11.2 Performance Method Changes Made In 2013

For the 2013 standards the Nonresidential Alternative Calculation Method (ACM) 'manual has been divided into two parts.  The Nonresidential ACM Approval Manual describes the application and approval process for submitted compliance software.  This document is adopted as part of the standards rule making process.  The Nonresidential ACM Reference Manual is approved by the Commission.  This document includes explanations of the rules that all compliance software programs 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.  The Nonresidential ACM Reference Manual is approved by the Energy Commission and, like the residential and nonresidential compliance manuals can be updated to allow for corrections and enhancements during the 2013 standards cycle.

11.2.1  Performance Concepts

The Warren-Alquist Act requires “performance standards,” which establish an energy budget for the building in terms of energy consumption per ft² 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. For compliance purposes, it also approves the use of privately developed computer programs as alternatives to the public domain computer program. The public domain computer program and the Commission-approved privately developed programs are officially called alternative calculation methods (ACMs). It's easiest to talk about these programs as "compliance software," and we will use that term throughout this 'manual.

11.2.2  Minimum Capabilities

Approved programs 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 and convective heat gain and heat loss through walls, roof/ceilings, doors, floors, windows, and skylights.

      Solar radiant heat gain from windows, skylights, and opaque surfaces.

      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.

11.2.3  California Energy Commission Approval







11.2.3   Alternative calculation methods

Alternative calculation methods must be approved by the California Energy Commission. Approval involves the demonstration of minimum modeling capabilities required input and output, and adequate user documentation. The program must be able to:

Automatically calculate the custom energy budget of the standard design.

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

Print the appropriate standardized compliance forms with the required information and format if a proposed building complies. Other reports that do not resemble forms may be printed for non-complying buildings.   Input and output requirements

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

11.2.4  Time Dependent Valuation (TDV)

Beginning with the 2005 Standards, the metric or “currency” for assessing building performance is time dependent valued (TDV) energy. TDV energy replaced source energy, which had been the compliance metric since the California Energy Commission first adopted 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 day- lighting, that are more effective during peak periods.

Reference Joint Appendix JA3 provides more information on TDV energy, and detailed TDV data is available from the California 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.”

11.2.4   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 acceptable or even necessary. In those instances, the compliance software user must exercise proper judgment in evaluating 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 professional judgment has been applied.   Two questions may be asked

Two questions may be asked in order to resolve whether good 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, 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. 

Figure 11-1 – Annual TDV Energy Use Summary (Sample of PERF-1)
[Note: Figure may vary due to different compliance software versions].


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.