What’s Your Home’s MPG?

Lacking accurate historical energy usage data, I needed a way to budget my new home’s heating oil costs. I devised a method analogous to how we evaluate miles-per-gallon for a car. Ultimately, this provided part of my economic justification for replacing oil heat with air-source heat pumps which are now saving me money and making my home more comfortable.

Shortly after we moved into our 1830s colonial 3 years ago, on Thanksgiving morning, we woke up to an ice-cold house as outdoor temperatures plummeted into single digits. The boiler had run out of oil. We had to wait until the next day for help. It took many hours of work from a technician from an oil delivery company to clear the line that was clogged with sludge and refill the ancient oil tank to get us back up and running. By mid-day Friday, we had heat again. And about 3 weeks later, we accepted delivery of another $700s worth of heating oil.

I knew the house was inefficient and the boiler in need of replacement, but I did not have any idea what the energy needs of our home were since we had just moved in.

I was able to find the historical usage from the oil company that had served the previous owners. But they had used the home as a business and I knew that our usage patterns would be very different. In one year, they had used 2100 gallons of oil! That seemed a bit excessive.

So I needed a way to project what our likely costs would be, using our actual experience. There are more sophisticated ways to model energy usage and heating needs, but I figured for my purposes if I could extrapolate actual usage from December into the rest of the year, this would give me a fair ballpark of what to expect.

Heating degree days are a measure of the daily deviation from some baseline temperature and provide a way to compare whether a given time period was warmer or colder.

I obtained the degree days data for the previous year. Of course, the numbers will vary from year to year as winters are warmer or colder, but I could at least project what the upcoming winter would be like if it was the same as the prior year. Then I recorded the gallons of oil purchased during that first winter to arrive at a number representing the degree days/gallon–kind of like you would calculate miles per gallon for your gasoline-powered car: if you know your car gets 30mph on a typical highway trip, then you can project that a 300-mile trip is going to require 10 gallons of gas.

My idea was to calculate the degree-days/gallon of heating oil to arrive at a metric for my home’s performance and our usage patterns and then extend that into the upcoming season. This would tell me that I might need X number of gallons for January-March, assuming same weather as last year. I created an excel spreadsheet to perform all the calculations and made adjustments as we re-filled the oil tanks. This confirmed the general accuracy of my approach–i.e. I knew how many gallons to expect when the tank was refilled.

My conclusion from this was that we’d expect to burn 1500 gallons of home heating oil per year, even with lower thermostat settings and turning off unused zones. This told me that I was looking at at least $5000 per year and maybe $7500 depending on weather and oil prices.


Leave a Reply

Your email address will not be published. Required fields are marked *