About Degree Days & Energy Estimate Calculator
The degree-days calculator computes heating degree-days (HDD) and cooling degree-days (CDD) from a base (balance) temperature and a series of mean outdoor temperatures, then estimates the heating and cooling energy a building needs over the period. Degree-days summarise how far, and for how long, the outdoor temperature sits below the base (heating) or above it (cooling).
Enter the base temperature and either a list of daily mean temperatures, a 12-month table of monthly means with day counts, or HDD/CDD values directly. Add the building heat-loss coefficient UA (W/K) and the system efficiency or heat-pump COP to convert the degree-days into estimated energy in kWh, and optionally an energy price for a cost figure.
How It Works
- Choose the data source: daily mean temperatures, a 12-month table, or HDD/CDD entered directly.
- Set the base (balance) temperature -- often 18 C / 65 F for heating, depending on the building.
- The tool sums max(0, Tbase - Tmean) for HDD and max(0, Tmean - Tbase) for CDD, weighting each month by its number of days.
- If you supply the building UA and system efficiency, it estimates energy as DD x 24 x UA / 1000 / efficiency (kWh), and applies an energy price for cost.
Worked Example
A site has a 30-day month with a mean outdoor temperature of 10 C and a base temperature of 18 C. Each day contributes 18 - 10 = 8 heating degree-days, so HDD = 8 x 30 = 240 K.day, with CDD = 0. For a building with UA = 200 W/K and a boiler at 90 percent efficiency, the heating energy is 240 x 24 x 200 / 1000 / 0.9 = 1280 kWh. At an energy price of 0.15 per kWh the heating cost is 1280 x 0.15 = 192.
Formulas
- Heating degree-days
HDD = sum over days of max(0, Tbase - Tmean_day)- Cooling degree-days
CDD = sum over days of max(0, Tmean_day - Tbase)- Monthly degree-days
HDD = sum over months of max(0, Tbase - Tmean_month) * days_in_month- Degree-day energy estimate
Energy (kWh) = DD * 24 * UA / 1000 / efficiency
Standards & References
- ASHRAE degree-day method (ASHRAE Handbook -- Fundamentals)
- CIBSE TM41 Degree-days: theory and application
- CIBSE Guide A Environmental Design
- ISO 15927-6 (degree-day data from temperature records)
Frequently Asked Questions
What base temperature should I use?
The base or balance temperature is the outdoor temperature at which a building needs no heating. A common default is 15.5 C (60 F) or 18 C, but the right value depends on internal gains and insulation. Well-insulated buildings with high internal gains have a lower heating base temperature.
How accurate is the degree-day energy estimate?
The steady-state degree-day method is a screening estimate, good for comparing periods, tracking consumption and rough sizing. It assumes a constant UA and ignores solar gains, intermittent operation and dynamic effects, so detailed work needs hourly simulation. It is widely used for energy monitoring and targeting.
Can I model a heat pump?
Yes. Enter the seasonal coefficient of performance (COP) as the efficiency. A COP of 3 means the building draws one third of the heat demand as electricity, so the energy figure is the demand divided by 3 rather than divided by a boiler efficiency below 1.
Why are heating and cooling degree-days computed separately?
Heating degree-days measure how cold it is below the base temperature, driving heating energy; cooling degree-days measure how warm it is above the base, driving cooling energy. A single day contributes to only one of them (or neither, if it sits exactly at the base temperature).