11.2      Performance Method Description

The Nonresidential Alternative Calculation Method (ACM) Approval Manual describes the application and approval process for submitted compliance software. The Nonresidential ACM Approval Manual is adopted as part of the Energy Standards rule making process. The Nonresidential ACM Reference Manual is approved by the California Energy Commission (Energy Commission) and includes explanations of the instructions that all compliance software programs must use to model the energy performance of the Proposed Design Building and the Standard Design Building. The reference manual also includes an explanation of the reference method and certification tests used by the Energy Commission to approve compliance software tools. Since the Nonresidential ACM Reference Manual is approved by the Energy Commission (just like the residential and nonresidential compliance manuals), it can be updated from time to time to allow for corrections and enhancements during the 2019 Energy Standards cycle.

11.2.1        Performance Concepts

The Warren-Alquist Act requires “performance standards” that establish an energy budget for the building in terms of energy consumption per square foot of floor space. This requires a complex calculation of the estimated energy consumption of the building and the calculation is only suited for a computer. The Energy Commission has developed a public domain computer program to do these calculations known as California Building Energy Code Compliance (CBECC). For compliance purposes, The Act also approves the use of privately developed computer programs as alternatives to the public domain computer program. The public domain computer program and the Energy Commission-approved privately developed programs are officially called alternative calculation methods. It is easiest to refer to these programs as "compliance software," which will be the term used throughout this manual.

11.2.2        Minimum Capabilities

Compliance software must simulate or model the thermal behavior of buildings including envelope surfaces, lighting, space conditioning and service water heating systems. The calculations take into account:

§ Conductive, convective, and radiative heat gain and loss through walls, roof/ceilings, doors, floors, windows, and skylights.
§ Solar radiant heat gain from windows and skylights.
§ Heat storage effects of different types of thermal mass.
§ Building operating schedules for people, lighting, equipment, and ventilation.
§ Space conditioning system operation including equipment part load performance.
§ Covered process mechanical equipment (kitchens, laboratories, parking garages, etc.).

11.2.3        California Energy Commission Approval    Alternative Calculation Methods (Compliance Software)

Compliance software must be approved by the Energy Commission. Approval involves the demonstration of minimum modeling capabilities, required input and output, and adequate user documentation. The compliance software must be able to:

1.  Automatically calculate the energy budget of the standard design.

2.  Calculate the energy budget of the proposed design in accordance with specific fixed and restricted inputs.

3.  Print the appropriate standardized compliance documents with the required information and format when a proposed building complies. Other reports that do not resemble documents may be printed for buildings that do not comply.    Input and Output Requirements

Input and output requirements and modeling capabilities are tested by using the compliance software to calculate the energy use of certain prototype buildings under specific conditions. These results are compared with the results from a reference computer program, which is EnergyPlus. This is explained in detail in the Nonresidential ACM Reference Manual.

11.2.4        Time Dependent Valuation (TDV)

Beginning with the 2005 Energy Standards, the metric or “currency” for assessing building performance is time dependent valued (TDV) energy. TDV energy replaced source energy that had been the compliance metric since the Energy Commission first adopted the Energy Standards in 1978.

As the name implies, TDV values energy differently depending on the day of the year and hour of the day that a specific type of energy is used. This means that electricity saved on a hot summer afternoon will be worth more in the compliance process than the same amount of electricity saved on a winter morning. The value assigned to energy savings through TDV more closely reflects the market for electricity, gas, propane and other energy sources and provides incentives for measures, such as thermal storage or advanced daylighting that are more effective during peak periods.

Reference Appendix JA3 provides more information on TDV energy and detailed TDV data is available from the Energy Commission upon request. §100.2 states: “TDV multipliers for propane shall be used for all energy obtained from depletable sources other than electricity and natural gas.”  A sample of the TDV values are shown below in Figure 11-1.

Figure 11-1: Annual TDV Energy Use Summary (Sample from NRCC-PRF-01-E)    Professional Judgment

Certain modeling techniques and compliance assumptions applied to the proposed design are fixed or restricted. That is, there is little or no freedom to choose input values for energy compliance modeling purposes. However, there are other aspects of energy modeling where some professional judgment may be necessary. In those instances, the compliance software user must decide whether a given input is appropriate.

Enforcement agencies have discretion to question a particular input if the permit applicant cannot substantiate the value with supporting documentation or cannot demonstrate that appropriate judgment has been applied.

Two questions may be asked in order to resolve whether appropriate judgment has been applied correctly in any particular case:

1.   Is a simplified input or assumption appropriate for a specific case? If simplification reduces the predicted energy use of the proposed building or reduces the compliance margin when compared to a more explicit and detailed modeling assumption, then the simplification is not acceptable. That is, simplification must reflect the same or higher energy use than a more detailed model and reflect the same or lower compliance margin when comparing the standard and proposed TDV energy.

2.   Is the approach or assumption used in modeling the proposed design consistent with the approach or assumption used by the compliance software when generating the standard design energy budget? One must always model the proposed design using the same assumption and/or technique used by the compliance software when calculating the energy budget unless drawings and specifications indicate specific differences that warrant energy compliance credits or penalties.

Any unusual modeling approach, assumption, or input value should be documented with published data and, when applicable, should conform to standard engineering practice.


Example 11-1


Three different sized windows in the same wall of a new one-story office building are designed without exterior shading, and they have the exact same NFRC-rated U-factors and SHGC values.  Is it acceptable professional judgment to simplify the computer model by adding the areas of the three windows together and inputting them as a single fenestration area?


Yes. The compliance software will produce the same energy results whether or not the windows are modeled individually or together as one area because the orientation, fenestration U-factors and SHGC values of the windows are identical. However, if overhangs and side-fins are modeled, the correct geometry of fixed shades must be modeled for each window.