Carbon Calculating: Estimating Your Home Energy Impact
The dozens of carbon calculators available as apps and on the web provide widely divergent estimates of your carbon footprint. Whether you choose to use one of the many calculators or want to assemble your own estimate — from finding and understanding your home energy usage to assessing a carbon calculator’s results — this article will break down how to track the impact of home electricity, natural gas, fuel oil, or propane use.
As mentioned in the first article in this series, carbon calculators use different formulas and estimated CO2 emissions. We entered the same data in five web-based carbon calculators and received strikingly different total emissions estimates, as well as radically different shares of the total carbon footprint attributed to home energy use. The differences raise significant concerns about the accuracy of each of the calculators.
Each calculator we tested has strengths and weaknesses. The reality is that you will probably need to use parts of several calculators to get the most accurate cumulative carbon footprint for your household. Before we explore which ones offer the most trustworthy home energy estimates, let’s walk through the information you need to have at hand.
If you are a DIYer, you will find the formulas for making your own calculations at the end of this article.
Collecting the Data
To get started, find your utility bills to help you determine your home energy use. Try to collect a full year’s worth of your electric bills as well as other energy bills, such as fossil fuel, natural gas, fuel oil, or propane. Carbon calculators ask for this information by month or as an annual total.
Electricity
Your electricity bill presents power consumption in kilowatt-hours, which represent the number off watts of energy needed to power a 1,000-watt microwave oven for an hour, for example. Look for “kWh” on your electricity bills, add up a year’s worth of bills to arrive at the annual total, then divide by 12 to get your monthly average electricity consumption. Whether the calculator you use asks for monthly or annual kilowatt-hours, you’re ready.
Most carbon calculators ask for electric data in kWh, but you may also find calculators that ask for your monthly or annual spending on electricity. Based on our experience, spending is a less reliable way to estimate electricity usage than exact usage based on kWh because local rates vary. Calculators that use spending must apply guesswork to estimate your usage.
In addition to your electricity usage, the source of your power makes a big difference to your environmental impact. If you get a significant percentage of renewable sources, such as hydro-electric, solar, or wind, the energy footprint is lower than someone who relies on fossil fuel generation.
Only two of the calculators we used, Doconomy and the EPA’s, asked about use of renewable energy. However, Doconomy asks only if you have renewable energy, not the percentage of electricity that is renewable, allocating CO2 emissions based on the assumption you either have access to all-renewable or no renewable energy. A simple yes/no question doesn’t deal with our realities. The EPA calculator, on the other hand, asks you to enter the specific percentage, which we recommend to ensure you get an accurate emissions report.
Local renewable percentages depend on your utility’s decisions about how to generate or where they buy energy. For example, where we live, in Western Washington, Pierce County enjoys 89% renewable energy while in Eastern Washington, Whitman County gets only 41% of its power from renewables. But statewide, the typical resident’s renewable energy percentage is 85% because most of the population lives on the western side of the state, where renewables dominate.
Many utilities will list the percentage of renewable energy you receive on your bill or in an annual customer report. If you don’t know your local renewable percentage, the U.S. Energy Information Administration provides state-level information about electric energy sourcing. Click on your state and choose the Electricity tab in the state report. You will have to do a bit of math to calculate your renewable percentage by mousing over the bar in the chart seen below to get the data about all your power sources.
Natural Gas
Methane gas, or “natural gas,” consumption is presented in several different units including therms (each therm equals 100,000 British thermal units, the equivalent of heat created by 29.3072 kWh of electricity), “ccf,” which represents hundreds of cubic feet of gas delivered per month, and “mcf,” which counts cubic feet by thousands.
You may need to convert ccf into mcf, or vice versa, to use a specific calculator — pay attention to the unit of measure requested because entering your gas usage in the wrong unit can throw your impact results off by a factor of ten. To convert ccf into mcf, divide by your total ccf by 10; going the other way, multiply your mcf by 10 to get total ccf.
Fuel Oil
Burning oil to generate heat in the home has been on the decline since the oil crisis of the 1970s. But several states still see significant use of oil, including the top five (in order): New York, Massachusetts, Pennsylvania, Maine, and Connecticut.
You will enter your fuel oil usage in gallons or by the dollar amount you spend — that data will be shown on your bill. Add up a year’s worth of bills and divide the total by 12 to get an average monthly fuel oil usage to use in your calculation. And because local rates vary, we recommend using the actual volume of oil and not the monthly spending for more accurate results.
Propane
This form of gas, which differs from methane chemically, is still in use in tens of millions of homes in the U.S. All the carbon calculators we tested that included propane as an option requested either a monthly or annual volume in gallons.
The Calculators
For this article, we focused on calculators offered on websites, but app-based calculators behave in similar ways. In the final article in this series, we will present a review of all the app and web calculators tested.
Environmental Protection Agency Carbon Calculator
Grades
Energy estimate: A-
Total footprint estimate: D-
The EPA Carbon Calculator asked for the most information, including the percentage of renewable energy in your home’s electricity supply, to estimate energy-related emissions. The renewable energy data we entered clearly reduced the total energy footprint estimate for our three-person household. When we compared the EPA’s energy results to our manual calculation, it was the closest estimate received, followed by the Wren calculator. Unfortunately, the EPA calculator does not cover all the categories of emissions necessary to provide a comprehensive footprint estimate.
Because it does not cover all the ways we generated emissions, offering estimates only for home heating and energy, driving, and the impact of recycling choices, the EPA calculator does not help individuals make many decisions that could reduce their environmental impact. It misses too many emissions.
CoolClimate Network
Grades
Energy estimate: C+
Total footprint estimate: B+
A project of the University of California, Apple, Meta, and the Nature Conservancy, the CoolClimate Household Impact Calculator provides the most detailed analysis of all aspects household carbon impacts in this review. While the home energy estimate includes entries for number of people in the household, percentage of renewable energy used, and volume-based entries for electricity and natural gas use, the estimated footprint for our home energy was clearly too low, representing only a third of the emissions actually generated.
The CoolClimate calculator does a great job by providing many detailed entries for air travel; number and type of vehicles driven; meat, dairy, and fish consumption; and spending on goods and services, for example. However, it also inserts arbitrary assumptions that cannot be adjusted for annual construction, furniture, and clothing spending that added 15% to our estimated total footprint. If those entries were customizable, the calculator would be even better.
Another shortcoming is a lack of a clearly defined methodology. CoolClimate points to a research paper that explains how models can predict carbon emissions but does not address how the estimate is generated. Note that this calculator asks for gas usage in cubic feet, not ccf or mcf. If your bill provides usage information in ccf, multiply by ten before you enter your data; if your bill displays mcf, multiply by 100.
TerraPass
Grades
Energy estimate: D-
Total footprint estimate: D+
In the first article in this series, we pointed out that calculators tied to the sale of carbon offsets typically make substantially higher estimates than calculators that do not offer offsets. TerraPass’ Individual Calculator results don’t make clear sense in two ways. Based on the same data we entered in the other calculators, TerraPass presents a total footprint that is almost twice the amount of the next highest estimate. Compounding our confusion, the TerraPass energy-related results account for 90.2% of the total emissions — home energy usage should reasonably be half or less of a total carbon footprint.
TerraPass does not account for use of renewable energy. And like the EPA calculator, TerraPass does not collect any food, shopping, or services data, which are essential to understanding your total footprint. On the other hand, the TerraPass estimates for driving and air travel are solid. They ask for specific information about what kind of cars you drive and the number of length of flights you take annually to deliver largely accurate estimate of travel impacts.
Doconomy
Grades
Energy estimate: D
Total footprint estimate: F
Doconomy and the United Nations partnered to create a European focused carbon calculator that covers a wide range of household impacts, including energy, driving, air travel, food, and shopping choices, as well as the emissions created by the services you use. However, every aspect of the tool produced surprisingly low emissions estimates. For example, Doconomy estimated that our annual home energy impact is only 3,840 lbs., and our food-related emissions only 540 lbs. That’s clearly incorrect and results in a very low estimate compared to the other calculators and our manual estimate of emissions. The low estimate may make you feel better about yourself, but you’ll be unprepared to make changes that lower your impact.
As noted in the discussion of electricity, Doconomy asks whether you buy renewable energy, treating your yes or no answer as representing 100% of the power used, according to its published methodology. Except for a home that relies only on its solar or wind generation capacity, Doconomy’s assumption about renewable energy is not applicable to anyone connected to the grid. One positive: Doconomy sells offsets but delivered the lowest estimated overall footprint among the offset vendors reviewed in this article. We’d like to see them refine and improve the formula and assumptions that drive their results.
Wren
Grades
Energy estimate: B-
Total footprint estimate: C
Another carbon offset seller, the Wren carbon calculator provides one of the most comprehensive assessment of a household’s carbon impact. In addition to home energy estimates, Wren calculates driving and air travel emissions, as well as food (including pet food), shopping, and services impacts. The site’s total annual estimate was the third highest, but far more conservative than TerraPass and Karma Wallet. However, Wren does not publish its methodology, so it is difficult to validate its estimates. Its home energy estimate is based in part on the ZIP code entered. In our case, adding the ZIP code lowered our estimated impact by 31.6%.
Karma Wallet
Grades
Energy estimate: F
Total footprint estimate: D+
The Karma Wallet calculator takes a completely different approach than the manual-entry calculators above. It analyzes your spending by tracking credit card and bank transactions. As we’ve noted about spending-based estimates, it is impossible to extrapolate with useful precision the amount of electricity, natural gas, or other units of energy used from the price paid without extensive context. Karma Wallet does not break out the individual categories of emissions, providing only a total emissions estimate. Our Karma Wallet results were 23.2% higher than the average reported by all the other calculators reviewed here.
Karma Wallet offers two way to take action. It suggests alternatives to your current spending, such as switching from one wireless carrier to another (with cashback offers), and provides offsets for purchase. The guidance about choosing more environmentally responsible companies to buy from would be useful if the actual impact of those choices was clearly presented, but the tool only compares a sustainability rating without explaining how a change reduces carbon emissions. This potentially useful tool requires the user to take too much on faith because there is no quantitative data about categories of your current carbon impact or how making changes will reduce emissions.
The DIY Way
Checking the work of third-party calculators can help you make confident decisions. But we know doing the math isn’t everyone’s cup of tea. If you’d like to do your own estimates, here are simple formulas for calculating the sources of each of the major types of home energy. For example, when calculating a carbon footprint for a home that uses electricity and natural gas, you’ll need to add the totals for all the energy sources to arrive your cumulative home energy footprint.
We’ve provided average emissions factors for each type of energy based on the EPA’s reporting. Keep in mind that local factors, such as the condition of your furnace and specific sources of gas, oil, or renewable energy affect your actual impact. Alas, estimates are not reality. We recommend asking your local utility for the emissions factors of their energy sources to improve your estimate.
Electricity, No Renewable Sources
Depending on what your utility burns to generate electricity, the emissions will vary. Here are formulas for three common fuels used to generate electricity in commercial plants.
The EPA reports the emissions factors for combustible fuels in millions of British Thermal Units (mmBtus). We included a step to convert the hours reported on your electric bill into the equivalent mmBtus, which involves multiplying the number of kWh by 0.0034095106405145. The EPA also provides emissions factors per mmBtus are provided in kilograms, so the final step, multiplying the result by 2.2046, converts the result in kilograms to imperial pounds. If the U.S. would embraced the metric system, this would be easier. When using a calculator, enter the values in BOLD below.
Mixed Coal
The energy industry typically uses mixed coal, which has an emissions factor of 95.52 kg per mmBtu, to power electricity generation plants. The formula to find your home’s energy emissions is:
CO2 emissions (in lbs.) = (YOUR TOTAL ANNUAL kWh) * (0.0034095106405145 MMBtu/kWh) * (95.52 kg CO2/MMBtu) * 2.20462
Natural Gas
Many utilities burn natural gas in peaker plants, which come online during periods of high demand. But some utilities rely on natural gas around the clock, accounting for 37% of all the natural gas burned in the U.S., about 11.27 trillion cubic feet in 2021, according to the EIA. If your utility sources most electricity from natural gas, here’s the calculation you need to perform:
CO2 emissions (in lbs.) = (YOUR TOTAL ANNUAL kWh) * (0.0034095106405145 MMBtu/kWh) * (53.06 kg CO2/MMBtu) * 2.20462
Biomass or Landfill Gas
Capturing and burning gasses produced in landfills and from processing organic waste, sometimes referred to as biogas or renewable gas, sounds pretty green. But burning fuels emit CO2, regardless of where they came from. These gasses accounted for about 0.2% of utility-scale electricity generation in 2021, according to the EIA. Here is the home energy carbon footprint formula for biogas:
CO2 emissions (in lbs.) = (YOUR TOTAL ANNUAL kWh) * (0.0034095106405145 MMBtu/kWh) * (52.07 kg CO2/MMBtu) * 2.20462
Electricity With Renewable Sources
The percentage of renewable energy you use can change your total energy footprint significantly. To find information about how much wind, solar, or hydroelectric power your utility uses, along with the source of the rest of your energy, refer to the EIA website we mentioned earlier. To find your adjusted emissions based on the mix of fossil fuel and renewable energy powering your home, we add a step to the previous fossil fuel calculation (we use mixed coal in this example), highlighted below in italics:
CO2 emissions (in lbs.) = (YOUR TOTAL ANNUAL kWh) * (0.0034095106405145 MMBtu/kWh) * (95.52 kg CO2/MMBtu) * 2.20462 * 1 – (1 – YOUR RENEWABLE ENERGY PERCENTAGE EXPRESSED AS DECIMAL)
Fuel Oil
A fuel oil furnace operates like a power plant without a mechanism for converting the heat into energy — it’s heating your house, instead. The formula is similar to the oil-fired electricity calculation above, and uses the CO2 emissions factor of 24.78 lbs. of CO2 per gallon:
Total CO2 emissions = YOUR ANNUAL OIL USE IN GALLONS * 24.76
Propane
Propane is easier on the atmosphere but still emits 12.68 lbs. of CO2 per gallon burned. To find your propane carbon footprint:
Total CO2 emissions = YOUR ANNUAL PROPANE USE IN GALLONS * 24.76
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