Our 2023 Carbon Footprint
A few calculations led to surprising conclusions about how to reduce our household’s impact.
The holiday break is a great time to catch up with friends, try out some new dishes on my fabulous induction stove, see a movie or two, and ponder some issues that I didn’t get enough time to think about during the year. Many years ago, I spent part of the holiday break measuring the electricity usage of all the appliances in my house, which I reported in our first blog post of 2014.
This year, I did some calculations about our household’s GHG footprint. I got to thinking about the subject while writing a paper on “energy hogs”, which I discussed in two blog posts in August. After analyzing our utility bills, odometer readings, and air travel, what I found made me rethink the best steps to reduce our contributions to climate change. (I considered using one of the many online carbon calculators, but they are so opaque and embed so many simplifying assumptions that I decided to do some calculations myself.)
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Energy use at home
Our household consists of just my wife and myself. The dog and cat don’t use much hot water, have expressed no opinion on the thermostat setting, and seldom drive anywhere, so we aren’t going to count them.
Our electricity usage in 2023 was 6314 kWh. Using the standard California marginal emissions rate of 0.0004 tonnes (metric tons) of CO2 per kWh, that translates to 2.5 tonnes. That likely overstates the emissions, both because California’s marginal rate is falling as more renewables come on the grid and because we try to focus our consumption on the hours when renewables are plentiful. Though, like all of these calculations, it does not account for upstream CO2 and methane emissions in upstream production of fossil fuels.
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Of the 6314 kWh, my best estimate is about 1100 kWh goes into our hot tub. I get the stink-eye from some folks for having a hot tub, though I have never been clear on why that particular consumption draws such ire. These calculations showed that the annual CO2 due to our hot tub is about the same as a one-way flight across the country (and I enjoy it a whole lot more).
The only thing we use natural gas for in our house is the furnace and the hot water heater. Our summer usage suggests that the furnace is likely over 85% of it. (The hot tub has its own electric heater.) Together, those consumed 677 therms of gas in 2023, which produced about 3.8 tonnes of CO2, though methane leaks in the distribution system may make its full greenhouse gas impact far larger. Still, even that is 50% more than all the electricity we used to run the clothes dryer, air conditioner (which actually only turns on 3-4 times a year), hot tub, fabulous induction stove, lights, computers, coffee maker, and everything else. Given that state and (current) federal agencies are all over the energy efficiency standards for these appliances, it makes me think that the biggest improvement may be beefing up insulation in our 1992-built house. Still, that is one of the most costly and challenging upgrades and the efficiency audit we got a decade ago suggested there isn’t much low hanging fruit there.
On the road
Compared to the average household, we don’t drive much. Between the two of us, and including mileage we put on rental cars and rideshares in 2023, we drove about 10,600 miles in total. We have hybrids that average around 40 MPG, so that is about 265 gallons of gasoline, which means a total CO2 emissions around 2.4 tonnes, roughly the same as our electricity.
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My wife’s car is a 2019, but I drive a 2008 Prius and have thought about whether I should replace it with an EV. Here’s the calculation that convinced me not to: I put about 4000 miles per year on my Prius, at about 40 MPG, so it burns around 100 gallons per year, which emits around 0.9 tonnes of CO2. Doing those same miles with an EV would take about 1000 kWh and emit about 0.4 tonnes. Even if you put a societal cost of $200/tonne on those CO2 emissions – the high end of current estimates – the 0.5 tonne annual savings would be worth $100 per year. It’s hard to see how society’s resources are well spent producing a new EV at a cost of $20,000+ in order to save society $100 per year.
Emissions at 30,000 feet
While we drive a lot less than the average US household, we fly a lot more, though I am pretty sure we are still below average among my academic and professional colleagues. Dividing total US passenger-miles (domestic plus international) by US population works out to about 3000 miles per person per year, or slightly less than one round trip between San Francisco and Houston.
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We had by far our heaviest travel year since before the pandemic, flying together to Europe, Michigan and Albuquerque, plus my wife took a trip with friends to Mexico City and I had work trips to Boston, Washington DC, and Calgary. In total, it added up to about 50,000 miles of air travel – all purchased before I had written the section of the energy hogs paper on air travel. Using the CO2 intensity numbers from that paper, our air travel amounts to about 8.3 tonnes of GHG, nearly as much as our entire emissions from home energy use and driving. (And it is probably worse than that, because the full impact of emissions from air travel should account for the additional radiative forcing due to high altitude non-CO2 emissions, such as nitrogen oxides, which may increase the climate impact by 90% or more.)
Our 2023 seems like a lot of air travel, and we are very unlikely to match it in the coming years. Still, even our 2023 works out to barely enough to qualify for the lowest frequent flyer status level on most airlines, and only if we were to do all of that travel on a single carrier.
And, unfortunately, low carbon air travel at scale is probably many years off.
What about everything else we consume?
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Of course, these are just our most direct energy consuming – and greenhouse gas emitting – activities. Overall, they amounted to less than 15% of our total expenditures in 2023. All the other goods and services we bought also created GHGs. Based on some quick calculations of the GHG intensity of everything in the US economy besides residential electricity/natural gas and personal vehicle and air travel, it looks like the emissions from our other expenditures may exceed the GHG emissions of our home and travel energy use. More on that in a future blog post.
What can we do to reduce?
Here’s what I take away from these numbers. California has a pretty darn clean grid, and most of our electricity use is for energy services that we value quite highly, so significant CO2 savings will be tough there.
When it comes to natural gas, we already keep the house at 68° during winter days, turn the heat off at night, avoid long showers, and have wrapped our hot water heater in insulation, so cutting here will also be a challenge. In the next few years, we will probably jump on the heat pump wagon, though the skyrocketing California electricity rates make that a tough choice.
In contrast, our air travel is mostly discretionary and largely recreational consumption. And it is, to me, a shockingly high share of our carbon footprint. Seeing family and friends will still be high priority, but I’m going to think much more carefully about tourist travel and about work trips that aren’t essential, especially when virtual participation is feasible.
We are not a “typical” household
This is our carbon footprint. Your emissions may vary, as they say. In fact, the vast majority of low- and middle-income households in the world don’t have “carbon fat” like discretionary air travel to cut. Reductions from their already-low emissions levels come straight from the meat of their standards of living. It seems only fair for households like ours to move first.
I resolve in 2024 to do less air travel and become more educated about all the ways in which my activities contribute to climate change. But I will not delude myself into thinking that we can save the planet by soliciting voluntary emissions reductions. So I will spend much of my time arguing for better regulation and financial incentives that bend the choices of all consumers towards a sustainable future.
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I am posting frequently these days on Bluesky @severinborenstein
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Suggested citation: Borenstein, Severin. “Our 2023 Carbon Footprint” Energy Institute Blog, UC Berkeley, January 8, 2024, https://energyathaas.wordpress.com/2024/01/08/our-2023-carbon-footprint/
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Severin Borenstein is Professor of the Graduate School in the Economic Analysis and Policy Group at the Haas School of Business and Faculty Director of the Energy Institute at Haas. He received his A.B. from U.C. Berkeley and Ph.D. in Economics from M.I.T. His research focuses on the economics of renewable energy, economic policies for reducing greenhouse gases, and alternative models of retail electricity pricing. Borenstein is also a research associate of the National Bureau of Economic Research in Cambridge, MA. He served on the Board of Governors of the California Power Exchange from 1997 to 2003. During 1999-2000, he was a member of the California Attorney General's Gasoline Price Task Force. In 2012-13, he served on the Emissions Market Assessment Committee, which advised the California Air Resources Board on the operation of California’s Cap and Trade market for greenhouse gases. In 2014, he was appointed to the California Energy Commission’s Petroleum Market Advisory Committee, which he chaired from 2015 until the Committee was dissolved in 2017. From 2015-2020, he served on the Advisory Council of the Bay Area Air Quality Management District. Since 2019, he has been a member of the Governing Board of the California Independent System Operator.
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Good for you! Thank you for doing your own math and sharing the insights.
Thank you for this. “You can’t manage what you don’t measure”, so it’s important for all of us to understand our own unique carbon footprints and what we can do to reduce them. One point you didn’t stress is the big impact of zero-carbon electricity. Several of our biggest loads — most notably car charging & water heating — can usually be completely shifted to times of 100% clean electricity. Zeroing out any of those big emission sources is huge.
Mr. Borenstein, here is another suggestion for how you and your colleagues at the Haas Institute can reduce your carbon footprint:
Support common-sense and effective climate policies.
In particular, with respect to air travel:
“A carbon price applied to aviation fuel at current EU ETS trading prices would raise air ticket prices, e.g. for a New York to London flight, by a paltry $10. Such a meager price incentive would not induce emission reductions in aviation consistent with climate stabilization objectives. However, if the carbon price were applied to global airline fuel consumption it would generate an annual revenue stream of over $23 billion, sufficient to fund a technology and commercial development program for sustainable aviation fuel (SAF) on the scale of the Manhattan Project. …”
(That is quoted from a paper I wrote in 2000; the numbers might need to be updated.)
papers.ssrn.com/sol3/papers.cfm?abstract_id=3470249
Severin
I recommend that you read Vaclav Smil’s new book “How the World Really Works as you prepare your future blog post on this subject. You may find that your estimated of GHG for your household to be considerably larger than your present calculations have revealed.
Gary Matteson
It is really suprising that an economist would get the definition of “marginal” so wrong. It is certainly true that the average carbon content of the California electricity is dropping, but at the margin it is not. That is, virtually all of the renewable electricity avialable is being utilizied is directly or via storage. So any time the total demand goes up the marginal generation is through fossil fuel, somewhere at sometime.
So that means if Severin replaces his gas furnace with a heat pump, he is actually increasing the average carbon content of the grid; that comes pretty close to a carbon wash with the local gas savings. Yet, it costs someone a lot of money to do that. To reduce carbon content it is better to invest it in on-grid PV in the desert or energy efficieincy improvements.
Your first paragraph correctly states the concept of marginal, and then applies it incorrectly empirically. It is widely reported that renewables are being curtailed at an increasing rate in California. (https://www.eia.gov/todayinenergy/detail.php?id=60822) Curtailment of renewables translate directly to the nobles being on the margin.
Your second paragraph then confuses marginal and average in a policy analysis. What matters with my installation of a heat pump is whether the marginal electricity it uses is cleaner than burning natural gas, not whether it increases the average carbon content. Building additional grid-scale PV is necessary, but at some point we need to use all of that electricity to phase out burning natural gas.
If curtailment of renewables is is a significant issue, it is not solved by fuel switching existing appliances unless they are only used during curtailment periods. I don’t imagine you cook, heat your home or water only when there is too much solar or wind generation. Rather better renewable utilization is fostered by storage and grid-level managment–not by individuals using heat pumps…especially for home heating uses which occur when solar is quite low. It would, however, suggest we should have incentives to get rid of any remaining gas air conditioning.
Renewable curtailment does complicate the marginal analysis, but not by much given the above. I would agree any curtailment is problematic and should be addressed as a priority–one that is more important than switching out gas appliances in existing homes.
Renewable curtailment is a truly tiny problem. Most excess renewable generation is exported from California, and even to in-state munis. It’s about 1% of the CAISO’s total generation. Further, a quite plausible low cost portfolio strategy is to over build renewables and curtail excess. Remember the actual “fuel” is free!
That contrasts with building gas use. It’s 10% or more of total state GHG emissions. It also causes significant indoor pollution. We need to encourage transitioning away from gas ASAP.
The question as to whether a furnace or heat pump generates fewer emissions currently is NOT the useful question, it seems to me. Rather, it is whether this happens over the life of the new furnace or heat pump appliance (I’m assuming, as does the Rewiring America folks in their calculations, that most will be replaced at end-of-life.) The goal is to continue to decrease carbon emissions from electricity generation. Assuming this will happen, it is a no brainer that when you replace your HVAC system you do so with a heat pump. Same with water heater.
However, on second thought, prices of NG and electricity are not based on the amount of energy available so who knows where you break even when fuel switching. The relative efficiencies of the two technologies stand though.
Maybe someday hot water heater companies will build a hybrid hot water heater with both natural gas and heat exchanger systems that could be controlled by the price of the commodity at that moment. We have time of day metering and electricity from 4:00 PM to 9:00 PM is outrageously high and that would be a great time to switch over to natural gas. They already have the Hybrid for an additional electric resistance element when the tank temperature gets too low so why not have that with natural gas as well, but they do not have any external control for pricing. Maybe because the utilities own the majority voting shares of the furnace and hot water heater companies and control the board of directors?
Newer Heat Pump Water Heaters will over-heat the water at the best time and use it as a thermal battery. The water is then mixed with cool water to lower the temperature to sanitary use values. This is common in some markets and it is now recommended in California.
That is not the only question, of course. If replacing gas appliances with electric ones strains the grid today, it is not sufficient to know that someday it won’t.
It is true that we need to look at the net present value of how we invest public resources, but that will mean that there are better things to invest in now (e.g. efficiency and renewables) and the replacing of gas appliciances would come later.
re: “In the next few years, we will probably jump on the heat pump wagon, though the skyrocketing California electricity rates make that a tough choice.”
Remember that heat pumps are 300-400% efficient while furnaces can only approach 100%. (https://www.technologyreview.com/2023/02/14/1068582/everything-you-need-to-know-about-heat-pumps/). So could electricity prices be three to four times NG prices before it actually gets more expensive to heat your house?
It depends on how you define efficiency. Heat pump salesmen will use such high numbers, but they don’t take into account distribution (and generation) losses. One has to include seasonal effects, site vs. source effects, equipment lifetime, performance differences, etc.
If you generate more electricity than you use, by all means consider a heat pump. Otherwise, just go through net present value calculations with expected utility rates and get the one that is most cost-effective. Don’t be fooledy by efficiency numbers.
Severin, agreed. The fundamental definition of marginal costs and emissions is that it is a change in total costs and emissions. Mathematically, if total costs or emissions are rising, then marginal costs or emissions must be above the average, and conversely if the total is falling then the marginal must be below the average. I illustrate this fundamental relationship in this blog post: https://mcubedecon.com/2022/12/20/the-fundamental-truth-of-marginal-and-average-costs/
Richard, your definition of the relationship between marginal cost and average cost is correct. But the application in your blog post to transmission is not. It assumes that the increase in average cost has a *causal* relationship to quantity. If marginal cost per unit is unchanged, but there is a huge increase in fixed costs, then average cost will rise at a time, but that does not imply that marginal cost is above average cost. That is what is happening in transmission and distribution systems now as we face drastically rising fixed costs of wildfires and grid hardening associated with climate change.
“Fixed costs” cannot “rise”–they are fixed. The capital return on existing investment is a fixed cost–the cost to replace that investment is not a fixed cost. Any incremental cost to the transmission system is a marginal cost, even if it is caused by a multitude of factors. Wildfire mitigation costs are not “fixed”–they are incremental costs incurred by existing customers (and new development on the WUI.) Grid hardening also is a marginal cost to existing customers created by changing conditions. This is no different than increasing fuel use at generators as performance degrades over time.
Unfortunately this discussion illustrates a lack of a rigorous discussion of what constitutes marginal costs in the electricity system. Rising total costs must be completely described mathematically. Heuristics won’t cut it.
Unfortunately, Richard’s comment completely misunderstands the standard economic definitions of fixed and variable costs. ANY textbook makes clear that fixed costs are fixed with respect to the quantity of output of the firm. Therefore, fixed costs most certainly can rise. If, for instance, in order to continue doing the exact same thing a company was doing before, the firm must pay an extra million dollars, that is a rise in fixed cost. That is exactly what has been happening with transmission and distribution as climate change has increased the risk of wildfires and required more grid hardening, vegetation management, and/or under grounding. Those costs are not going up because people are consuming more. They are an increase in fixed costs. Likewise, costs for public policies, such as energy efficiency programs, EV charging stations, subsidies for rooftop solar, and subsidies for low income households all add fixed costs that are being recovered through volumetric pricing. These costs do not increase when I consume additional electricity at my house. It is completely different from the fuel used at a generator. The basic understanding of efficient pricing depends on understanding this difference.
Furthermore, as the work Meredith Fowlie, Jim Sallee and I have done shows, by collecting the revenues to cover these costs through the volumetric price of electricity, we are disproportionately burdening low income households with these costs.
PG&E boasts a 91.5% carbon free electrical output in in Northern California not including rooftop solar that is just separate sourced energy. Where a Gas Hot water Heater is 100% natural gas, a heat exchanger hot water heater is just8.5% natural gas in its energy mix. Not all utilities can boast such a high non-fossil fuel output, there is some satisfaction for those choosing heat exchanger hot water or even heating of their home to being greener than the those up the street that still uses fossil fuels. Being ready for the future fusion reactor generated energy will favor the bold that switch now.
After my home burned in a fire, PG&E cut both my gas and electric lines so my engineer designed both heat exchanger hot water and heating and natural gas into the re-build of my home and now it is up to PG&E to give me back the two energy sources I once had or do I go completely OFF-Grid with all electric Solar and Batteries. I had both on grid solar and a fully functioning off-grid solar and battery system before the fire, so I know Off-Grid works and so does heat exchanger and induction cooking works.
Severin,
Another excellent blog. Your final two sentences hit the nail on the head: “But I will not delude myself into thinking that we can save the planet by soliciting voluntary emissions reductions. So I will spend much of my time arguing for better regulation and financial incentives that bend the choices of all consumers towards a sustainable future.”
You should not focus on reducing your carbon footprint; you should focus, as you do, on creating and advocating for policies that will reduce the emissions of others. You should not hesitate to fly to Washington or Brussels to discuss energy policy with stakeholders and decision-makers. If your back aches from the long flight, jump into the hot tub. If owning an EV helps you think about policies encouraging price-responsive demand, buy one. Your contributions to energy policy are essential.
Cramton,
Regulation and sustainable flying are being discussed here-
https://www.thebignewsletter.com/p/its-time-to-nationalize-and-then/comments
I was unaware that a PhD in “sustainable aviation engineering” is available.
Reading Auffhammer’s blog from 2021, I’m not sure why he expected improved comfort from the insulation that he put in (which buy the way is probably saving him $2/day given the large gas and electric rate increases since then, and those are going to grow.)? He’s maintaining the same temperature at a lower cost. Isn’t that a level set in econ speak? His payback has probably shrunk to close to a half dozen years now–that’s pretty good.
Meanwhile, China, India & Indonesia burn 8.5 billion tons of coal per year.
The combined populations of China, India and Indonesia are 10 times that of the USA when combined. The fossil fuel per capita is however less than that of the USA. Our wealth allows us to build solar and wind power plus purchase electric cars even though China builds and sells more Solar and Electric cars than we do here in the USA. We have yet to sign the “Paris Climate Accord” and now we are curtailing Rooftop Solar with NEM3.0 in California. Our obsession with “RED MEAT” gives us the highest production of livestock methane production of any country in the world and who does not own at least one gas guzzling internal combustion car? Just installing one LED or CFL light bulb replacing an incandescent lamp is doing something positive and the rest of the world is way ahead of us on that account. I am not saying you are wrong because your points are valid and need to be said but America is not doing this alone. This is a global effort and all of us on the planet need to do more.
Severin, I appreciate your rational assessment of energy-related issues overall but have a single clarifying question regarding the GHG emissions of air travel. That is, whether the emissions calculations you use represent the average emissions per passenger mile or the incremental emissions from adding a passenger, with luggage, to an existing flight? Unless there is a significant incremental emissions impact of adding 100 kg or so to an already partially full airplane, wouldn’t your flying result in a lower emissions level per passenger overall and your not flying wold only be beneficial if it resulted in the flight being canceled?
Please see the section on airline travel emissions in the Energy Hogs paper (p. 20-21), https://haas.berkeley.edu/wp-content/uploads/WP341.pdf
Focusing solely on the small increment of fuel used as a true measure of “marginal” reflects a larger problem that is distorting economic analysis. No one looks at the marginal cost of petroleum production as the energy cost of pumping one more barrel from an existing well. It’s viewed as the cost of sinking another well in a high cost region, e.g., Kern County or the North Sea. The same needs to be true of air travel and of electricity generation. Adding one more unit isn’t just another inframarginal energy cost–it’s an implied aggregation of many incremental decisions that lead to addition of another unit of capacity. Too often economics is caught up in belief that its like classical physics and the rules of calculus prevail.
Severin, does this analysis suggest we should emphasize getting high speed rail finally built in CA in order to create an alternative to flying, at least regionally?
It suggests that the carbon emission benefits could be significant, but that doesn’t speak to the costs, which are also significant. I don’t think I know enough to say how that nets out.