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Take a home needing 12,000 kWh of heat a year. A gas boiler at ~90% efficiency burns about 13,333 kWh of gas → ≈ 2.8 t CO₂e. A heat pump at a seasonal COP of 3 uses about 4,000 kWh of electricity → ≈ 0.54 t. That's roughly 81% less, or about 103 trees a year, on the SAP 10.2 factors used for Part L 2021 work today.
The factors move the answer, and every official set puts the heat pump well ahead: on SAP 10.3's — the Future Homes Standard's, for new homes notified from 24 March 2027 — the saving is about 88%; on DESNZ's 2026 company-reporting factors — the gas as burned, the electricity with the grid's transmission losses — about 76%.
And the gap widens over time: gas stays gas, while the electricity a heat pump runs on keeps getting cleaner as the grid decarbonises. TreeMonkey's Compare tab lets you set your own heat demand.
Source: SAP 10.2 and SAP 10.3 emission factors (BRE); DESNZ greenhouse gas conversion factors 2026; illustrative boiler efficiency 90%, heat-pump SCOP 3. Figures are illustrative UK equivalents.
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Frequently asked questions
What COP should I assume?
A well-designed air-source heat pump typically achieves a seasonal COP (SCOP) around 3, sometimes higher with low-temperature emitters. TreeMonkey uses 3 for illustration; a higher COP widens the saving.
Does the grid factor change the answer?
Yes — the cleaner the electricity, the bigger the heat-pump advantage: about 81% on SAP 10.2, 88% on SAP 10.3 and 76% on DESNZ's 2026 factors. The live grid is often cleaner still.