Is the way you think about emissions from purchased electricity wrong?

February 26, 2013, by Michael Gillenwater

Electrons flowing down a wire. How many times have you heard this description in discussions on how electric power grids functions? Our greenhouse gas (GHG) accounting framework for indirect emissions from purchased electricity (i.e., Scope 2) is built around this mental model — the idea that electrons in the electric grid are analogous to water or natural gas in a pipe where we just replace molecules with electrons. Indeed, it is hard to find a reference in our field on the topic of indirect emissions that does not lean on or allude to this description of the physics of electricity.

Well, in the spirit of earlier semantic alerts I’ve posted on misused terminology in GHG accounting (see previous entries on “real” and “counterfactual”), in this blog post I’m going to come after another esoteric GHG-related linguistic bubble with a pointy stick.

First, let’s start with the basics, electricity generators that supply energy to a grid do not produce electrons that flow down wires where they eventually reach your house, building, or factory. Physical electrons exist in these wires. And these electrons (e.g., held in atoms of copper or aluminum) do move through the wire, but at a speed of roughly 0.03 cm/sec (or about 1 meter per hour, a snail’s pace …though only if you’re dealing with a particularly slow snail). In other words, there are moving electrons at play here, but that’s not the main story. It’s the electric field that propagates at the speed of light through the wire, and that does the work (in the thermodynamics sense of the word). The result: when you throw a switch it takes 1/1000 of a second for the electrons 300 km away to start moving at 0.03 cm/sec.

But, components of an electric field are not individually identifiable like electrons are (and according to Heisenberg, it’s not clear how easy it is to identify individual electrons, but I probably just got myself in heap of trouble with the physicists out there; quantum theory and engineers rarely mix). There is just more or less total field in any given place. Everything physically connected to the wire is contributing to the field. The consequence of this fact is that it is impossible to supply electricity from one generator to you and from another generator to your neighbor. All generators and grid operators are able to do is toss more or less energy from each generation source onto the grid and let everyone collectively tap into it. (It even gets a little more complicated because where you toss it in matters as well as other factors.)

This little physics of electromagnetism lesson is relevant to GHG management because the oversimplified and mistaken electrons-in-wires-as-water-in-a-pipe model has led us to an inappropriate framing of a key question. The question concerns how we should allocate indirect emissions from electricity consumption to end users. By approaching the problem with a mental model of electrons flowing down a wire, we have established what I would argue has grown into a full-fledged red herring.

Although the first law of thermodynamics does hold (i.e., total energy is conserved), this is very different from applying a conservation of mass framing to this question. A conservation of mass model would view the system as a bunch of generators contributing to a pool of electricity which then mixes and flows down wires and is sucked out —as if with a straw— by individual consumption devices. But this analogy that every “kilogram” of electricity that enters the grid also leaves the grid is the wrong way to think about it. “Electrical mass” is not conserved in this way. Instead energy enters and leaves the system through many different routes in the form of both work and heat (e.g., line losses) and is affected by many different variables (temperature, material properties, interference by other electromagnetic sources, distance between generators and consumers, etc.). So in the end, what goes into the lines from generators is not exactly the same as what is drawn out to light our houses and office buildings.

At this point you might find yourself nodding, while at the same time thinking I’m missing the point. After all, science is often simplified for policymaking. And while today’s model may not meet the muster of an academic physicist, by many measures it is “close enough” of a system: line losses are small (~7-10%); it’s convenient to ignore other, often immaterial, factors; and not least of all, it’s conceptually useful to bypass the minutiae of electromagnetics to the more infographic-friendly model pervasive in GHG accounting today. The problem? Basing GHG accounting on an incomplete understanding of science leads to poor reasoning, which in turn leads to ill-fitting policies. Case and point is the very supposition that green power markets and programs have been built upon: that every MWh of electricity is fungible with any other regardless of when or where it was added to the grid. The elaboration of this concept being, that this pool of “electrons” can just be added up over a year and their “ownership” allocated to individual consumers, ignoring the realities of space and time and the other variables I listed above. I get to claim the wind electrons and you get stuck with the coal electrons.

The point here is admittedly technical and esoteric (if you read the early posts in my series on misconceptions in GHG accounting, then the erudite nature of my conclusion should come as no surprise to you). But, as I have said before, definitions matter and so do concepts. Call me a purist, but I firmly believe it’s critically important to ground our reasoning when we create GHG accounting frameworks on a sound technical understanding of the underlying mechanics leading to emissions. As GHG professionals, it is important that we establish a rigorous tradition of technical credibility.

Working on implementing climate programs, we face enough adversity as it is. The last thing we need is scientists laughing at us for getting the basic physics wrong. If we want to compromise by assuming something slightly at odds with a purist interpretation of the science, then we should do so explicitly and with transparency regarding the implications of our simplifying assumptions.

So, the next time you hear a GHG accountant describe the “flow of electrons” to their facility, politely intervene and correct them. You may risk sounding a bit pretentious, but you will have taken a baby step to elevate the scientific reputation of our profession in the process.

Note: If you are interested a quick reference on the topic, see the following:



9 Responses to “Is the way you think about emissions from purchased electricity wrong?”

  1. Tom says:

    I completely understand the discussion (I have an engineering degree and have done GHG accounting projects). However, I don’t think the description of how electrical grids work supports that conclusion.

    The grid is a shared distribution system, but shared distribution does not preclude segmented ownership. The fact of line losses doesn’t change this and doesn’t really hurt the analogy to mass. Indeed, if I’m shipping physical product there are often losses during distribution due to spoilage, damage or theft.

    Let’s say I produce widgets which are completely indistinguishable from my competitor’s. However, I produce mine with sustainably sourced inputs, while the competitor uses polluting sources and child labor. We pack our widgets in unlabeled boxes for distribution. Neither I nor my competitor have our own trucks and distribution centers. When 10% of the widgets are lost during transit the shipping company credits us in proportion to how much we shipped.

    Our customers order directly from us and we only sell what we product, net of losses, even if the final shipping to the customer is from a third party. So when my customers buy from me, they know they’ve caused a widget to be made using my standards. There is no doubting this (unless I commit fraud). Even if my customer receives a widget made from my competitor, it is I and my system that benefits. (From a sales and marketing perspective this would be a problem, because most consumers want the psychological benefits that come from the “purity” of their purchase.)

    Anyway, it’s fine to explain to people that flowing electrons aren’t doing the work and that all the generation is intermingled, but I don’t see how it matters to the electricity consumer.

    A much better “food for thought” article would be to consider grid average vs. marginal electricity consumption! (Hint: electric vehicle owners DO NOT want to understand this topic.)

  2. Rod Sobin says:

    Further, most (but not all) electric grid transmission and almost all distribution is alternating current so if one wants to use the image of electrons flowing, they should be moving back and forth (60 times a second in USA and Canada). Plus there are all those transformers that get in the way.

    The mental model of electricity being like water–hydraulic analogies–goes back a ways to when capacitors were condensers and vacuum tubes were valves.

  3. John Kazer says:

    An interesting analogy might be found in fair trade chocolate. If 10% of the chocolate bars I make are fair trade, you might think that all the chocolate in those bars is actual, physical, fairly traded chocolate.
    But you’d normally be wrong – because most companies also make regular chocolate and all their beans tend to get mixed together. So long as I buy 10% total during the year, the claim sticks – as per Tom’s note, by purchasing fair trade chocolate you guarantee that you’ve caused fair trade production, just not necessarily eaten the result!

  4. Alex says:

    Hi Michael

    Isn’t that the reason why there are scope 3 emission factors for T&D losses?

    Although it is my understanding that most mandatory GHG reporting schemes only include scope 1 and 2 emissions, it seems as if this can be accounted for.

    In this context, the question I would like to ask then is who would/should account for it, i.e., who should be held responsible?

    Should it be the end consumer who is ultimately responsible (i.e., scope 2 & 3) for the emissions because of their electricity demand? But then what about the operators of T&D networks? Ultimately, they can influence the level of T&D losses through technology/infrastructure used. Or should it be the generators because they are burning fossil fuels and because of the T&D losses need to generate more than there is demand?

    From a policy maker perspective it would seem that accounting for both scope 1 & 2 or scope 2 & 3 would provide a more accurate (and necessary approach to accounting for electricity emissions) result then just relying on scope 2 emissions from users – even though it is the end consumer who drives power generation.

  5. Michael Gillenwater says:

    My point is that the physics at play in this case are not most accurately represented by a conservation of mass-type models. If we want to assume that anyway, then we just need to be honest and clear that we are making that assumptions and make sure we understand the implications of the simplification. I think it is first necessary to recognize the problem then we can decide how to address it. My point was to get people to think more critically about this issue before jumping to conclusions.

    To your larger point about whether and how we can also apply the model from sales of products to electricity is another question. Even in restructured electricity markets.

  6. Michael Gillenwater says:

    Great point on AC current, which just reinforces the point. I chose not to make this post even more technical, getting into AC vs DC and frequency, etc. etc. Although it was a toss up.

  7. Michael Gillenwater says:


    Well guarantee is not the same as cause, if that is your objective. It is possible that 10% of the coffee the company produces meets the fair trade standard simply by default (due to other factors). And that this would not change regardless of how many people paid a premium for it. In such a case then only if there is a willingness of customers to pay premium for more than 10% of sales would you start to have an effect on production of fair trade (although even this might not have an effect if the response was instead to just raise the premium price rather than increase production). This is the situation we have in most jurisdictions with green power, which typically receives subsidies already from one or more government programs.

    Further, we have the complicating issue of unbundling in a more extreme way because, at least with RECs, the “attribute” is traded completely separately from the energy service delivery.

    But, this blog post was not intended to get into all the complexities of scope 2 accounting.

  8. Michael Gillenwater says:


    I approach these questions with a regulator’s mindset. It is what we call point of regulation. And I focus on two factors: 1) ability to control/manage the emissions (or causing emissions); and 2) our ability to monitor and verify the activity and/or emissions for compliance purposes. You might think that end-users are the ones to regulate for scope 2 emissions, but the problem is that there are millions of end users. The complexity of monitoring and enforcing rules on so many actors is tough. This is why regulators tend to focus on the few big actors in the chain of causation, because it is more tenable from a implementation perspective.

    And with respect to accuracy, the main issue is what is a fair accounting and what is less likely to be gamed by participants in the system. It does not need to be perfect.

  9. Andrea Smith says:

    hi – I read papers by you and an article by you in environmental finance magazine some years ago. I thought you were in favour of allocating emissions via tracking mechanisms. after reading this, I wonder if you have changed your mind? what you have written rings true in terms of the science, but what kind of working model can we use to marry the physical phenomenon of electricity with financial transaction of purchasing grid electricity other than the one we have, admittedly flawed?

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