The uncomfortable arithmetic of fuel switching
A site burning 1 GWh of natural gas a year emits about 184 tonnes of CO₂e. Here is what the obvious options do to that number, on published UK factors:
| Option | t CO₂e/yr | Change |
|---|---|---|
| Natural gas (today) | 184 | — |
| Electrify | 207 | +13 % |
| 10 vol % hydrogen blend | 178 | −3 % |
| Biomass | 12.5 | −93 % |
| Green hydrogen | 0 | −100 % (at point of use) |
Two of those deserve a second look.
Electrification makes it worse. Not forever — the grid is decarbonising, and at some point the arrow flips. But swapping a gas burner for a resistive electric one today, at 0.207 kg CO₂e/kWh, increases emissions and multiplies the fuel bill by about five. The exception is a heat pump: at a COP of 3 you need a third of the electricity, and the carbon case turns strongly positive. If electrification is on your roadmap, the COP is the whole argument, and this screening tool does not model it.
A 10 % hydrogen blend is not a 10 % cut. Hydrogen carries roughly a third of the energy per cubic metre that methane does, so 10 % by volume is only about 3.2 % by energy — and carbon follows energy. This is the single most common mistake in hydrogen-blending claims, and the calculator makes it explicit every time you move the slider.
Why efficiency comes first
Efficiency and heat recovery cut carbon regardless of what you burn or which grid you are on, and they shrink every option that comes after them. A 10 % efficiency improvement on that 1 GWh site removes 18 t CO₂e a year on its own — and if you later electrify, it is 10 % less electricity you have to buy at five times the price per kWh.
The order that almost always holds:
- Use less — controls, insulation, leak repair, shutting things off.
- Recover what you already paid for — economisers, recuperators, waste-heat exchangers.
- Then switch what remains to a lower-carbon source.
Doing it in the other order means paying to decarbonise energy you should not have been using.
What the model does
Energy today is your consumption converted to kWh with a typical calorific value for the fuel, annualised from whatever basis you enter. Carbon today is that energy times the fuel's emission factor.
After the levers:
Eafter=Etoday×(1−ηeff)×(1−fsolar) CO2after=Eafter×ftargetEfficiency is capped at 40 % and the solar fraction at 30 %, because beyond that a screening estimate is not the right tool. Emission factors are indicative typical values in the range of the UK DESNZ 2024 conversion factors (gross CV, Scope 1 point of use), and grid electricity is carried at 0.207 kg CO₂e/kWh.
Scope 1, Scope 2 and what "zero" means
The calculator reports point-of-use carbon, and the distinction matters when you report:
- Electrifying does not eliminate emissions, it moves them from Scope 1 to Scope 2. They are still yours to report, and they fall as the grid decarbonises.
- Green hydrogen is zero at the point of use. Its lifecycle carbon depends entirely on how it was made — grey hydrogen from unabated methane reforming can be worse than burning the gas directly.
- Biomass is counted as biogenic and near-zero at point of use. Whether that is genuinely true depends on sourcing and land use, and it brings particulate and NOₓ obligations that gas does not.
The assumptions, stated plainly
This is a screening tool, and it assumes:
- equal combustion efficiency across fuels — in reality a biomass boiler, a gas burner and a hydrogen burner do not have the same efficiency, and the fuel energy you actually need changes with it;
- that the solar fraction you enter is achievable — the real number depends on site irradiance, your load profile, available area and storage;
- that a hydrogen blend is safe on your plant — burner design, flame speed, materials compatibility and safety cases all decide the practical limit, which is why the slider stops at 10 %;
- published average prices, which are not your tariff.
It also does not model heat-pump COP, capital cost properly, grid decarbonisation over time, or the £ per tonne abated — which is the number that actually ranks options against each other on a board paper. Use this to see the shape of the decision and to kill the options that do not work. The costed, staged roadmap is the engineering study.