Cooling & Heating Degree Days (CDD / HDD) Calculator
Weather-normalize building energy consumption, calculate annual Cooling Degree Days ($CDD_{65}$) and Heating Degree Days ($HDD_{65}$), estimate seasonal HVAC kWh and gas therms, and determine building Energy Use Intensity (EUI).
Climate Zone & Building Envelope
Annual Energy Consumption & Operating Cost
Weather Normalization Rule of Thumb
Utility bill changes between consecutive years are predominantly driven by outdoor degree-day deviations rather than appliance degradation. If heating degree days increase by 12% during a severe winter polar vortex, a 12% increase in gas therm consumption indicates perfectly normal thermal performance.
Annual Temperature Climatology vs 65°F Balance Point ($T_{ ext{base}}$)
Live Degree-Day IntegrationASHRAE Climate Zone Degree Days & Energy Baselines
| Climate Zone | Representative City | Annual $CDD_{65}$ | Annual $HDD_{65}$ | Dominant Load | Average Residential EUI |
|---|---|---|---|---|---|
| Zone 1 | Miami, FL / Honolulu, HI | 4,200 | 150 | 100% Cooling Dominant | 38 kBTU/(sq ft·yr) |
| Zone 2 | Houston, TX / Phoenix, AZ | 2,900 | 1,400 | Cooling Dominant | 42 kBTU/(sq ft·yr) |
| Zone 3 | Atlanta, GA / Dallas, TX | 1,850 | 2,800 | Mixed / Dual Season | 46 kBTU/(sq ft·yr) |
| Zone 4 | St. Louis, MO / Washington, DC | 1,400 | 4,500 | Mixed Heating Heavy | 52 kBTU/(sq ft·yr) |
| Zone 5 | Chicago, IL / New York, NY | 950 | 6,100 | Heating Dominant | 58 kBTU/(sq ft·yr) |
| Zone 6 | Minneapolis, MN / Burlington, VT | 680 | 7,850 | Severe Heating Heavy | 66 kBTU/(sq ft·yr) |
| Zone 7 | Duluth, MN / Grand Forks, ND | 320 | 9,600 | Sub-Arctic Heating | 78 kBTU/(sq ft·yr) |
Degree-Day Integration & Thermal Math
1. Daily Cooling and Heating Degree Days:
For any 24-hour day with mean outdoor temperature $ar{T} = rac{T_{max} + T_{min}}{2}$:
$$ ext{CDD} = max(0, ar{T} - 65^circ ext{F}) quad ext{and} quad ext{HDD} = max(0, 65^circ ext{F} - ar{T})$$
Across the annual profile, the site accumulates 950 CDD and 6,100 HDD.
2. Building Seasonal Thermal Loads (MMBTU):
Overall building conductive heat loss/gain coefficient: $ ext{UA} = A imes U_{ ext{factor}} = 2,400 imes 0.30 = mathbf{720 ext{ BTU}/( ext{hr}cdot^circ ext{F})}$.
$$Q_{ ext{cool}} = rac{ ext{UA} imes 24 imes ext{CDD}}{1,000,000} = rac{720 imes 24 imes 950}{1,000,000} = mathbf{16.4 ext{ MMBTU}}$$
$$Q_{ ext{heat}} = rac{ ext{UA} imes 24 imes ext{HDD}}{1,000,000} = rac{720 imes 24 imes 6,100}{1,000,000} = mathbf{105.4 ext{ MMBTU}}$$
3. Electrical & Fuel Sizing Conversions:
$$ ext{Cooling Electricity} = rac{Q_{ ext{cool}} imes 1,000}{ ext{SEER2}} = rac{16.4 imes 1,000}{15.2} = mathbf{1,080 ext{ kWh}} quad ($173)$$
$$ ext{Gas Furnace Fuel} = rac{Q_{ ext{heat}} imes 10}{ ext{AFUE}} = rac{105.4 imes 10}{0.96} = mathbf{1,081 ext{ Therms}} quad ($1,459)$$
5 Fatal Traps & Degree-Day Modeling Pitfalls
⚠️ Trap 1: The 65°F Universal Base Assumption Blindspot
Base 65°F was standardized in the 1930s when homes had uninsulated walls, incandescent lighting, and few internal electronics. In modern, highly-insulated homes with massive internal heat gains (computers, refrigerators, televisions, cooking, occupants), the actual thermal balance point where indoor heating is needed drops to 55°F to 60°F. In commercial office buildings with high plug-loads, cooling is required even when outdoor temperatures are 50°F! Modeling modern buildings with standard Base 65°F drastically overestimates heating fuel and underestimates cooling electricity.
⚠️ Trap 2: Degree Days Completely Ignore Latent Humidity Loads
Cooling degree days are derived strictly from dry-bulb temperatures. In humid climates like Houston, Miami, or New Orleans, 30% to 45% of total air conditioning electrical power is expended condensing airborne moisture (latent heat of condensation) rather than lowering air temperature. Two summer days with an identical 82°F dry-bulb mean will yield the exact same 17 CDD, but an 80% relative humidity day will consume twice the air conditioning kilowatt-hours of a dry 30% RH desert day.
⚠️ Trap 3: Thermostat Setpoint Drift & The 3% Per Degree Exponential Rule
The degree-day formula assumes an occupant maintains an unyielding constant 68°F winter and 75°F summer setpoint. In practice, bumping a winter thermostat up by just 2°F (from 68°F to 70°F) or dropping a summer thermostat down by 2°F (from 74°F to 72°F) increases annual heating/cooling energy consumption by 6% to 10%. Failing to calibrate degree days to actual user thermostat setpoints produces large discrepancies between modeled predictions and actual utility bills.
⚠️ Trap 4: Solar Radiation & Fenestration SHGC Weather Distortion
Degree days treat cloudy overcast days and clear, cloudless sunny days identically if mean temperatures match. However, unshaded south-facing and west-facing windows admit immense solar heat gains (up to 200 BTU/(hr·sq ft) of glass). On a crisp 40°F sunny winter day, passive solar heat gain can completely heat a house with zero furnace operation; on an identical 40°F overcast day, the furnace runs continuously.
⚠️ Trap 5: Heat Pump Auxiliary Strip Heat Penalty in Extreme Cold
Standard air-source heat pumps suffer significant COP degradation below 25°F. When ambient temperatures plunge below the thermal balance point (typically 15°F to 25°F), the system engages emergency electric resistance heat strips (10 to 15 kW of toaster-wire coils with COP = 1.0). If a degree-day model assumes a constant HSPF2 efficiency across all 6,000+ heating degree days without modeling the auxiliary strip transition, winter electric heating costs will be underestimated by 30% to 50%.