How we work out the figures

Our online calculations for energy use, travel and flights are based on the following sources:

Office energy and company travel

To work out the CO₂ emissions from your office energy and company travel, we have based our calculations on the UK Government’s published conversion factors for calculating greenhouse gas emissions. These can be found in the Annexes to Guidelines for Company Reporting on Greenhouse Gas Emissions, which were last updated in 2007.

To work out energy emissions from your office floorspace, we have used average figures per m2. These are published for four types of office building in "Energy Use in Offices" published by the Energy Efficiency Best Practice Programme Revised March 2003.

Air Travel

We are often asked why there are so many different figures out there when it comes to the climate impact of taking the a plane. Unlike driving a car (where the calculation is based on the fuel used) in the case of air travel there are many more variables. These include the type of plane, the number of seats, the amount of cargo and of course the distance travelled. We also have to account for the effects of "non CO₂" greenhouse gases on the climate.

To work out the emissions from your air travel, we have based our formulae and calculations on a review of the CO₂ emissions from air travel we commissioned from University of Oxford’s Environmental Change Institute – “Aviation Emissions and Offsets”

Car, Home & Transport

Calculating the CO₂ emissions from our home energy use and day-to-day travel is relatively straightforward. Defra (the UK government department in charge of climate change) publishes conversion factors for calculating CO₂ emissions using our energy consumption or travel mileage. These were last updated in 2007 (Annexes to Guidelines for Company Reporting on Greenhouse Gas Emissions) and form the basis of our calculations.

The emissions from electricity depend on how it is generated. Defra publishes a figure of 0.547 kg of CO₂e per kWh in the UK which is the figure we use (as of August 2009). This represents the blend of coal, gas, nuclear and renewable energy we use in the UK. If you use renewable electricity, you may elect not to include it in your calculations.

For emissions calculations based on the amount you spend on gas and electricity or the size of your car, there is a subjective element in converting these to kWh or litres of fuel. We have made assumptions based on energy prices and general fuel consumption figures.

The energy use for small, medium and large houses are published by OFGEM - the gas and electricity markets regulator.