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Cooling Degree Days

Free Cooling Degree Days calculator with clear step-by-step results.

About Cooling Degree Days

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.

Formulas

Cooling degree days

CDD = max(0, mean outdoor temp - base temp) x days

Electricity

kWh = UA x CDD x 24 / 1000 / COP

Worked example

With the default values below, Cooling Degree Days returns Cooling degree days of 240.0. Change any input above and every figure here is recalculated in your browser.

Inputs used

Mean outdoor temperatureaccepts -20–50 C
26 C
Base temperatureaccepts 5–30 C
18 C
Number of daysaccepts 1–366
30
Building heat loss coefficientminimum 1 W/K
250 W/K
Cooling system COPaccepts 1–8
3.2

Results

Cooling degree days
240.0 Kd
Degree days per day
8.00 Kd
Cooling energy delivered
1,440.0 kWh
Electricity consumed
450.0 kWh

Step by step

  1. 1Values used
  2. 2Cooling degree days
  3. 3Electricity
  4. 4Cooling degree days= 240.0 Kd
  5. 5Degree days per day= 8.00 Kd
  6. 6Cooling energy delivered= 1,440.0 kWh
  7. 7Electricity consumed= 450.0 kWh

Assumptions and limits

  • The result is derived only from the 5 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why is the base temperature below room temperature?

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.

Can I use monthly averages?

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.

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Cooling Degree Days – Calculate Cooling degree days