Comparing Your Home’s Energy Use to Your “Neighbors”

This post first appeared on the Green Building Advisor Website

Part of an energy auditors’ job is energy analysis, which includes analyzing historical energy consumption data to determine if usage is as expected.  One method is to compare a home’s energy use to some average (the local rural electricity provider I contract with calls this “compared to your neighbor’s”).  This average could be the national average, but it’s better to compare at a more local level, such as by state.  There are a few sources of this information, one that I have been using is: U.S. Energy Information Administration – EIA – Independent Statistics and Analysis.  This dashboard provides me with several different energy consumption metrics, expressed in MMBtu’s and kWh, broken down by state and also displays national averages for the US (sorry my Canadian friends).

Before we dive into the analysis, we should look at two different ways we compare energy usage, Million British Thermal Units (MMBtu) and kilowatt-hours (kWh).  One Btu is defined as the amount of energy required to raise the temperature of one-pound of water one-degree.  One MMBtu would be 1,000,000 Btu’s.

A watt is a unit of power, one joule per second.  As an energy auditor, I’m looking for electricity consumed, which is billed by kWh’s, or 1,000 watts per hour.  We can convert kWh’s to Btu’s and visa vera.  One kWh is equal to 3,412 Btu.  One MMBtu is equal to a little more than 293 kWh.

Now that we have a basic understanding of the metrics we use for an energy analysis, let’s use the data to perform a basic energy assessment.  We’ll use my home as an example.  The home is located in Northern Minnesota, climate zone 7.  It has a conditioned floor area of 2054 square feet.  It’s primary source of heat is a natural gas forced air furnace (the only gas burning appliance in the home).  There is also a central ducted air source heat pump.  The ASHP is not a cold climate version, it is used for heating spring and fall and summertime air conditioning.  The home has a standard electric resistance water heater, an older (1990) 50-gallon model.

The natural gas forced air furnace consumed 565.5 ccf over the past 12 months (ccf stands for 100 cubic feet, a measurement of how natural gas is sold.  Natural gas has roughly 103,700 Btu/ccf.  To convert the ccf to MMBtu, we multiply my usage, 565.5 ccf, times the amount of MMBtu per ccf, 0.1037.  This results in 58.64 MMBtu).

My home also used 10,934 kWh of electricity over that same time period.  (To convert kilowatt hours to MMBtu, we multiply the 10,934 kWh times the number of Btu per kW, which is 3412.  This results in 37.30 MMBtu.)  My home had a total energy consumption of 95.95 MMBtu over the past 12 months.

If I reference the Residential Energy Consumption Survey Dashboard (from the above link), I’d find the average home in Minnesota is 1,994 square feet and consumes 100.3 MMBtu of total energy per year.

So, my home is slightly larger than the Minnesota average in size and uses slightly less than the average amount of total energy per year (at least for the past 12 months).  I can further breakdown the numbers to analyze space heating from electricity use.  The chart above shows 59.3 MMBtu is used for space heating; the chart below shows the average Minnesota home uses 9331 kWh in electricity consumption.  My home used less energy for heating than average, 58.64 MMBtu, but more electricity than average, 10,934 kWh.  Part of the electrical increase would be added to the space heating because the all-electric air source heat pump is a heat source that I cannot separate from the general electrical usage without a dedicated meter or other way to track usage.

If you really want to get nerdy, you can also break out water heating, air conditioning, refrigeration, and “other” end use consumption.  Sometimes these other energy using products can be broken out of energy costs, sometimes you can only estimate actual consumption.

Improvement Recommendations

If I were working with a homeowner on an actual energy audit, I would be looking at what I call the three-leg stool (occupant, equipment/appliances, and the building enclosure).  I would be asking many questions to determine how the homeowner operates and lives in the home.  I would also record equipment and appliance information and, depending on the type of energy audit, testing the home to determine performance (air leakage rates and verifying insulation levels and type).   But this is my home and I already know it inside and out.

I have a very high air leakage rate of 12 ACH50.  I’ve estimated the cost of the air leakage at 7 MMBtu per year, nearly 15% of my total energy usage for heating and cooling.  My hope is to reduce the overall air leakage of the home by 2/3, to roughly 4 ACH50.

Another issue with my home is ventilation, I rely on spot ventilation only in the form of a bath and kitchen range fans.  I have my furnace fan set to cycle for 20 minutes per hour to filter and hopefully improve the indoor air quality.  The furnace fan consumes 400 watts of power, operating at 20 minutes per hour, my attempt at air filtration consumes roughly 1,100 kWh per year, or about 10% of my electricity usage.  My plan is to install a balance mechanical ventilation system (HRV) and operate the unit based on the requirements of ASHRAE 62.2.  This should reduce overall electricity consumption of the furnace by 50%, plus I should see an improvement in overall indoor air quality.

Lastly, my current water heater is nearly 35 years old, well past its life expectancy.  Replacing the unit with an air source heat pump water heater should reduce my water heating costs, though I’m struggling with “stealing” heat from my already cool basement (where my office is located).  There are solutions, I need to have a good plan.

Using energy analysis is one tool in the toolbox for helping homeowners see the big picture of their energy consumption.  I recommend hiring a trained and experienced energy auditor, (not always easy to find), a good energy audit goes well beyond an energy analysis and can help find ways to reduce total usage.