Graddags

Graddags, often referred to as degree days in English, is a meteorological index used to estimate the energy needed for heating or cooling a building based on outdoor temperatures. It quantifies the deviation of the outside temperature from a chosen reference indoor temperature.

Calculation and definition: For a given day, the graddag value is usually defined as the positive difference between a base temperature and the day's mean outdoor temperature, i.e., graddag = max(0, T_base − T_mean). The base temperature (T_base) is a reference comfort temperature, commonly 18°C (65°F), though it varies by climate, building type, and regulation. If the mean temperature is higher than the base, the day contributes zero graddags. Cumulative graddags over a period summarize the heating demand; some systems also track cooling degree days with analogous formulas for cooling needs.

Uses and applications: Graddags are used to normalize and compare energy consumption across different weather years, forecast energy demand, and model heating requirements in building design and performance analyses. They are published by meteorological services and widely adopted in Nordic countries and other temperate regions as a practical, simple proxy for temperature-driven energy consumption.

Regional variations and limitations: The base temperature and interpretation of graddags can differ by country and sector, reflecting local climate and building characteristics. Graddags do not account for humidity, solar gains, occupancy, or insulation quality, so they are a simplified tool often used in conjunction with more detailed energy analyses.