Calculate cooling degree days for energy use.
Cooling degree days turn a temperature record into a measure of how much cooling a building actually needed. Each day contributes the amount by which its mean temperature exceeded a balance point — usually 18 C, the outdoor temperature at which internal gains from people, lighting and appliances are enough to keep a building comfortable without mechanical help. Days at or below the balance point contribute nothing, which is why cooling degree days cannot be negative. Because air-conditioning energy is roughly proportional to the temperature difference it has to bridge, degree days scale almost linearly with consumption, making them the standard tool for weather-normalising energy bills and comparing one summer with another.
Cooling degree days
Mean temperature = (daily high + daily low) / 2; cooling degree days per day = max(0, mean temperature - balance point); total = per-day figure x days in the period
Because the balance point is an outdoor temperature, not a thermostat setting. Body heat, lighting, cooking and equipment typically lift the inside of a building three or four degrees above ambient, so cooling starts to be needed once outside air passes about 18 C.
Only with a coefficient for your own building. Divide a past period's measured cooling kWh by its degree days to get kWh per CDD, then apply that to any other period — this is exactly how weather-normalised energy comparisons are made.
It is the standard method and works well for typical days. It slightly underestimates the load when a day spikes far above its mean, so hourly integration is preferred for detailed engineering work.