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    Air systems / economiser

    Economiser saving calculator

    An air-side economiser replaces chiller energy with outdoor air whenever conditions allow. The saving is real but not free: the fan has to move that air. This calculator nets the two and shows when the economiser stops being worth it.

    Load100 kWCOP3.2VerdictWorth it
    Chiller energy avoided
    6.250e+4 kWh
    2000 free-cooling hours
    Net energy saved
    5.250e+4 kWh
    After 1.000e+4 kWh of fan energy
    Annual cost saving
    $7,875
    At $15.00 per 100 kWh
    Free-cooling share
    2283.1%
    Of the 8,760-hour year
    Saving build-up
    Chiller energy avoided less the fan energy the economiser adds.
    5.250e+4 kWh
    Saving build-up
    TermValue
    Coil load100 kW
    Free-cooling hours2000 h
    Chiller COP3.2
    Energy avoided6.250e+4 kWh
    Fan penalty1.000e+4 kWh
    Net saving5.250e+4 kWh
    Cost saving$7,875

    A high fan penalty can erase the benefit — the economiser must not add more fan energy than the chiller it displaces.

    Inspect the model
    Choose a quick result view, a scaled comparison, or the complete diagnostic set.
    Saving against free-cooling hours
    Linear, until the fan penalty dominates at very low load.
    Saving against free-cooling hours$0$3.5k$7.1k$10.6k$14.1k$17.6k004004008008001200120016001600200020002400240028002800320032003600360040004000Cost saving
    Energy avoided against fan penalty
    The net figure is what shows on the bill.
    Energy avoided against fan penalty-1.580e+42470.002.074e+43.901e+45.728e+47.555e+4Chiller avoidedChiller avoidedFan penaltyFan penaltyNetNetkWh
    Canonical equations
    The exact relationships implemented by this engine.

    Q_cool × hours / COP · Chiller electricity displaced.

    fan kW × hours · Extra air-side energy to move outdoor air.

    avoided − fan · The number that decides the business case.

    engineering methodology

    A clear path from inputs to results.

    An air-side economiser replaces chiller energy with outdoor air whenever conditions allow. The saving is real but not free: the fan has to move that air. This calculator nets the two and shows when the economiser stops being worth it.

    Step-by-step methodWorked exampleAssumptions explained

    How to use

    1. Enter the coil load.
    2. Set the free-cooling hours available at your climate.
    3. Enter the chiller COP and the fan penalty.
    4. Read the net saving and its value.

    The formula

    Enet=Q⋅hCOP−Pfan⋅hE_{net} = \frac{Q \cdot h}{COP} - P_{fan} \cdot h

    Reference: ASHRAE air-side economiser analysis; screening estimate.

    Worked example

    100 kW coil, 2,000 free-cooling hours, COP 3.2

    Inputs
    • Coil load 100 kW.
    • 2,000 free-cooling hours.
    • COP 3.2, fan penalty 5 kW.
    • Tariff $0.15/kWh.
    Calculation path
    1. 1

      Chiller energy avoided: 100 × 2,000 / 3.2 = 62,500 kWh.

    2. 2

      Fan penalty: 5 × 2,000 = 10,000 kWh.

    3. 3

      Net saving: 52,500 kWh.

    4. 4

      Value: $7,875 a year.

    5. 5

      Free cooling covers 22.8% of the year.

    Result

    52,500 kWh net · $7,875 a year.

    Interpreting the result

    The fan penalty is the term that decides marginal cases. A badly designed economiser with long, restrictive outdoor-air ducts can consume most of the chiller energy it displaces, which is why the net figure — not the gross one — belongs in the business case.

    Assumptions & limitations

    • Screening estimate with fixed free-cooling hours.
    • Constant chiller COP and fan power.
    • No changeover control losses or damper leakage.
    • Tariff excludes demand charges.

    Frequently asked questions

    How many free-cooling hours do I have?

    It depends on the changeover enthalpy and the climate; use bin data from a weather file.

    What about humidity?

    An enthalpy economiser accounts for it; a dry-bulb changeover can import moisture.

    Does this apply to a data centre?

    Yes, and they are the classic case — high internal load, year-round cooling.

    Methodology last reviewed 2026-10-21. Results are informational and not financial or engineering advice — see our disclaimer.