2013-12-25

How to Calculate the Carbon Footprint of Software Products

This post gives a short overview of how a carbon footprint may be calculated for software products. It is loosely based on our scientific paper “Integrating Aspects of Carbon Footprints and Continuous Energy Efficiency Measurements into Green and Sustainable Software Engineering”1.

A carbon footprint (abbr. CF) has been defined as:
“A measure of the total amount of carbon dioxide (CO2) and methane (CH4) emissions of a defined population, system or activity, considering all relevant sources, sinks and storage within the spatial and temporal boundary of the population, system or activity of interest. Calculated as carbon dioxide equivalent (CO2e) using the relevant 100-year global warming potential (GWP100)”2
A regular carbon footprint covers the life cycle stages from the extraction or acquisition of raw materials to recycling and disposal of waste3.

For an adaption of such a life cycle for software products, see http://www.green-software-engineering.de/en/reference-model/life-cycle.html.
Further on we refer to it as the Life Cycle of Software Products (abbr. LCSP).

The Functional Unit

A suitable functional unit for a Software Development Life Cycle (abbr. SLDC) is CO2e/PM (carbon dioxide equivalents per person month).

However, for a whole software product, you may choose CO2e/License or CO2e/User or some other more suitable definition. For a single user desktop application, CO2e/License may be a good choice, but for a SAAS contract, CO2e/User may be more appropriate.

Carbon dioxide equivalents are a unit with which you can compare the radiative forcing of a greenhouse gas (e.g. methane) to carbon dioxide. More specifically, it considers the global warming potential of emissions on a 100-year basis. So you should not confuse CO2 with CO2e, because CO2e can also be the global warming potential of CH4. Anyway, the CO2e factor of CO2 is 1.

Prerequisites

  • Define an adequate functional unit for your software product.
  • Calculate the office space ratio per employee. The office space ratio partially includes corridors, meeting rooms, coffee kitchens, etc.
  • Acquire information about total electricity consumption, energy consumption for heating and hot water. This includes also the electricity consumption of your office building, your server infrastructure necessary to drive your SDLC, and other related support processes.
    It should not include electricity that is necessary to drive your production related (e.g. SAAS) or distribution related IT infrastructure (e.g. download servers). If this cannot be separated simply by measurements or electricity bills, you have to apply an allocation method that allows a separation.
  • Acquire CO2e coefficient data for your electricity mix according to your electricity supply contract.
  • Acquire CO2e coefficient data for your heating fuel according to your supply contract and your type of boilers.

Calculate the Functional Unit for the SDLC

  • Calculate the CO2e emissions of all software development activities: electricity and heating energy for used floor space in office buildings, electricity for the required server infrastructure (as long it is not already included in the electricity consumption of the floor space).
  • Transform the total electricity values to CO2e according to your electricity mix and the heating energy according to your heating fuel data.
  • Calculate the CO2e/PM by dividing the total CO2e per year by the total number of person months that can be “invoiced to customers” (the person months of common corporate departments are not included on their own).
  • It is also possible to apply a more specific allocation method to distribute the CF of common corporate departments between different development projects.

Calculate the CF of the Development Phase of the LCSP

  • Calculate the CF of the development phase (pre-maintenance phases of the SDLC) by multiplying the functional unit CO2e/PM with the expected (forecast) or real number of person months.
  • Calculate the CF for the maintenance phase by multiplying the functional unit CO2e/PM with the expected number of person months for maintenance over the expected product life time.

Calculate the CF of the Distribution Phase of the LCSP

Calculate a forecast according to expected sales figures for:
  • the CF in CO2e of the material product parts (e.g. data medium, manuals, packaging, average transportation). The CF of the material product parts includes also the disposal phase of the product.
  • the CF of the operation of the download servers and required IT infrastructure (electricity measured in your facilities, CF of data transport, CF of download client usage)

Calculate the CF of the Usage Phase of the LCSP

The usage phase of a software product depends on whether the software product is driven in-house (e.g. SAAS) or whether the software product is directly operated by your customers. Base your approximation on your expected sales objectives.
In-house:
  • Approximate the emissions of your servers per year (possibly based on historic figures). Distribute the emissions with a suitable allocation method between the software products operated in your premises. Consider the proportional emissions of your product.
  • Approximate the emissions of the users clients using the specific software product and the necessary network transmission while using it.
Ex-house:
  • Approximate the total emissions of servers on the site of your customers. Assume an appropriate mix of minimal, average, and maximum hardware configurations.
  • Approximate the emissions of client computers the software product is installed on the site of your customers while using the specific software product.

Calculate the Total CF of your Software Product

Sum up the CF of the different phases of the LCSP to calculate the full CF of the whole software product.

The total CF should be reported in such a manner that it is possible to distinguish clearly between real and approximated/assumed figures.


  1. Kern, Eva; Dick, Markus; Jakob, Drangmeister; Hiller, Tim; Naumann, Stefan; Guldner, Achim: Integrating Aspects of Carbon Footprints and Continuous Energy Efficiency Measurements into Green and Sustainable Software Engineering. In: Page, Bernd; Fleischer, Andreas; Göbel, Johannes; Wohlgemuth, Volker (eds.): EnviroInfo 2013 - Environmental Informatics and Renewable Energies. 27th International Conference on Informatics for Environmental Protection. Proceedings of the 27th EnviroInfo 2013 Conference, Hamburg, Germany, September 2-4, 2013. Aachen: 2013, pp. 300-308.
  2. Wright, Laurence; Kemp, Simon; Williams, Ian: ‘Carbon footprinting’: towards a universally accepted definition. In: Carbon Management, February 2011, Vol. 2, No. 1, Pages 61-72. doi:10.4155/cmt.10.39
  3. PAS 2050:2011: Specification for the assessment of the life cycle greenhouse gas emissions of goods and services. http://shop.bsigroup.com/upload/Shop/Download/PAS/PAS2050.pdf

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