graddag

Graddag, or degree day, is a meteorological and energy planning index that quantifies how much outdoor temperature departs from a reference temperature, indicating potential heating or cooling demand for buildings. It is commonly used to estimate energy needs and to adjust energy use for weather variations. Graddag can be applied over daily, monthly, or annual periods to assess how climate affects consumption.

For heating, the heating degree day (HDD) is calculated daily as max(0, T_base − T_mean), where T_mean is the day's average outdoor temperature and T_base is a chosen reference temperature. For cooling, the cooling degree day (CDD) is calculated as max(0, T_mean − T_base). Daily HDDs or CDDs are summed over a period to produce total HDD or CDD values. The base temperature T_base is commonly 18°C (65°F) but varies by region, building type, and usage patterns. For example, if T_mean is 10°C and T_base is 18°C, the HDD for that day is 8 degree-days.

Applications and use: Graddag is used by energy suppliers, building designers, and researchers to forecast heating or cooling needs, compare climates across locations, and normalize energy consumption for weather differences. It is often reported as monthly or annual totals and is a common tool in weather normalization and energy-performance assessments in Nordic countries and elsewhere.

Limitations: The base temperature choice influences results, and graddag is a simplified proxy that does not account for humidity, wind, solar gains, occupancy, or insulation. Consequently, it is best for trend analysis and rough planning rather than precise energy forecasting.