Power planners rarely talk about temperature directly. They convert it into degree days, a measure built to match how buildings actually consume energy.
A degree day counts distance from a base temperature
A cooling degree day accumulates when the day's average temperature exceeds a base value, traditionally set near the point where buildings need neither heating nor cooling.
The size of the accumulation reflects how far above the base the day ran. A day well above base contributes several times what a marginally warm day contributes.
Heating degree days work the same way below the base, which lets a single framework describe winter and summer demand in comparable units.
Buildings respond to the gap, not the peak
Air conditioning works to hold an interior temperature against heat leaking in from outside. The rate of that leak scales with the indoor-outdoor difference.
A hotter day therefore demands more energy per hour rather than merely more hours of operation, which is why the measure multiplies degrees by time.
Because the relationship is close to proportional over normal ranges, degree days correlate with consumption far better than daily maximum temperature alone.
Consecutive hot days compound the load
Building materials store heat. After several hot days the mass of a structure is warm throughout and equipment starts each morning already behind.
Nights that fail to cool prevent the overnight reset, so a run of warm minimums raises consumption even when afternoon highs hold steady.
That is why utilities watch multi-day heat events rather than single hot afternoons when assessing whether a grid will be stressed.
Humidity adds load the temperature does not show
Cooling equipment removes moisture as well as heat, and condensing water vapor consumes a substantial share of the energy an air conditioner uses.
In humid regions the same degree day count therefore corresponds to higher consumption than in dry regions, so some analyses substitute humidity-aware measures.
The correction matters most in the Southeast and along the Gulf, where summer dew points keep latent load high through the night.
The base temperature is a convention, not a law
The traditional base reflects older building stock and older assumptions about insulation, occupancy and equipment efficiency.
Better insulated buildings with more internal heat from lighting and electronics begin cooling at outdoor temperatures below the conventional base.
Utilities increasingly fit their own base values to observed consumption in their territory, which produces a measure that describes their customers rather than a national average.