Estimates are that most people use their furnace when the mean daily temperature drops below 65F. Heating degree-days are determined by subtracting the mean temperature for the day from the reference temperature. On days when the mean temperature is above the reference temperature, there are no heating degree-days. Therefore, the lower the average daily temperature, the more heating degree-days and the greater the consumption of fuel. Cooling degree-days are used during warm weather to estimate the energy needed to cool indoor air to a comfortable temperature. Mean daily temperature is converted to cooling degree-days by subtracting the reference temperature from the mean.
Higher values indicate warm weather and result in a high power production for cooling.
Knowledge of the number of cooling degree-days in an area in the summer gives power companies a way of predicting the energy demand during peak energy periods. A summary of heating and cooling degree-days can give a practical indication of the energy needed over the year. Growing degree-days are used as a guide to planting and for determining the approximate dates when a crop will be ready for harvesting. If the base temperature for beans was 50F, then the beans would accumulate 30 growing degree-days. Theoretically, beans can be harvested when it accumulates a total of 1200 growing degree-days.