What Are Heating and Cooling Degree Days?

Heating Degree Days (HDD) and Cooling Degree Days (CDD) measure how much heating or cooling is required to maintain indoor comfort. Instead of simply tracking temperature, they quantify how far temperatures deviate from a baseline of 65°F.

How Degree Days Are Calculated

Degree days are based on the difference between the daily average temperature and 65°F:

• HDD = 65 − average temperature (when below 65°F)
• CDD = average temperature − 65 (when above 65°F)

If the average temperature is exactly 65°F, both HDD and CDD are zero — meaning no heating or cooling demand.

Real-World Example

• Average temp = 50°F → HDD = 15, CDD = 0
• Average temp = 80°F → HDD = 0, CDD = 15

Why 65°F Is the Standard

65°F is considered the “balance point” where most buildings maintain comfort without heating or cooling. It accounts for internal heat sources like people, appliances, and sunlight.

Why Degree Days Matter

• Energy forecasting: Utilities predict demand
• Cost estimation: Higher HDD/CDD = higher bills
• HVAC sizing: Engineers design systems
• Agriculture: Tracks seasonal development
• Finance: Used in weather derivatives trading

How to Interpret the Charts

• Each bar = one day
• Higher HDD → colder weather → more heating
• Higher CDD → hotter weather → more cooling
• Weekly total shows overall energy demand trend

Limitations of Degree Days

While widely used, degree days are estimates:

• They don’t account for humidity or wind
• Building insulation varies
• Actual energy usage depends on behavior

Advanced Insight: Why Average Temperature Matters

Using only daytime highs can misrepresent energy demand. This page improves accuracy by averaging daytime and nighttime temperatures, giving a more realistic daily energy estimate.

Frequently Asked Questions

Are HDD and CDD used worldwide?
Yes, though base temperatures may vary slightly by region.

Can I estimate my energy bill?
Yes — higher totals generally mean higher energy use.

Why do mild days have low values?
Because they stay close to 65°F, requiring little heating or cooling.