Posted by: paul | January 17, 2009

Measuring the Efficiency of a House

We all know a simple way to measure the efficiency of our cars: Miles per Gallon, or MPG. It is easy to do – just measure how many miles you travel between fill-ups, note how many gallons it takes to fill up, and divide the miles by the number of gallons. So, if you reset your trip odometer when you fill up, then the next time you fill up you’ve gone 200 miles, and you put 10 gallons in, then you were getting 20 MPG (200 miles / 10 gallons).

But how do we measure the efficiency of a house?

Gallons per Year, Cords per Year, etc.

The easiest measurement is simply how much fuel you use in a year. If you burn oil you’ll measure gallons per year, if wood then cords, if natural gas then cubic feet.

This is a fine way to compare two houses in the same area for the same year, but what if you want to compare the same house in different years? Perhaps you added an addition, or did some insulation work. Sure, you can compare the total fuel consumption before and after, but if one year was warmer than the other, your comparisons really aren’t valid.

Heating Degree Days

We need a standard way to compare how warm or cold a given year is. One way to do that is to use heating degree days. A heating degree day is a statement of how many degrees below 65 F a day’s average temperature is. So, if the average temperature on Monday is 40 degrees, you would say there were 25 heating degree days on Monday. Heating degree days accumulate over time, so if the average temperature on Tuesday was 45, then you would say that there were 45 heating degree days on Monday and Tuesday.

NOAA‘s National Climate Data Center has heating degree data for each state. There are lots of ways to get this data, but for now, one of the easiest ways is from here. Pick the years you are interested in and find your state. The top line states how many heating degree days there were for each month. The second line is a running total, so the total for the heating year is the value for June on the second line. (Heating years run from July to June.) The third line compares the running total for that year to the average value – so a number less than 100 indicates that the year so far is warmer than average, while a value greater than 100 means it was colder than average. Below is a snippet of the data (for Maine) in 2006/2007. We see that there were 7689 heating degree days, and that particular heating year was only 96% of the average. (There are ways to get more accurate data for your region, rather than a statewide average. I’ll address that in a future post.)

2006/2007 10 81 365 103
2006/2007 10 91 7586 7689
2006/2007 28.6 97.8 96.2 96.0

Heating Degree Days per Gallon

If we know how many heating degree days were in a year and we know how much fuel we used in a year, then we can calculate how many heating degree days per gallon (assuming the fuel is oil). For example, if you lived in Maine and used 700 gallons of oil during the 2006-2007 heating season, then your house “got” 11 heating degree days per gallon (7689/700). Or, if you burned 10 cords of wood, then your house “got” 769 heating degree days per cord (7689/10).

That is closer to something like Miles per Gallon, but it doesn’t let us compare the house that burns oil to one that burns wood. If you burn both, then you can’t even compare years.

BTUs and

To compare oil, wood, gas, etc, we need a common unit. In the US, the most common unit used in the heating industry is the BTU. A BTU is a pretty small unit of energy when it comes to heating a house, so an easier number to use is the therm, which is 10,000 BTU. We can convert our heating degree days per gallons, cords, etc into heating degree days per therm by dividing by the following numbers: (data from Fuel and Energy Conversion and Equivalence Chart)

Fuel Divide by:
Cubic foot of Natural Gas 0.0103
Cubic foot of Propane 0.025
Gallon of #2 Heating Oil 1.38
Cord of Wood 200
Ton of Wood Pellets 165
Kilowatt-hour of Electricity 0.0341

So, how many heating degree days per BTU did our two examples get?

The oil-heated house (which got 11 heating degree days per gallon of oil) got 8 heating degree days per therm. (11/1.38  =  8 ) Our cordwood heated house got 4 (769/200  =  4) heating degree days per therm. So the oil heated house is performing better – perhaps it is better insulated or smaller.


So there you go – a way to compare the same house across multiple years, or multiple houses by calculating the Heating Degree Days per Therm. Lets call it HddPT (“head pit”?) and make it a standard value when we buy and sell homes, when we compare them, etc.

This concept is nothing new or earth-shattering. Many heating oil companies essentially calculate your HddPT , and compare it to the number of heating degree days to date to know when to schedule your next delivery.

To calculate it, you just look up how many heating degree days there were, divide that by how much fuel you used, and then divide that by the conversion factor for your fuel above. That is,

HddPT = (heating degree days / amount of fuel ) / conversion factor

Wouldn’t it be great if it were as common a number as MPG?



  1. […] Degree Days This is a follow-up to a previous post, Measuring the Efficiency of a House, in which I discuss measuring the energy consumption of your house per heating degree […]

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