In a recent post I compared a number of carbon footprint calculators that are now available on the web. In conducting my research I was interested to find out whether they agreed or not. They all ask different questions so I’d expect some difference in the results. I did, however, try to give consistent answers as far as I could. Some give an option to give a broad estimate or supply more detailed numbers. In this case I always chose the more detailed information option.

Without hanging around too much, here are the overall results:

	Total Tonnes CO2 equivalent per year
WWF	10.2
PawPrint	10.7
Climate Independent	9.1
Carbon Footprint	7.4
Carbon Stewards	7.0

So, as expected, I did get different answers. If we try to present it in the worst light we can say that the biggest (10.7) is over 50% higher than the lowest (7.0) which sounds a lot! Each calculator makes a lot of assumptions and I know, from how they are described or the information that they are working on. It wouldn’t have surprised me if the results had been even more different.

At first sight it looks as if the calculators divide into two groups. The first two give similar higher results and the last two are also fairly close but much lower. The third sits in between. My initial reaction is that the first two ask more general questions than the others and maybe this is the explanation. To explore this we need to break down the results into the different areas. All of the calculators allow a fairly similar summary of the data so we can compare this with the little bar chart below.

Home (gas and electricity combined)

The emissions from home use (gas and electricity in my case) show the same grouping as the overall totals. The grey, yellow and green columns are all very similar in height which, given that I’ve supplied all three with the same readings from my electricity and gas bills, is reassuring (this also extends to their separate estimates for gas and electricity). WWF (blue) guessed at my energy usage from a couple of questions about how big my house is. I think this over-estimates my footprint because although I live in a big house I’m very careful with how I use my heating system. I probably use less energy than the average person living in a house of that size but the calculator has no way of knowing this. It’s not really clear to me why the brown (Pawprint) column is so high. (Although Pawprint asked me to put my meter readings in categories rather than type in the actual figure, they actually fell at round about the middle of the category which I would ahve thought, would lead to a fairly consistent result – the difference remains a mystery.)

Transport

The figures for my car use are all fairly similar which is good. Rather bizarrely Pawprint allocates 1/2 a tonne to public transport despite me stating that I don’t use buses or trains, perhaps it’s acknolwedging the emissions from the infrastructure that is there whether I use it or not. It is a lot though according to this website I would have to travel 12,000 km on national railways to emit 1/2 a tonne of CO2. I haven’t flown for over four years now so can’t really comment on how the different calculators handle flights.

Food

Four of the calculators give very similar results for emissions related to my diet (I’m vegan so these figures are quite low). The WWF caculator gives a value almost twice as much as the others and it’s not at all clear why. I did answer that I don’t pay much attention to where my food comes from but surely that can’t account for over a tonne of CO2?

Other spending

The biggest discrepancy comes in “other spending” which doesn’t surprise me as calculating the carbon cost of something like the average hotel stay is really tricky. Carbon Independent looks to be the highest but it includes 1.1 tonnes per year for general government spending (health, education, armed forces etc). If you remove this then the bar comes more into line with others on the chart. It does seems valid to include this (although there is little any of us can do to change it) but none of the other calculators do.

If you take this into account then Pawprint gives the highest value. I’m not quite sure why, but it might be that the calculator estimates based on questions about whether my spending is below, on, or above average. The other calculators ask me to guess how much I spend each year or month on different things. I suspect it is quite easy to under-estimate the amounts we spend on this stuff in a year and wonder if Pawprint’s approach might be more valid if less detailed. I do notice looking back at the analysis that if I total up all the spending I’ve “declared”, it does come to significantly less than my annual credit card bill, so maybe this is the explanation. Of course I could go through my credit card bills and work out the actual amount spent in the different categories but I’d have to be quite a hard core footprint nerd to go through a process like that!

I was particularly interested in the comparison between Carbon Stewards and Carbon Footprint. These have a very similar in overall methodology and look yet the Carbon Stewards emissions for “other spending” is about half of that of Carbon Footprint. Becasue of the way they are structured its realtively easy to compare results and the difference seems to result primarily from two items. Carbon Footprint calculate emissions relating to my morgage interest payments over almost 0.6 tonne per year. None of the other calculators even ask about this. The other item is computers and other electronic equipments. I’ve given the same overall expenditure (£500 for a new laptop and phone I purchased last year). Carbon Footprint calculate emissions (nearly 0.6 tonne) that are about ten times those calculated by Carbon Stewards (0.07 tonne). When I look at other sites (like this one) Carbon Stewards fiugre does look very low and I wonder if there is some mistake here.

Summary

As expected different carbon calculators do calculate different carbon footprints. With the exception of the “other spending” category, those that ask for more detailed information tend to give more consistent results (which are lower for me). The “other spending” category shows most variation. Here it may be that I’ve had a tendency to under-estimate what I spend on stuff when asked to put in an amount in pounds spent and that the Pawprint question about how I compare with “average” might give a more realistic result (unless I’ve got time to go through and work out the actual figures).

The differences are sufficiently large that it’s clearly not sensible to compare the results of one calculator with those of another to compare my footprint with some body else’s. It’s also clear that if I want to chart my progress over a number of years that I need to sue the same calculator each year.

Even though they give different overall results, the calculators are in broad agreeement over the pattern of my emissions across the main areas (home, travel, food and other spending). If I want to use them to work out where I am most likley to reduce my emissions in future then all the calculators will point me in the same direction.