How is my carbon footprint calculated?
For a general overview of what carbon emissions are, and the difference between direct and indirect emissions, please read what are carbon emissions?
Once you've registered with the Carbon Account, you'll see that there are two footprint figures: your estimated footprint, and your recorded footprint. The reason your recorded footprint starts off at zero is because you haven't entered any meter readings yet (read on for more info).
The first thing to note is that both of your footprint figures depend on the number of people in your house. The Carbon Account is for individuals, and so when we calculate your individual user footprint, we divide the figure by the number of adults you told us you live with. We count children as half an adult (it's up to you to decide when your children are grown-up!). You can see your home's overall footprint, not divided by the number of people who live in it, on the Houses page.
This figure is based on the answers you gave to the questions during registration. It includes estimates of how much energy is required to heat your house, cook your food and power your lights and appliances. If you entered your vehicle registration and estimated an annual mileage, then we calculate the amount of carbon released as a result. The estimated footprint assumes that you haven't flown in the last year.
The estimated footprint is a very approximate figure because of the subjective nature of the questions asked, and the large effect that behaviour has on the amount of energy used in the home (for example, how much cooking is done in the oven versus the microwave, how often you take baths, how much the TV or Playstation is on etc.). Often, people don't have a clear idea of how much they drive in a year.
Unlike other carbon calculators on the web, the Carbon Account asks you to come back and enter meter readings, your car mileage, and details of any flights you take. You can enter readings as regularly as you want, and the more often you do so, the more accurate your graph becomes (we recommend adding a reading at least once a month to get a meaningful output).
As we've seen above, when you first register, your recorded footprint will appear as zero. This is because we need you to enter actual readings to start calculating a footprint that is based on your real behaviour rather than an estimate. The components of your real footprint are the items you see appear when you click the 'Add a reading' button. These are usually flights, gas, electricity and vehicles (which are identified by the numberplate you entered).
Apart from a flight, which leads to a fixed emission based on the particular journey, you need to add two readings for each component in order to start building an accurate recorded footprint. For example, if you add two readings for gas without entering any for electricity, your recorded footprint will only reflect gas. Similarly, if you enter just one reading for each component, your recorded estimate will stay at zero because we need to take the difference between two readings in order to work out how much carbon was emitted in a given time period.
In order to convert your energy data into carbon emissions, the Carbon Account links with AMEE to get conversion factors for each fuel type. For electricity, we detect the fuel mix for individual suppliers from electricityinfo.org.
Comparing your recorded footprint with your estimate
The idea of having an estimated and a recorded footprint is that you can see how you're doing against a pre-determined baseline. This only really becomes relevant after you've been entering data for a full year, because in the winter months your recorded footprint will be higher (because it's cold and dark) and in the summer it will be lower (because it's warm and light). In general, if your recorded footprint is less than your estimate, then you are doing well at keeping your emissions down.
You'll notice on your graph that for the period where you've not entered any readings, your graph is hashed. This is so that you can easily tell what your estimate is versus your real data. In this example, there is a big drop-off from the estimate because the real readings were started in summer, when very little gas was being used for heating the house. As it got colder the gas emissions went up, but this user is still staying underneath their estimate, even in winter. The annual footprint figures will reflect this.
Ideally the the estimate part of the graph would reflect the seasonal differences so that it's easier to see how you are performing against your estimate, regardless of what time of year you start entering data.
When we've got a decent amount of people entering data in the Carbon Account, we can start comparing users with houses of a similar size. This will be interesting, because as well as comparing against estimates of energy use for your house size, we'll be able to see how you're doing against people entering real data.
There are lots of layers of complexity, such as where in the country you are (e.g. a four bedroom house in sunny Cornwall will use less energy than one in the cold Highlands of Scotland) but having a tool as precise as the Carbon Account opens up some exciting possibilities in designing our response to climate change.