EPC CO2 emissions vs energy rating: why they sometimes contradict each other

What the two ratings measure

Why they diverge — and when

The key is that different fuels have different relationships between cost and carbon output. The divergence is most dramatic when comparing gas-heated and electrically-heated properties.

The gas vs electric paradox

Consider two properties:

• Property A: Gas-heated, well-insulated, EPC band B. Cheap to run — gas is ~6p/kWh. But gas combustion emits CO2 directly. Environmental impact rating: C.
• Property B: Electrically-heated (storage heaters), less well-insulated, EPC band D. Expensive to run — electricity is ~25p/kWh. But the UK grid is increasingly renewable. Environmental impact rating: C or D — similar to or better than Property A in carbon terms.

The energy efficiency rating says Property A is significantly better. The environmental impact rating says they're comparable. Both are correct — they're measuring different things.

The direction of travel

As the UK electricity grid decarbonises — more renewables, less gas generation — the carbon intensity of electricity falls. A property heated by electricity or a heat pump becomes progressively cleaner over time without any changes to the property itself. A gas-heated property's environmental rating doesn't improve unless you change the heating system.

This is why the government's policy trajectory is increasingly focused on the environmental impact rating, not just the energy efficiency rating. Future regulation is likely to tighten CO2 requirements — not just energy cost requirements.

The CO2 figures on the certificate

Below the rating bars, the EPC lists estimated CO2 emissions in kg per year — typically broken into current and potential (if improvements were made). These figures let you compare properties directly:

• A typical D-rated gas semi-detached: 3,500–5,000 kg CO2/year
• A B-rated gas property: 1,500–2,500 kg CO2/year
• A heat-pump-heated A-rated property: under 1,000 kg CO2/year

These numbers are also relevant if you have personal or company sustainability commitments — increasingly common among buyers who work for or run ESG-conscious organisations.

When the gap matters to buyers

Green mortgages

Current green mortgage products are based on the energy efficiency rating (A or B), not the environmental impact rating. So a gas-heated B doesn't lose its green mortgage eligibility. But as regulation evolves, this may change.

Future saleability

Policy direction suggests that environmental impact will become increasingly regulated alongside energy efficiency. Buying a property with a high CO2 rating — even if the energy efficiency band looks reasonable — may create a future compliance liability, particularly for buy-to-let investors.

Comparing gas vs heat pump properties

When choosing between a gas-heated B-rated property and a heat-pump-heated B-rated property at similar prices, the environmental impact rating reveals a significant difference in CO2 output that the energy efficiency rating masks.

How to read both ratings together

A useful framework:

• Both high (B/A on both): The gold standard. Well-insulated and low-carbon. Usually heat pump or new build.
• High efficiency, moderate environmental: Typical gas-heated well-insulated property. Good bills, but still burning gas. A heat pump upgrade would close the gap.
• Moderate efficiency, moderate environmental: Most D-rated gas properties. Both need improvement.
• Low efficiency, lower carbon: Electrically-heated properties — expensive to run but relatively low CO2. Bills need addressing, carbon profile is better than it looks.