Dear Tom,

What is meant by the term heating degree day units?

—Frank Lizak, Porter, Ind.

Dear Frank,

With the cool days of September rapidly approaching, it is a good time to revisit the topic of heating degree days. Heating degree day units assess heating usage. They are easy to calculate using a base temperature of 65 degrees. Each degree that a day's average temperature falls below 65 is one heating degree day. A day with an average temperature of 40 would log 25 heating degree day units. Heating degree units are tallied on a seasonal basis running from July 1-June 30, giving a one-number snapshot of seasonal heating costs and allowing quick season-to season comparisons. In a similar manner, cooling degrees units are calculated for each degree the day's average temperature exceeds 65, with the cooling seasonal totals kept on a calendar-year basis.