Calculate Your Building's Heating Energy Intensity

by ‐ Tags: alternative energy, building science

Unlike last year, it felt like we had a real winter this year in Boston.  This has kept heating costs in the forefront of peoples' minds, mostly because they're wishing they were lower.  Since this winter is much colder, it makes it difficult to compare last year's heating bills to this year's, or the ones from the year before.  So if you made changes that you think will save you money, how do you know if they did, even though your heating costs have gone up?  What you want to know is what your gas usage would have been if the weather was essentially the same every year.  A great way to do this is to compare heating energy intensity for the two years. Keep reading to learn how to calculate this. 

It will take a little time, but when you're done, you'll have a better way to compare your bills from year to year.  

To get started:

  1. Gather at least a year of your utility bills. 
    If you use gas for heat, get those. If you use electric for heat, you'll want those.  If you use oil...  You get the picture.

  2. Download the heating degree days for the time period that you're analyzing.  
    I usually use http://www.degreedays.net/. Assuming you don't know your nearest weather station ID, enter your zip code or address. You'll see a list of nearby weather stations. Hovering over one will tell you how much data is available from that weather station and how accurate it is. For all the other fields, leave the defaults, except for the Period Covered, if you want more than a year of data.  
    Heating Degree Days

  3. Determine your heating energy.
    You want to figure out what portion of your usage is for heating and what can be attributed to baseloads. Baseload refers to the energy that you use all year, no matter what, such as water heating and cooking energy.  If you use gas or oil and only use it for heat (no hot water, no cooking), then you're done with this step and can move on to the next one.  

    For the rest of you, you'll want to identify the 3 months with the lowest usage. Average these together, which will give you your baseload. In our example below, which is for a building with gas heat and hot water, the baseload is 21.7. Multiply this by 12 to get your annual baseload usage (260 for our example).

  4. Determine your heating energy.
    Add up all your usage for the year. Then, if you weren't one of the lucky ones who got to skip the previous step, subtract your baseload usage from this number. For our example, the annual usage was 959 therms. We subtracted the baseload usage of 255.6 therms to get our heating related usage of 703.4 therms.

  5. Calculate your heating energy intensity.
    Now we need three numbers--the conditioned square footage of the building, the annual heating-related usage, and the annual heating degree days. First we're going to convert our therms to BTU (British Thermal Units). This is because otherwise we will end up with a really  tiny number. A couple quick conversions for you:

Now that you have BTU, divide this by your annual heating degree days. Finally, divide it by your conditioned square footage.  This is your BTU/HDD/Conditioned Sq Ft, otherwise known as heating energy intensity.

Example 

Since Lily invited everyone to see how she's doing at reducing her gas bill this year, we'll use her house.  It uses gas for heat and hot water and is 1288 conditioned square feet.  Her annual gas usage is 959 therms, 699 of which was for heating.  

Baseload: 19 + 24+ 22 = 65.  Average = 21.7.  Annual = 260

Heating energy: 959 annual therms - 260 therm baseload = 699 therms for heating energy

699 therms (annual heating usage) x 100066.96 (factor to convert to BTU) = 69,946,805 BTU

69,946,805 BTU / 5026 HDD (annual heating degree days) = 13,917 BTU/HDD

13,917 / 1288 conditioned square feet = 10.8 BTU/HDD/Conditioned sq ft

calculations

Once you have this number, it makes it easy to determine your actual savings from retrofits and compare your building to your neighbors. If you are in a cooling dominated climate you can run the same calculations for your cooling energy.  Alternatively, WegoWise can do it for you.

weather normalized heating savings