Decarbonising your home or car - which saves more?

The global carbon budget is extremely tight and we all need to do what we can, as soon as we can, to reduce the carbon emissions from our home and other life choices. I was recently asked – will I save more by replacing my gas boiler with a heat pump or replacing my car with an EV? I was surprised to find the results were fairly well balanced. Which saves most depends quite a lot on how much gas you use and how much fuel you typically use in your car. Here are some typical figures and example calculations. 

Also there is at least one other factor to consider: lock-in. If you buy a new gas boiler today you probably will not want to replace it for 15 years. On the other hand you might buy a second hand car and expect to replace it in 5 years. So the decision on a new gas boiler might have more impact over the lifetime of the boiler even if the annual carbon savings are higher for the car.

The headline figures

Headline results, savings in green

Carbon savings based on latest carbon factors for electricity generation.

Heating	102g/kWh on your gas bill	1.2 tonnes/year assuming 12,000 kWh/year
EV	86 g/mile	0.9 tonnes/year assuming 10,000 miles/year

The figures above are based on the latest carbon factors for electricity generation (233 g/kWh), as stated by the government in their guidelines for company reporting [1] - a rolling average of the last three years. This is actually an overestimate, since the renewable energy content of our power has been increasing rapidly – in 2019 it was down to 217 g/kWh [2] - and will continue to do so. If you take care to charge your car at times when the grid carbon is low, you can do considerably better that way. Plus, low carbon is usually cheaper, if you go for a dynamic tariff like Octopus Agile. Or you may consider your electricity to be zero emissions anyway because you use a 100% renewable tariff. In the limit case, the electricity use is zero carbon and the savings are much greater.

Carbon savings based on zero carbon for electricity generation

Heating	173g/kWh on your gas bill	2.1 tonnes/year assuming 12,000 kW/year
EV	167g/mile	1.7 tonnes/year assuming 10,000 miles/year

Other options: insulation, waste heat, renewable gas, hydrogen

This assumes that your home is suitable for a heat pump - that it is reasonably well insulated and has a suitable place to put the heat pump . There are other approaches. Insulating your home will reduce your gas use and hence your emissions, but for most houses it will take a hugely expensive retrofit to get this even close to zero. There are examples - look for EnerPHit homes. However there is still heat needed for water. Insulation is worth considering, though, and a less extreme retrofit can make the heat pump conversion much easier.

If you live in a reasonably compact housing area with a convenient source of waste heat nearby (such as a power station) then you can hope that someone will lay on a district heating network for you. Or you can switch to renewable gas, (usually made from waste biomass,) but there is not going to be nearly enough of this to go round. Or you can wait for the gas networks people to convert the gas grid to hydrogen. Unfortunately there are some big problems with this approach. Perhaps the biggest is simply that it will take too long. It has to be done segment by segment and it will be at least a decade before even the first part is converted. We need to decarbonise faster than this!

Having said this, some homes are going to be difficult to convert to heat pumps. I recently gave a talk on different ways to decarbonise your heating - perhaps that will be the next blog post. 

For comparison, UK greenhouse gas emissions are 5.3 tonnes per person.

To put these savings into perspective, UK carbon emissions as of 2019 were 5.3 tonnes per person [6] This is the emissions produced within the UK geographic boundary - if you consider consumption emissions adding imports and subtracting exports, the emissions are quite a bit higher. But either way, car and heating add up to a substantial chunk.

I mentioned the carbon factor for electricity mentioned above; here are the other assumptions I have used.

Assumptions for the electric heating calculation.

Heating

• Typical gas bill 12000 kWh/year
• Gas carbon factor 183 g/kWh [1]
• Gas boiler efficiency 85%
• Air source heat pump SCOP (Seasonal Coefficient of Performance) 2.8 – quite a conservative figure and you can do a lot better than this
• Heat pump manufacturing carbon 1600 kg [3] (assumes a 10 kW heat pump. The boiler carbon is negligible in comparison, because it is physically smaller and simpler.
• Heat pump lifetime 15 years at typical use 12000 kWh/year

Based on these assumptions, the heat pump emissions are 9g/kWh for the embodied carbon and 71g/kWh for the electricity equivalent to your gas use. 

EV assumptions

I have assumed that you are starting with a diesel vehicle. For a petrol vehicle the savings would be a bit more because diesels are usually more efficient.

• Typical annual mileage 10,000 miles
• Diesel 2546 g/litre [1]
• Efficiency 50 miles per gallon, or 11 miles/litre
• EV efficiency 0.35 kWh/mile [4]
• EV extra embodied carbon equivalent to 40 g/km [5] taking an average of a US battery [30 g/kWh] and an Asian battery (50 g/kWh]
• About half these emissions are related to energy use. Over time they can be expected to decrease too.

For the calculations, it is easiest to view them in a spreadsheet - here is a link to Google Sheets. You should be able to copy the sheet and edit the assumptions for your situation.

[1] Government conversion factors for company reporting of greenhouse gas emissions (www.gov.uk)

[2] Digest UK Energy Statistics 2020 (www.gov.uk) - figure for 2019 multiplied by 1.095 to allow for transmission and distribution losses. 

[3]  The embodied CO2e of sustainable energy technologies used in buildings: A review article Finnegan et al (2018)

[4] Electric vehicle database - a typical estate car 

[5] Factcheck: How electric vehicles help to tackle climate change (Carbon Brief) 2019

[6] Analysis: UK’s CO2 emissions have fallen 29% over the past decade (Carbon Brief) 2020