Carbon Footprint vs. Carbon Intensity: What Should You Track?

We talk all the time about climate change and how it’s affecting the Earth. And while most of us have a vague sense of what that means, let’s dive a little deeper into how this works with energy and how you can track your emissions. 

First, here’s a quick refresher on what we mean by global warming. Greenhouse gases, such as carbon dioxide and methane, cause the Earth’s overall temperature to increase. Unfortunately, human activity causes these gases to be released into the atmosphere. To counteract our greenhouse gas generating behavior, it’s important that we use our resources more wisely and find ways to reduce emissions. But how do we figure out the easiest way to reduce the most emissions? 

How to track electricity emissions: Carbon Footprint vs Carbon Intensity

Many articles have discussed that tracking your carbon footprint can be a good way to reduce carbon emissions. But, what do we mean by “carbon footprint?” And should we use it to track emissions at home?

A person's or household's carbon footprint is the total amount of carbon emissions they cause, directly or indirectly, through all of their actions. But each separate action has a different carbon cost, so it can be helpful to consider the amount of carbon emissions per action: that's the carbon intensity. The difference between carbon footprint and carbon intensity is like the difference between the amount of gasoline we use each week and the fuel efficiency of our car. For many activities it can be hard to know the carbon intensity, but luckily for electrical power usage, it's a little bit easier.

So, what’s carbon intensity? The carbon intensity of electrical energy is a measure of how much carbon dioxide is emitted into the atmosphere to produce an amount of electrical energy. It changes over time, as different sources of electricity — like wind, solar, natural gas, coal — produce more or less energy. And it can come in two different flavors: total carbon intensity, and marginal carbon intensity.

To better explain this, let's jump into the science!

Total carbon intensity is the total amount of CO2 emissions from the electrical grid, divided by the total amount of electricity being produced. 

Total amount of CO2 emitted / total amount of electricity produced = carbon intensity

In our example, two kinds of power plants are emitting a combined 600 lbs of CO2 during one hour, while producing a combined 1000kW (or 1MW) of electrical power during that hour. Therefore, the total carbon intensity is 600 lbs CO2 per megawatt-hour. Our example house uses 2kW for one hour, so we are “responsible” for 2/1000 of those emissions, or 1.2 lbs of carbon dioxide emissions. This is the “carbon footprint” for our house’s electricity usage.

To help explain marginal carbon intensity, let’s say we cut our energy use in half during that hour (so instead of 2kW, we only use 1kW). How will the power-plant operators respond?

The wind farm will still produce as much as it can, because it doesn’t cost any more money to produce electricity from a wind turbine once it’s constructed. There is no fuel cost for wind energy. But because natural gas costs money, during that hour the natural gas power plant will reduce its electricity production by 1kW, and reduce its CO2 emissions by 1.2 lbs. The total carbon intensity of the grid is now 598.8 lbs CO2 per 999 kWh (or 599.4 lbs CO2 / MWh), and our carbon footprint is now 1/999th of those 598.8 lbs, or about 0.6 lbs for the hour.

Now wait a minute! Our action (reducing our energy output by half) prevented 1.2 lbs of CO2 emissions, but our carbon footprint only shrank by half that? And every other house on the grid is getting credit — through a slightly smaller carbon footprint — for our change in behavior? 

This is why we measure marginal carbon intensity. Marginal carbon intensity is the amount of CO2 emitted for each extra amount of energy used. 

Marginal Carbon Intensity (lbs of CO2 / MWH) = extra emissions generated (lbs CO2) / extra electrical energy used (MWH)

In this example, our decision to reduce our electricity consumption by 1 kW for one hour reduced the CO2 emissions of the grid by 1.2 lbs. The marginal carbon intensity of our example grid is 1.2 lbs / kW / h (or 1200 lbs / MWh). But, as you can see, the carbon footprint only decreased by 0.6 lbs for the hour. This is why it’s more accurate to measure carbon intensity, especially when it comes to seeing how your individual actions reduce carbon emissions due to energy production.

In short, the cheapest times of the day have the lowest carbon intensity. You can save money and reduce carbon emissions at the same time. Pretty cool, right?

Now, let’s talk about curtailment.

When the sun and wind are producing more electricity than people need or want, the extra energy is considered “curtailed,” or wasted. During periods of curtailment, everyone should crank up the AC (or heat), charge their devices, run the laundry and dishwasher, whatever they can, because otherwise the opportunity to generate carbon free power will be wasted; it’s not something we can store. When curtailment occurs, the marginal carbon intensity of electricity is zero. 

The tricky thing is that the marginal carbon intensity doesn’t really have a middle ground here: If any amount of the grid’s demand is supplied by natural gas generators, from 100% all the way down to 1%, then the marginal carbon intensity of the grid is 1200 lbs CO2 / MWh. But the moment that wind power and other carbon-free sources start providing 100% of the grid’s demand, marginal carbon intensity plummets all the way to zero. And then, when demand grows above carbon-free supply and the natural gas generators start up again, the marginal carbon intensity leaps right back up to 1200 lbs CO2 / MWh. It’s not exactly as simple as that, but it’s close:

What’s the easiest way I can have an impact on my personal emissions? 

One way to track the real-time marginal carbon intensity is through the Octopus Energy app. It can help you decide when the best time to use energy is, especially while running higher energy-using products like your laundry or dishwasher, or even when you should shift your energy to charge your electric vehicle. The best part about this is, switching to renewable energy doesn’t require a lot of noticeable action on your part. 

A household can certainly decide to use less electricity, but that’s a lot harder than, say, shifting their electricity use by an hour here or there. And the good news is that electricity is cleaner at different times of the day, so that by shifting usage from times when power is “dirty” to times when power is cleaner, one household can make a real impact. We now know that tracking your carbon footprint may not give you the most accurate measurement of any behavior changes you may make, because it distributes part of your carbon impact to everyone else (and distributes theirs to you). Use the Octopus Energy App to track real-time carbon intensity. In the future, we plan to add a forecasted carbon intensity meter to the app (for now, you can track this using our forecasted pricing graph, remember that it correlates with carbon intensity). 

Think of it like gamifying your energy use. Open the app before you start to do something that requires a lot of energy (think laundry, dishwasher, EV charging) and see if it’s a good time. If you’re in the red, it might be best to check back later. If you’re in the green, go ahead! You’ll use more renewable-generated energy, which can make a legitimate difference to overall carbon emissions.