Unlike vehicles where it is easy to compare efficiency by comparing the mileage (expressed in either miles per gallon or litres per 100 km), comparing efficiency of two houses - or comparing efficiency before and after improvements to a house - is much more difficult. If a homeowner makes an investment in improved energy efficiency it is difficult to compare the effect of the improvement from one month to the next due to seasonal changes. Comparing a months worth of post-improvement energy consumption with the same month of the previous year or even the average of that month over the previous X years is an improvement but still doesn't really provide a direct “apples to apples” comparison as the month immediately following the improvement may have been warmer or colder than usual.
Just as we discuss automotive efficiency “per 100 km”, we need a way to measure our homes energy consumption “per unit” and the unit we use is the Heating Degree Day or HDD. The premise of a Heating Degree Day is that when the outside temperature is 18 degree Celsius (or warmer), you do not need to heat your house and so a day where the temperature averages 18 degrees counts as zero HDDs. If the temperature averages 17 degrees Celsius for a whole day, it counts as 1 HDD. Fractional values are allowed so that a day where the average temperature was 14.5 degrees Celsius would count as 3.5 HDDs. Obviously, the lower the temperature drops, the more energy is needed to keep your house warm and the higher the value of the HDD.
Using Heating Degree Days, you can compare energy consumption taking into account the variability of the outdoor temperature. For example, From April 9 to May 7, 2008 my house used 6.980 GJ of natural gas (as shown on my gas bill) and during this period Edmonton had 435.6 Heating Degree Days. Dividing the later into the former gives 0.01602 GJ per HDD. To make it a more human-friendly we can convert from GJ to MJ to get 16.02 MJ per HDD.
For comparison, from December 6, 2008 to Jan 10, 2009 (after my renovations where complete), my house used 14.212 GJ of natural gas. Initially this looks like a significant step backwards until you realized that during this period Edmonton had 1,323.7 Heating Degree Days. Expressed in terms of MJ per HDD, the energy consumption of my house has now dropped to 10.74 MJ per HDD – a reduction of 32.96%!
In tabular form, the same results are:
|Pre-Reno||Apr 9, 2008||May 7, 2008||6.980||435.6||16.02|
|Post-Reno||Dec 6, 2009||Jan 10, 2009||14.212||1323.7||10.74|
So the question is: How do you find the Heating Degree Days? Unfortunately it's not a really convenient – in fact the only way I could get the information is a convoluted pain in the backside. Here's the method I use:
|First:||Browse to http://www.climate.weatheroffice.ec.gc.ca/climateData/canada_e.html|
|Second:||Click on the city you're interested in – in my case: Edmonton. This takes you to a page that displays hour by hour weather data for your city.|
|Third:||Click on the link for “Daily Data” (at the very bottom of the page).
This page shows a day by day summary of the current month which includes, among other things, the heating degree day value for each day.
However, since your gas bill spans two months we need to dig a little deeper.
|Fourth:||Click on the link for “Bulk Data” (again, at the very bottom of page). This link downloads the current “year to date” data to your PC.
OK, so you've got data for the current year but how do you get data for previous years? The next step is to . . .
|Fifth:||Extract the “StationID” from the URL you just used to get the current years data. For example, Edmonton's StationID is 1865:
|Sixth:||Put your StationID and the year you want into the following URL:
Like I said: pain in the a**.
Attached to the bottom of this post is a spreadsheet for Edmonton's climate data starting at Jan 1, 2000.
You can take this even one step further by dividing the MJ/HDD by the size of the house in order adjust for the size. Suppose you are comparing two houses: House 1 is 158 m2 (1700 ft2) and consumes 12 MJ/HDD. House 2 is 112 m2 (1206 ft2) and consumes 10 MJ/HDD. Dividing the house size into the MJ/HDD gives the following:
|Property||MJ/HDD||Size (Sq. M)||MJ/HDD/sq M||KJ/HDD/sq M|
Notice that I've added another column to switch from MJ/HDD/sqM to KJ/HDD/sqM in order to make the numbers a little more human readable.
This shows that House 1 uses more energy in absolute terms but less on a per square meter basis. Does this make the larger house more efficient? No. House 1 uses more energy, period. I mention this only as another option for analyzing energy efficiency.