Cost Per Degree of Heating Calculator
Use this tool to understand how efficiently your heating system performs relative to the heating demand of a period. Heating Degree Days (HDD) quantify this demand based on how much the average daily temperature falls below a baseline (typically 65°F or 18°C).
Enter your Total Heating Cost for a specific period (e.g., a month or heating season) and the corresponding Total Heating Degree Days for your location and that same period.
Enter Heating Data
Understanding Cost Per Degree Day
What is Cost Per Degree Day?
Cost Per Degree Day is a metric used to assess the energy efficiency of a building's heating system over a specific period. It standardizes your heating cost by dividing it by the heating demand experienced during that time, as measured by Heating Degree Days (HDD).
The formula is simple: Cost Per HDD = Total Heating Cost / Total Heating Degree Days
A lower cost per HDD generally indicates better heating efficiency (less money spent to satisfy the same heating demand). Comparing this value over time or across different periods (with similar conditions, if possible) can help you track efficiency improvements (like adding insulation, upgrading a furnace) or identify potential issues.
What are Heating Degree Days (HDD)?
Heating Degree Days are a measure of how much and for how long the outside air temperature was below a specific base temperature, used to estimate the demand for heating. The base temperature is commonly 65°F (18°C) in the U.S. and 18°C in many other countries. For a given day, HDD is calculated as the difference between the base temperature and the average daily temperature, if the average temperature is below the base. For a period (like a month or season), the total HDD is the sum of the daily HDD values.
Daily HDD = max(0, Base Temperature - Average Daily Temperature)
Total HDD for a period is the sum of Daily HDD for each day in the period.
Cost Per HDD Examples
Click on an example to see the calculation:
Example 1: Cold Winter Month
Scenario: Calculate the cost per HDD for a cold January.
Known Values: Total Heating Cost = $250.00, Total HDD = 1200.
Calculation: Cost Per HDD = $250.00 / 1200 HDD
Result: Cost Per HDD ≈ $0.2083 / HDD
Interpretation: It cost about 20.8 cents per degree day of heating demand this month.
Example 2: Milder Month
Scenario: Calculate the cost per HDD for a milder November.
Known Values: Total Heating Cost = $80.00, Total HDD = 400.
Calculation: Cost Per HDD = $80.00 / 400 HDD
Result: Cost Per HDD = $0.2000 / HDD
Interpretation: This month was slightly more efficient (20.0 cents/HDD) than the cold January in Example 1.
Example 3: Entire Heating Season
Scenario: Calculate the average cost per HDD for a full heating season.
Known Values: Total Heating Cost (Seasonal) = $1200.00, Total HDD (Seasonal) = 6000.
Calculation: Cost Per HDD = $1200.00 / 6000 HDD
Result: Cost Per HDD = $0.2000 / HDD
Interpretation: The average cost over the entire season was 20.0 cents per degree day.
Example 4: Tracking Efficiency Improvement
Scenario: Compare efficiency after adding insulation. Last year vs. This year (similar HDD).
Known Values: Last Year: Cost = $1000, HDD = 5000. This Year: Cost = $900, HDD = 5000.
Calculation: Last Year: $1000 / 5000 HDD = $0.2000 / HDD. This Year: $900 / 5000 HDD = $0.1800 / HDD.
Result: Cost Per HDD decreased from $0.2000 to $0.1800.
Interpretation: The insulation seems to have improved heating efficiency, reducing the cost per HDD by 2 cents.
Example 5: High Cost, Low HDD
Scenario: A month with unexpectedly high cost for low heating demand.
Known Values: Total Heating Cost = $100.00, Total HDD = 200.
Calculation: Cost Per HDD = $100.00 / 200 HDD
Result: Cost Per HDD = $0.5000 / HDD
Interpretation: A high cost per HDD ($0.50) suggests potential issues like a malfunctioning furnace, air leaks, or inaccurate meter readings, especially if this value is much higher than typical.
Example 6: Low Cost, High HDD (Unlikely but for illustration)
Scenario: Calculate cost per HDD with very low cost and high HDD.
Known Values: Total Heating Cost = $50.00, Total HDD = 1500.
Calculation: Cost Per HDD = $50.00 / 1500 HDD
Result: Cost Per HDD ≈ $0.0333 / HDD
Interpretation: An extremely low cost per HDD might indicate very high efficiency or perhaps an error in the input values (cost too low or HDD too high).
Example 7: Comparing Fuel Types (Conceptual)
Scenario: Compare the relative efficiency of two different heating fuels in similar periods (requires careful data collection).
Known Values (Fuel A): Cost = $300, HDD = 1000. Known Values (Fuel B): Cost = $280, HDD = 1000.
Calculation: Fuel A: $300 / 1000 HDD = $0.3000 / HDD. Fuel B: $280 / 1000 HDD = $0.2800 / HDD.
Result: Fuel A Cost/HDD = $0.3000, Fuel B Cost/HDD = $0.2800.
Interpretation: In this example, Fuel B appears slightly more cost-efficient per degree day of heating demand during these periods.
Example 8: Efficiency During a Specific Month
Scenario: Calculate cost per HDD for a specific moderate month.
Known Values: Total Heating Cost = $120.50, Total HDD = 625.
Calculation: Cost Per HDD = $120.50 / 625 HDD
Result: Cost Per HDD = $0.1928 / HDD
Interpretation: The cost per degree day for this specific moderate month was approximately 19.3 cents.
Example 9: Checking for Seasonal Variation
Scenario: Compare cost per HDD in early vs. late heating season (assuming similar rates).
Known Values (Early): Cost = $150, HDD = 700. Known Values (Late): Cost = $200, HDD = 950.
Calculation: Early: $150 / 700 HDD ≈ $0.2143 / HDD. Late: $200 / 950 HDD ≈ $0.2105 / HDD.
Result: Early Season Cost/HDD ≈ $0.2143, Late Season Cost/HDD ≈ $0.2105.
Interpretation: In this case, the efficiency seems relatively consistent between the two periods.
Example 10: Using the Calculator to Spot Anomalies
Scenario: You usually see a cost per HDD around $0.22. This month it's $0.35.
Known Values (Current Month): Total Heating Cost = $245.00, Total HDD = 700.
Calculation: Cost Per HDD = $245.00 / 700 HDD
Result: Cost Per HDD = $0.3500 / HDD
Interpretation: This significantly higher value ($0.35 vs typical $0.22) flags a potential issue needing investigation (e.g., spike in energy price, thermostat set higher than usual, efficiency problem).
Frequently Asked Questions about Cost Per Degree Day
1. What does "Cost Per Degree Day" mean?
It's a measure of how much money you spend on heating for each "degree day" of heating demand. A lower number means you're spending less money to satisfy the same amount of heating need, indicating better efficiency.
2. How do I find my Heating Degree Days (HDD)?
You can typically find historical Heating Degree Day data for your specific location and time period from weather websites, energy provider reports, or national climate data centers (like NOAA in the US).
3. What is a typical value for Cost Per HDD?
There's no single "typical" value as it varies greatly based on factors like fuel type and cost (natural gas, electricity, oil, propane), furnace efficiency, home insulation, thermostat settings, window quality, and climate zone. The value is most useful for comparing *your own* performance over time or year-to-year under similar conditions.
4. Can I compare my Cost Per HDD to someone else's?
You can, but be cautious. Differences can be due to many factors beyond efficiency, like different fuel costs, building sizes, insulation levels, thermostat habits, or even different local HDD calculations. It's best used for self-comparison or comparison with very similar homes in the same area using the same fuel type.
5. Should I use Fahrenheit or Celsius for HDD?
HDD are calculated using a base temperature (65°F or 18°C). You must use HDD values calculated using the *same* temperature scale consistently. The calculator works regardless of the scale used for HDD, as long as your HDD value corresponds to the correct method for your location/data source.
6. Why did my Cost Per HDD go up this month compared to last month?
Reasons could include: an increase in fuel/energy price, setting your thermostat higher, inefficient furnace operation, air leaks developing, or simply variations in how degree days are calculated day-to-day (though this is usually minor). Tracking this metric can help pinpoint such issues.
7. What units should I use for the Total Heating Cost?
Enter the cost in your local currency (e.g., US Dollars, Euros, Pounds Sterling). The resulting Cost Per HDD will be in Currency Unit / HDD.
8. Can I use this for any heating system?
Yes, this calculator works for any heating system (furnace, boiler, heat pump, electric heat) as long as you have the total cost for the period and the corresponding total HDD for the same period and location.
9. What if my Total Heating Degree Days is zero?
If the total HDD for your period was zero, it means the average temperature never dropped below the base temperature, and effectively there was no heating demand based on the HDD model. The calculator cannot compute a "cost per degree day" in this scenario, as it would involve division by zero.
10. How accurate is this metric?
It's a useful comparative metric but an estimate. It doesn't account for factors like solar gain, internal heat sources, wind speed, or humidity, which also affect heating needs. However, over longer periods (monthly, seasonally) and for comparisons of the same building, it provides a good standardized measure of efficiency.
