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    HVAC / Energy

    Building Energy Estimate

    Annual heating and cooling energy from the degree-day method: envelope conductance times degree-days, adjusted for internal gains, divided by plant efficiency. It is the quick estimate an engineer runs before a full hourly model, not a substitute for one.

    Annual energy7.557e+4 kWhHeating6.800e+4 kWhCooling7570 kWh
    Annual energy
    7.557e+4 kWh
    76 kWh/m²·yr
    Heating
    6.800e+4 kWh
    90% of the total
    Cooling
    7570 kWh
    COP 3.5
    Annual cost
    $12,091
    At $0.16/kWh
    Heating energy against degree-days
    Cooling is held at the current CDD.
    Heating energy against degree-days
    HDDHeating (kWh)Total cost ($)
    10002.720e+45563
    15004.080e+47739
    20005.440e+49915
    25006.800e+41.209e+4
    30008.160e+41.427e+4
    40001.088e+51.862e+4

    A degree-day estimate ignores part-load performance and solar gains.

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    Choose a quick result view, a scaled comparison, or the complete diagnostic set.
    Annual energy against heating degree-days
    Linear in HDD, which is the assumption the method makes.
    Annual energy against heating degree-days0.0002.607e+45.213e+47.820e+41.043e+51.303e+5100010001500150020002000250025003000300040004000kWh
    Canonical equations
    The exact relationships implemented by this engine.

    UA·HDD·24 / (1000·η) × (1 − gains) · Degree-day method.

    UA·CDD·24 / (1000·COP) × (1 + gains) · Internal gains raise the cooling load.

    annual kWh / floor area · Compare with the benchmark for the building type.

    engineering methodology

    A clear path from inputs to results.

    Annual heating and cooling energy from the degree-day method: envelope conductance times degree-days, adjusted for internal gains, divided by plant efficiency. It is the quick estimate an engineer runs before a full hourly model, not a substitute for one.

    Step-by-step methodWorked exampleAssumptions explained

    How to use

    1. Set floor area (m²).
    2. Set envelope ua (w/k).
    3. Set heating degree-days (k·day).
    4. Set cooling degree-days (k·day).
    5. Read the headline metrics, then check the table for the detail.

    The formula

    E=UA HDD 241000 η(1−g)+UA CDD 241000 COP(1+g)E = \frac{UA\,HDD\,24}{1000\,\eta}(1-g) + \frac{UA\,CDD\,24}{1000\,COP}(1+g)

    Reference: ASHRAE Fundamentals, degree-day method.

    Worked example

    At the default inputs

    Inputs
    • Floor area (m²): 1000; Envelope UA (W/K): 1200; Heating degree-days (K·day): 2500; Cooling degree-days (K·day): 800; Internal-gain offset (%): 15; Heating efficiency (%): 90; Cooling COP: 3.5; Energy price ($/kWh): 0.16
    Calculation path
    1. 1

      Envelope loss → 7.200e+4 Plant efficiency → 6.800e+4 Cost → $12,091

    Result

    Annual energy: 7.557e+4 kWh · Heating: 6.800e+4 kWh · Cooling: 7570 kWh

    Interpreting the result

    The envelope loses about 6.800e+4 kWh of heating energy and needs 7570 kWh of cooling, for 76 kWh/m²·yr and $12,091 a year.

    Assumptions & limitations

    • One zone with constant UA.
    • Plant efficiency and COP are constant.
    • Internal gains are a fixed percentage offset.

    Frequently asked questions

    Is this an hourly model?

    No. It is a degree-day estimate; a full model needs bin or hourly weather data.

    Why does UA matter more than floor area?

    Envelope loss scales with UA, not floor area.

    Methodology last reviewed 2026-10-23. Results are informational and not financial or engineering advice — see our disclaimer.