Hopefully you are not actually driving your home, because RVs get horrible gas mileage. In all seriousness though, one of the challenges today is figuring out how to compare the energy efficiency of homes in a way that can be readily understandable by home owners and potential buyers or renters.
For cars, the EPA’s miles per gallon (MPG) fuel economy guideline is well established and well known. And while gallons per hundred miles might be a better comparison metric, people understand MPG. What should we do for our homes to compares two similar homes and let’s us know which is more energy efficient? What is your Home’s “MPG?”
Last year, while attending a workshop at NESEA’s Building Energy 2011Conference, we met Nancy Hazard, who introduced us to the term Home MPG. The term and concept has really resonated with us ever since. So what does “Home MPG” mean and how do we calculate it?
The measure of energy intensity (and therefore also efficiency) commonly used for commercial buildings, is kBTU/SqFt/Yr (kilo-BTUs per square foot per year). Basically it is the measure of the total energy consumed in a building over the entire year divided by the size of the building. Amazingly this applies very nicely to homes as well. To calculate your Home MPG, you need your utility bills (and oil/propane delivery) totals for a year and the size of your home.
Calculate the Annual Energy Use in Your Home
Gather one year of utility bills such as electricity and natural gas. Be sure to include any fuel deliveries like oil, propane, wood pellets, etc.
Most electric bills have your usage for the past 13 months. Can you figure out when we installed our solar array?
Hint: Most utility bills (electricity & natural gas) include a year’s worth of usage history for comparison purposes that makes it easy to total that up. I find using the January bills the easiest because then you have a calendar year, but as long as you are covering the same time period, any month will work. You may also be able to get this information online from your utility company, possibly even going back a couple years which can be helpful in seeing how your usage has changed over time. For deliveries, consider calling your supplier as they should have records of all deliveries to share with you.
As an example, here is our home energy use. Our electricity usage for 2010 totals to 6,411 kWh. Our natural gas usage for the same period was 486 therms.
Figure out the Size of Your Home
We know that the size of our house is 1,600 sqft of conditioned space. Conditioned space is the area that is heated and/or cooled for comfort and livability. An unfinished basement would not count as conditioned space.
Hint: If you aren’t sure of your home’s square footage, look up your address on Zillow.com using the Zestimate. Keep in mind this may not reflect any renovations you have done since the house was last sold.
Now that you have the usage and size, you can calculate your Home MPG.
Calculate Your Home MPG The next step is to convert all of your energy usage into kBTU (kiloBTU). For anything measured in therms, that is really easy because 1 therm = 100 kBTU. For our 486 therms of natural gas, we get an annual usage of 48,600 kBTU.
Converting electricity to kBTU is a bit trickier. Most people when computing kBTU/sqft/year use what is termed Site Energy because it is easier to calculate and independent of location. I find that computing kBTU/sqft/year in terms of Source Energy more useful and appropriate for Home MPG. For an explanation of site versus source energy, read our post:”Site vs Source Energy: What Is It and Why Do I Care?“For electricity, regardless of whether you use site or source energy, you must be consistent for your comparisons to be valid.
Calculate Source Energy for Electricity:
see Comparing Site Energy vs Source Energy and Why You Should Care? for more details and your regional conversion factor for electricity to kBTU
- Heat Input / Net Generation from EPA’s eGrid for your region
- e.g., New England 5.929 kBTU = 1 kWh
Calculating Site Energy for electricity:
Use a conversion factor of 3.412 kBTU/kWh. For calculating your Home MPG either will work. Even though site energy is much easier to calculate, I prefer using source energy as that helps address the overall energy consumption related to green house gas emissions.
Put it Together into Your Home MPG
To show how to calculate Home MPG, let’s take a look at our home for 2011. We use natural gas and electricity.
- Natural Gas usage for 2011: 551 therms = 55,100 kBTU
- Electricity usage for 2011: 1,142 kWh = 6,770 kBTU source or 3,897 kBTU site
We produced 83% of our electricity from solar in 2011 and only purchased 1,142 kWh from the electric company. - Conditioned Area: 1,600 sq ft
Our HomeMPG was 39 kBTU/SqFt/Year for 2011
Before we added the solar panels at the end of 2010, our Home MPG was 67 kBTU/sqft/year.
Our HomeMPG in 2010 after insulating & upgrading boiler but before our Solar Awning
Before we insulated our walls and upgraded our boiler to a high efficiency unit, our Home MPG was over 100 kBTU/sqft/year.
Our HomeMPG in 2005 before any energy efficiency work
Just like the EPA’s estimated MPG for your car doesn’t always equal what you get when you actually drive a car, Home MPG does fluctuate with behavior and weather.
What Home MPG can tell you is that the home we live in today is significantly more efficient than just a few years ago when we started down the green path. Home MPG is an excellent way to measure your own behaviors changes and efficiency improvements.
If you’re looking to buy or rent a home, you might ask to see the previous resident’s utility bills to get a sense of what you might pay in that home. With that information you can calculate the unit’s Home MPG. However, you should be aware that this is not taking into account the number of people that live in the home, whether they are home a lot or out of the house, and what their position is on using or wasting energy. Maybe in the future, Home MPG will be a standard part of home appraisals.
Happy Greening!
Jon
Do you think HomeMPG for the past three years should be available when buying a home or renting an apartment? Why or why not?
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Update: I just ran the numbers for 2012 and we are down 70% in kBTU & 73% in CO2 emissions since 2005. Our HomeMPG for 2012 is 33 kBTU/sqft/yr!