How Does Your House Compare?
Ideally every house would get an energy audit at a nominal cost, courtesy of our electric utility — analyzing the home’s inefficiencies, with suggested corrections. We could also get free help in deciding our best options, help with finding the right contractor, and low interest financing.
Alas this is no ideal world. But at least I can show you how your house stacks up against other houses and against the world’s best performing: Passive House. All you need is the heated floor area of your home, your energy bills, and some basic grade-school level math. Since we must have a common basis for comparison that holds true regardless of the size of each house, we normalize all data to a common unit of floor space, i.e. “per square foot” or “per square meter”.
I prefer international (metric) units, because the most common unit of power, the kilowatt-hour (kWh) is already familiar from our electric bills, and the numbers are smaller and easier to visualize than the English unit equivalents. To wit: “If a 2,450 square foot home burns 1,100 gallons of fuel oil , it consumes 1,100 x 138,690 BTU/gal = 152,559,000 BTUs per year for heating. Divide this by 2,450 and get 62,269 BTUs/square foot of floor space”.
I’d rather multiply the 2,450 sq.ft. by 0.0929, to get a (rounded) 228 sq. meters (m²). Burning the same 1,100 gallons of oil, the house now consumes 1,100 x 40.6 kWh/gal = 44,660 kWh per year for heating. Divide this by 228 and get 196 kWh per m² of floor space. — I hope you agree that 196 is a lot easier to visualize (not mention remember) than 62,269.
Here are the metric fuel values for Firewood 5,860 kWh/cord; Wood pellets 4,834 kWh/ton; #2 fuel oil 40.6 kWh/gallon; Kerosene 39.6 kWh/gallon, Propane 26.8 kWh/gallon; and Natural gas 29.3 kWh/therm. (These values are subject to conversion efficiency adjustment, but let’s not complicate things). As long as we compare apples to apples, we will be OK.
Let’s base our comparison on one or more of the following values and assume that the houses compared are all heated to 20°C (68°F)):
- Heating energy consumed in kWh/m² living space/ year: Compare to 15 for Passive house and 185 for the median Maine house. Based on this measure, the example house above, at 196, consumes more than 13 times the heating energy than a Passive House of similar size and 6% more than the median Maine house.
- Primary energy consumed, based on your energy bills. This is the total amount of energy your house consumes per year, including heating. Compare to 120 for Passive house and 203 for the median Maine house.
- Air leakage (if you can test with a blower door at 50 Pascals pressure): Compare to 0.6 air changes per hour for Passive house and 11 ACH for the median Maine house.
Next time we will explore some basic know-how about home energy makeovers every homeowner should understand.