. It does not run the split. A verifier accredited for UK CBAM must still verify the figure.

• Why grid electricity is not counted
• Steam bought from a neighbouring plant
• Standard fuel emission factors
• UK CBAM for Indian exporters

Sources: HMRC, Carbon Border Adjustment Mechanism (CBAM): technical guide for monitoring, reporting and verification, HMRC, The Carbon Border Adjustment Mechanism (Emissions and Verification) Force of Law Notice (Draft)

Last reviewed 2026-11-06. The technical guide section D4.8.4 and the draft force of law notice section G.2.2.3 (Emissions and Verification, draft) were read on 6 November 2026. HMRC may change both before 1 January 2027, and has not yet shown the reference efficiencies in the text we read.