In the world of home energy, “degree days” are a critical metric used by fuel suppliers—including Collett Propane—to estimate how much heating or cooling a building needs. In this article, we explain what degree days are, how they relate to propane usage, and how we use them to time your deliveries intelligently.
What Are Degree Days?
Degree days are a way of quantifying how cold or hot it has been relative to a “balance” temperature—commonly 65 °F in the U.S.—which is considered the baseline above or below which heating or cooling energy is needed.
There are two types:
- Heating Degree Days (HDD): When the mean daily temperature falls below 65 °F, the difference is counted as heating degree days. For example, if the day’s average is 40 °F, then:
HDD=65−40=25
That day contributes 25 heating degree days. - Cooling Degree Days (CDD): When the mean daily temperature is above 65 °F, the difference is counted toward cooling degree days. Example: average = 78 °F ⇒
CDD=78−65=13
That day contributes 13 cooling degree days.
Over a season, you add up all the HDD or CDD to get total degree-days for that period.
Degree days allow energy usage comparisons across different temperature periods.
How Degree Days Relate to Propane Usage
In propane delivery and forecasting, degree days help us predict how much fuel a home will use. The colder it is (i.e. more heating degree days), the more propane is burned to maintain indoor comfort.
But each home is different. That’s where the K-factor comes in. The K-factor is essentially the ratio of degree days to gallons of propane used over a given period. Whichever number of degree days occurs, that home burns a certain number of gallons per degree-day.
Here’s how it works in practice:
- Track a delivery interval. Suppose we make a delivery when cumulative heating degree days (HDD) is 2,500. The next delivery occurs when cumulative HDD is 3,800.
- Measure the gallons delivered. Let’s say we delivered 200 gallons in that interval.
- Compute the K-factor:
That means this home uses 1 gallon of propane per ~6.5 degree days.
Once we know your K-factor, we can forecast future usage by plugging in forecasted degree days. That helps us decide when your next delivery should happen without relying on your tank gauge alone.
| Interval Metric | Value in Example |
| Degree days between fills | 1,300 |
| Gallons delivered | 200 |
| Calculated K-factor | 6.5 (degree days per gallon) |
If over the next few days we estimate another 500 HDD will accumulate, then your home is likely to consume ~500 ÷ 6.5 ≈ 77 gallons in that span. That helps us decide to schedule a delivery before you run too low.
Propane Delivery Timing & the “80/20” Rule
It’s important to understand how safe propane tank filling works:
- For safety, residential propane tanks are filled to about 80% capacity during delivery. This allows room for thermal expansion as the gas warms.
- For will-call customers, we suggest ordering when your gauge reads ~20–30% remaining.
- For automatic delivery customers, we use your K-factor plus degree-day forecasts to time deliveries so that you never have to watch your gauge.
So in short: “For safety, tanks are filled to about 80% during delivery; we plan deliveries so you don’t have to watch the gauge, but as a rule of thumb, will-call customers should order at ~20–30% remaining.”
Because the K-factor is unique to each customer (based on house size, insulation, usage patterns, etc.), this method is far more accurate than a one-size-fits-all schedule.
Even though our degree-day and K-factor systems accurately predict when you’ll need propane, some customers prefer extra visibility into their fuel levels. That’s where tank monitors come in. These wireless devices attach directly to your tank and let both you and Collett track real-time propane levels through a secure online or mobile app connection. It’s a great option for vacation homes, busy households, or anyone who simply likes to keep an eye on their fuel usage for added comfort and confidence.
Why This Matters to You
- Better reliability & fewer runouts: You don’t have to worry about freezing nights or unexpected snowstorms leaving you without heat—because your deliveries are scheduled based on data, not guesswork.
- More efficient delivery routes: We can plan our drivers better, combining routes to reduce costs and keep pricing fair.
- Adaptation to weather swings: In a mild winter, fewer heating degree days means you burn less propane and need fewer deliveries. If a cold snap hits, the system adjusts accordingly.
- Transparency & fairness: You pay for what you use, and your delivery schedule is based on your own historical usage—not arbitrary rules.
In Summary
A “degree day” is a unit that measures how far, and for how long, the outdoor temperature deviates from a reference (typically 65 °F). We use heating degree days and cooling degree days to estimate energy demand. Paired with each home’s K-factor, degree-day calculations allow Collett to forecast your propane usage and schedule deliveries precisely. We never wait until your tank is 80% full—we always deliver beforehand, and fill only to about 80% to allow for expansion. Will-call customers should request a fill at ~20–30% remaining.
If you’re interested in having us manage your propane deliveries and take the guesswork out of your tank gauge, let’s connect. Contact us via our online form, and we’ll be glad to set up automatic delivery for your home.