Carbon calculator methodology

Household size

The household size questions determine whether the questionnaire is being completed by an individual or for a household. This is needed for sharing out your energy usage between the members of your household.

Household energy

The age of construction and building type of a dwelling has consequences for energy performance. Therefore, the type and age of the building can be used to estimate your household fuel usage.

Buildings built pre-1919 are generally less easy to heat. After 1919 the introduction of cavity wall construction led to improvements in energy efficiency. In 1982, minimum levels of energy efficiency and airtightness were introduced which led to another step change in preventing heat loss. Types of dwellings can differ in terms of the sizes, shapes and areas of exposure which will impact the energy efficiency. We have used the figures given in the latest Scottish House Condition Survey (2018) to estimate the carbon footprint for different dwelling ages and dwelling types, however it should be noted that there is also a good deal of variability within these groups and these figures can only give an average of a typical dwelling of that type or age.

The latest UK Greenhouse gas reporting: conversion factors 2020 gives emission factors for most fuel types. Where possible we have used these to calculate the carbon footprint of the energy that you have told us that you use in your house. Where available well-to-tank (WTT) emissions factors have been included used to ensure that all emissions including the production, processing and delivery of a fuel or energy vector are included. A Well-to-Tank emissions factor, also known as upstream or indirect emissions, is an average of all the GHG emissions released into the atmosphere from the production, processing and delivery of a fuel or energy vector. For peat, the emission factor comes from the IPPC Good Practice Guidance and Uncertainty Management in National Greenhouse Gas Inventories.

If you have a wind turbine or solar panels any reduction in energy use will be reflected in your household electricity bills. We have not considered any exported energy in terms of carbon offsetting.

Any energy used for charging an EV should be included here as we will make an estimate of the energy used in the personal transport section and assign it to your personal transport total at the end of the calculator.

Emission factors

Emissions factors for electricity will vary depending on your supplier. If your supplier has given you this, you can choose to apply this factor in the calcualtor. If you have a green energy tariff, we have assumed that the majority of the supply will be from onshore wind since over 73% of renewable energy in Scotland was from onshore wind in 2019 (Energy Trends: UK renewables) and an emission factor to reflect this (Life cycle costs and carbon emissions of wind power, 2015).

If you do not know what the emission factor for your electricity is, then average emission factor for electricity generated in Orkney is applied or a typical emission factor for wind energy if you have a 100% renewable electricity tariff. An emission factor of 0.015kg CO2e/kWh is applied if you have a 100% renewable electricity tariff (Life cycle costs and carbon emissions of wind power, 2015). Orkney produces a large proportion of its own electricity from onshore wind turbines and is also connected to the UK grid via subsea cables. The average emission factor for Orkney’s electricity has been falling over the last decade as more onshore wind turbines have been installed in addition to the carbon footprint of electricity imported into Orkney falling. This figure has been estimated to be around 0.017kg CO₂e/kW (OREF, 2021). This figure assumes that the Scottish average emission factor is applied to the imported electricity (Scottish greenhouse gas emissions annual target report: 2017) and that a factor of 0.015kg CO₂e/kW is applied to renewable energy produced in Orkney of which the majority is from wind turbines (Life cycle costs and carbon emissions of wind power, 2015).

Housing

The carbon footprint of the bricks and mortar of the house itself will vary widely depending on the age and materials used. According to a paper by The Empty Homes Agency average carbon emissions for a new build would be around 50 tonnes of embodied CO₂e whereas the average carbon emissions for refurbishing an old home were around 15 tonnes of embodied CO₂e. For example, an average house over a 100-year lifespan it accounts for 50 tonnes when it is built and if refurbished twice in that time that would give 0.8 tonnes CO₂e per year per household.

Consumables

Most products and services that we consume will have a carbon footprint generated either through direct emissions across the supply chain or in the energy required for their production, transportation, and disposal.

To estimate your carbon footprint from these products and services (up to the point of disposal), you should estimate the amount of money spent on each type of product or service category. We have then multiplied the amount of money spent (on the various goods and services listed) by the factors for these selected goods and services given in the 2012 Guidelines to DEFRA / DECC's GHG Conversion Factors for Company Reporting. To keep the calculator simple, we have grouped similar goods and services together and used an average emission factor for each. As these factors are based on cost, they have been corrected to account for inflation between 2012 and 2019 using the Bank of England Inflation Calculator.

We have set the default values on these spending categories based on Office of Nation Statistics data on average household expenditures on goods and services in Scotland. As the carbon footprint of your housing, travel and diet is included elsewhere please exclude any spend on these items.

Waste

The latest waste statistics for Orkney (for 2018) are available from SEPA. We have calculated the carbon impact of waste stream that are not recycled using the Carbon Metric developed by Zero waste Scotland. The Carbon Metric gives a whole lifecycle metric but as we are only interested in the carbon footprint of the disposal (as other upstream impacts are accounted for above) we have only used the relevant factors for (Incineration/Landfilled/Other diversion). This gives a total of 0.111 tonnes CO₂e per person.

Food

Although figures are available as to the relative carbon footprint of all food types and ingredients, in order to keep the calculator simple a more simplistic approach was taken that estimated carbon footprint based on the type of diet utilising the findings of a 2014 report, Dietary greenhouse gas emissions of meat-eaters, fish-eaters, vegetarians and vegans in the UK which provides daily carbon footprints for varying levels of meat-eaters, fish-eaters, vegans and vegetarians taking account of productions, transport, storage, cooking and wastage of food.

An adjustment is made for any non-adult members of the household by calculating the relevant carbon footprint as a percentage of the adult diet based on NHS calorie guidance (which estimates a calory intake for child older than 12 to be 90% of an adult's calorie intake, child between 5 and 12 to be 70% of an adult's calorie intake and a child under 5 to be 50% of an adult's calorie intake).

The emissions associated with out diet are due to a variety of factors: land use change, on farm emissions (use of fertilizers, methane, farm machinery), animal feed, processing, retail & packaging as well as transport as shown here: https://ourworldindata.org/environmental-impacts-of-food.

Especially for foods with a large footprint, transport as a share of the food’s total emissions is small and therefore contrary to most people’s expectation eating local only slightly reduces your emissions as shown here: https://ourworldindata.org/food-choice-vs-eating-local#where-do-the-emissions-from-our-food-come-from which uses data from the largest meta-analysis of global food systems to date, published in Science by Joseph Poore and Thomas Nemecek (2018).

Personal transport

We have used the UK Greenhouse gas reporting: conversion factors 2020 for these calculations which include all direct as well as indirect emissions (i.e., tailpipe emissions, emissions from electricity as well as Well-to-Tank emissions). These emission factors are multiplied by your annual mileage to give an estimate the carbon footprint for different modes of transport. For battery electric and plug-in hybrids these conversion factors have been adjusted to reflect the use of average emission factor for electricity generated in Orkney (see household energy section above).

The latest UK Greenhouse gas reporting: conversion factors 2020 gives emission factors for most fuel types. We have used these to calculate the carbon footprint of the fuels used for recreational activities. Well-to-tank (WTT) emissions factors have been included to ensure that all emissions including the production, processing and delivery of a fuel or energy vector are included. A Well-to-Tank emissions factor, also known as upstream or indirect emissions, is an average of all the GHG emissions released into the atmosphere from the production, processing and delivery of a fuel or energy vector.

Users are likely to be less familiar with their annual mileage when using public transport so you can either input your annual mileage or estimate it by entering the amount of time you spend using different types of transport and the average speed for each mode to work out how many miles you travelled.

We have given estimates of what these speeds are likely to be for different road types of public transport options. According to Vehicle Speed Compliance Statistics car, motorbikes, and buses tend to travel close to the speed limit for the road being used. Therefore, we have assumed an average speed of 50mph for local buses as this is the speed limit on single carriage ways, 60 mph for coaches as this is the speed limit on dual carriage ways, 25 mph for black cabs and regular taxies as the speed limit in towns is generally 20 or 30 mph.

Trains can travel at a variety of speeds depending on the track and train type. As an example, the train from Thurso to Inverness travels 149 miles in 3h 48min (48 mph). Therefore, we have used an average speed of 50 mph for this calculation. For light rail and trams, we have used the average speed given by TFL of 20 mph.

If you are regularly travelling on other routes, you can get an estimate of the distance by looking up the route details online.

Air travel

You can either choose the simple option which asks you to input the number of flights you have taken in the past year or the more detailed one which asks about the time you spent flying.

For the simple option you are asked to input the number of flights and we assume a flight time based on the flight type as follows:

• Interisland flights - starting and ending in Orkney and assumed to be 0.5 hour flying time.
• Domestic flights - starting and ending in the UK and assumed to be 1 hour flying time.
• Short-haul - fights between the UK and Europe (or those of similar distance, up to a 3,700km maximum) and assumed to be 2 hours flying time.
• Long-haul - flights between the UK and non-European destinations (or for flights over 3,700km) and assumed to be 8 hours flying time.

To calculate the distance travelled we have taken the average flight length given in the UK Greenhouse gas reporting: conversion factors 2020 and calculated average flight speed from representative journeys of that length. The flight speed for the interisland flights was obtained from Loganair.

For the more detailed option you are asked to input the flight length for each of the categories above. To calculate your carbon footprint, we multiply the amount of time you spend flying by the average speed of relevant flight type a plane to work out the distance you travelled. This distance is multiplied by the relevant emissions factors per km from the UK Greenhouse gas reporting: conversion factors 2020 (flight type selected based on an average passenger). The emissions factors used are based on an average passenger rather than on a specific seat type (i.e. economy/business/first class). We have used emissions factors which include the effect of radiative forcing to capture the full climate impact of aviation on climate change.

Sea travel

Similarly to the calculation for air travel above the calculation uses the amount of time spend travelling by sea to estimate the distance travelled this is based on an average for the various route within and to/from Orkney (26.2 km per hour) and multiply it by the relevant emissions factors per km from the UK Greenhouse gas reporting: conversion factors 2020 (for the passenger type selected).

Government services

There are also emissions associated with services provided by government. Statistics by end user sector produced by Department for Business, Energy & Industrial Strategy for 2019 show greenhouse gas emissions from the public sector were 12.2 million tonnes CO₂e. Dividing this by the estimated UK population mid-2019 of 66.8 million give a carbon footprint per person of 0.203 tonnes CO₂e per person. This includes things like public administration, defence, social security, education and health and social services.