What does a personal carbon price look like?

Beyond the global or regional carbon pricing strategies I've already discussed, I'd also like to work out a personal carbon price.

A personal carbon price is effectively the price that I, personally, am willing to pay to offset a tonne of my own carbon emissions.

It's a concept I've been thinking about more and more recently because I need tools my personal arsenal to counter the message of "consume more" that's pushed out in adverts by companies. One astute commenter on a Guardian article¹ pointed out that it's not that consumers aren't "consider[ing] the impact" of emissions from flying, it's simply that to most people, the concept of "holiday" is synonymous with "flying". There is no considering to be done, because there's no viable alternative in people's minds.

Understanding a personal carbon price would be very helpful in trying to counter this mindset. I would be able to use it to determine the most cost-effective measures I can take to reduce my own emissions.

It's a difficult value to work out with 100% certainty, but I can make estimates of it and use these values to inform any future actions I take.

Estimation of a "personal carbon price"

A personal carbon price can be estimated from any two equivalent actions, each of which result in the same final outcome, but which could be carried out by different means, emitting different amounts of carbon dioxide, at different costs.

Although this sounds complex, there's several easy-to-understand examples. The most obvious one would be taking a long journey by either train or by plane. Both journeys achieve the same goal of moving you to your destination, but they will have different costs and different carbon footprints. The difference between price and carbon emissions can give you a personal carbon price.

In reality, these two journeys aren't 100% equivalent. They will take different amounts of time, have differing levels of comfort and convenience, and offer an overall different travelling experience. For these reasons, any estimate that I make of personal carbon pricing is just that – an estimate. Each time I try and estimate my personal carbon price it will likely be different.

Hopefully though, ballpark figures can be found which are still useful for planning my route to net-zero.

Specific estimation example – what is my personal carbon price?

We were recently heading to the south of France for a hiking holiday.

We could have either taken a plane or taken the train. These two modes of transport have very different carbon emissions associated with them associated with them and also come at different costs. The journey is about 1,200 km (about 750 miles).

To go by plane would involve a relatively expensive coach transfer to the airport in the UK, flying from Gatwick to Geneva, and then another coach transfer from Geneva airport to take us to our final destination.

To take the train would require a cheaper coach to London, a Eurostar to Paris, then a direct train to Geneva followed by a coach transfer from Geneva city centre to our final destination.

In the end, our two possible journeys could be summarised like this:

Despite my best intentions to reduce these journeys to just numbers, it's never quite that straightforward. In practice, the journey by train also included an evening meal overlooking the lake in Geneva which is not something you can get if you have to hang around Geneva airport for your coach transfer. We could also get out of bed earlier for the train journey which is always a bonus!

Still, if we look at just the numbers, we have three clear conclusions:

• The train would take longer than the plane
• The train would be more expensive than the plane overall, even factoring in the extra luggage fees we'd have to pay for the hiking equipment
• The train journey would emit a lot less carbon dioxide than the equivalent journey by plane

In the end we took the train. It's not something I regret at all, despite the higher cost and longer time.

Even though the train journey was longer, it wasn't as much of a difference as I'd initially expected. The average speed door-to-door was 95 km/h (60 miles/h) by plane and 83 km/h (52 miles/h) by train. Planes fly fast, but you spend a long time waiting around at airports in a way you don't for trains.

Subjectively, I also prefer travelling by train to plane. We had spacious seats, good views of the French countryside, could keep our luggage with us all the time, and had city-centre to city-centre connections, as well as Wi-Fi en route.

Importantly, I also have all the numbers I need to estimate my personal carbon price: we paid £173.57 extra for a journey that emitted 664 kg less of carbon dioxide.

How does this compare to other carbon prices? Well I can re-plot the carbon market price graph I used before and add in an additional set of bars for our personal carbon price:

In this graph, the UK ETS, USA federal pricing, and EU ETS represent carbon prices in three major global markets and the Aviation Offset is the greenwashing "price" of carbon dioxide for aviation.

The price for Rennert et al. is an independent estimate of the "total societal cost" of carbon dioxide emission¹. This value is much higher than the official values in any of the major markets.

My personal price is higher still than any of the other estimates – between 3-7x as high as the official markets, 1.5x higher than Rennert's total societal cost, and an astonishing 500x higher than the pointless Aviation Offset.

I can only speculate on why my personal price is higher. Maybe I was more willing to pay for the train journey because I got to see Paris and Geneva en route, rather than just the airports. Maybe I'm coming from a position of relative personal stability and I feel that I'm able to pay more to address a pressing global problem. Maybe also, this estimate is simply wrong. It could just reflect a single trip with values that aren't reproducible on other journeys.

I will continue to refine this estimate as I gather additional data, but for now, it's the best I have.

What to do with a personal carbon price?

Now I have an estimate I can use it to inform future actions. For example:

• I can work out the cost-effectiveness and carbon-effectiveness of planned changes to my house
• I can work out how much I'm willing to pay to get a more fuel-efficient car or appliance
• I can decide between different modes of transportation for different trips, not only based on price, but also based upon carbon-effectiveness

Example 1 – choosing between different modes of transport

Let's consider an example journey between Oxford and High Wycombe.

Is it better to take the train or to drive?

By car, it's 85 km there and back (53 miles). I've already worked out that my car emits 173 g CO₂/km which means that the journey would be responsible for 14.7 kg CO₂ emissions.

I also know that my car costs £0.15/km to run (£0.24/mile). This price includes fuel, of course, but also includes other "per km" costs including servicing, tyres, distance-based depreciation etc. The true cost of running the car is higher if I include fixed costs like insurance that don't vary if I drive a different distance. At this price, the return from Oxford to Wycombe costs £15.70.

For the train, it's easier to work out. The return distance is 124 km (78 miles)² – longer than by car because the train takes a more circuitous route via Banbury. At 45.5 g CO₂/km³, the train journey would be responsible for 5.7 kg CO₂. A typical return fare for this route is £16.70.

We now have a train journey that emits less carbon dioxide and costs more. The fact that the train journey costs more than the car is a national travesty and is something that urgently needs addressing, but that's beyond the scope of this calculation.

Putting all the numbers together, the train journey is £1.00 more expensive to save 9 kg CO₂, giving a carbon price of £111/tonne. This value is less than my personal carbon price of £261/tonne which means I should take the train.

Example 2 – electric immersion heater vs gas boiler

Burning gas in my boiler to heat water will always emit a fixed amount of carbon dioxide. Using electricity to heat the same water will emit a variable amount of carbon dioxide, depending on the emission intensity of the grid at the time the water is heated.

There's plenty of times when it's more carbon-effective to heat water by electricity than by gas. That being said, electricity is more expensive per kWh than gas. This is exactly the type of situation where you can work out the carbon price of an action – two identical outcomes, but with different costs and different associated carbon emissions.

Burning gas always emits 203 g CO₂/kWh ⁴. With a boiler efficiency of 90% this corresponds to a final carbon intensity of 226 g CO₂/kWh of hot water. My current gas price is £0.076/kWh at the meter, or an efficiency-adjusted £0.085/kWh.

Electricity carbon intensities vary, with values in the UK falling between 35-335 g CO₂/kWh in 2022. Electric heating is 100% efficient, and my current electricity price is £0.303/kWh.

Let's work out a carbon price difference for the greenest electricity as this represents the best-case scenario. The difference in carbon dioxide emission for 1 kWh is 226-35 = 191 g CO₂. The difference in cost for this carbon saving is £0.303-0.085 = £0.21. This converts to £0.011/g, or £1,099/tonne CO₂. This is greater than my personal carbon price of £261/tonne which means it's not carbon-economical for me to use electricity instead of gas to heat my water.

In fact, electricity would have to emit negative carbon to ever be worthwhile (-600 g CO₂/kWh in fact). What does this mean? How can it make sense that it's never worth switching from gas to electricity from a carbon-saving point of view? Well, it means that it's more economical to spend your money elsewhere to achieve carbon reduction.

In the longer term, I anticipate removing the gas boiler completely, replacing it with a heat pump. This changes the economics considerably. The majority of my gas is burned to heat the house, not water. The overall carbon effectiveness of removing the boiler should be considered, especially as the heat pump increases the efficiency of electricity use far beyond 100%.

Conclusions

Both these examples are simplistic and make many assumptions about the journeys being otherwise equal. It may be critical that you have the car at the far end, or it may be that you want a drink in the destination, making the car less attractive than the train.

The point of this type of thought process isn't to provide hard-and-fast rules for life, but to guide and inform your decision-making process.

I'm well aware that making this type of calculation for every trip or household decision is arduous. I think that the end goal should be an online tool or an app that allows these figures to be prepared in real time. Even if they're not used for individual trips, the incorporation of carbon pricing into national strategy for transport, heating, power etc. is a critical tool in our journey toward net zero.