How much carbon would a heat pump actually save compared with your gas boiler? This calculator answers it with the official SAP 10.2 emission factors that sit behind Part L 2021 — mains gas at 0.210 kgCO2e/kWh and grid electricity at 0.136 kgCO2e/kWh — which is exactly the arithmetic an energy statement presents to your council.
Heat pump carbon calculator
The calculator needs JavaScript enabled. The factors it uses are the SAP 10.2 values described in our UK heat pump statistics guide.
How the maths works
- Your annual gas use × boiler efficiency = the useful heat your home actually needs.
- That heat ÷ the heat pump's seasonal efficiency (SCOP) = the electricity a heat pump would use for the same warmth.
- Each fuel × its SAP 10.2 factor = annual carbon, compared side by side.
Because grid electricity's factor (0.136) is now far below gas (0.210), and a heat pump delivers roughly three units of heat per unit of electricity, the carbon saving is typically substantial — which is why heat pumps dominate the Be Green stage of the energy hierarchy in planning submissions, and why UK installations keep breaking records.
Why SAP factors and not real-time grid carbon?
The real grid's carbon intensity changes hourly; compliance doesn't. Part L 2021 assessments and planning energy statements must use the SAP 10.2 factors, so that is what this tool shows. It also means the number you see here is directly comparable to the CO₂ savings tables in an energy statement — including the London Plan's 35% on-site reduction target, where heat pumps usually do the heavy lifting.
Honest limits
This is a screening comparison for a typical heating-and-hot-water pattern. A full energy statement models your actual dwelling fabric using the SAP 10.2 notional building method, applies Be Lean and Be Clean measures first, and calculates savings at each stage — a heat pump's SCOP also depends on emitter sizing and flow temperatures, which is design work, not a slider.
How Fortress Associates can help
Our automated energy statement service produces a planning-ready statement in minutes, free — Part L 2021 baseline via SAP 10.2, the full Be Lean / Be Clean / Be Green hierarchy with CO₂ savings at each stage, and the London Plan 35% target with carbon-offset calculations for London schemes. Try the rest of our free planning tools, browse all services or get in touch.
Sources & further reading
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