Free Cooling Degree Days calculator with clear step-by-step results.
Cooling degree days accumulate the amount by which mean outdoor temperature exceeds a base temperature, which is the point above which a building needs mechanical cooling. Multiplying by the building's heat loss coefficient and 24 hours converts degree days into a cooling load, and dividing by the system COP gives electricity.
Cooling degree days
CDD = max(0, mean outdoor temp - base temp) x days
Electricity
kWh = UA x CDD x 24 / 1000 / COP
Occupants, lighting and appliances contribute internal heat, so a building typically needs no cooling until outdoors passes roughly 18 C. Well-insulated buildings with high internal gains use a lower base still.
For a rough figure, yes, but averaging hides days that swing above and below the base and understates degree days. Daily means give a materially better result.