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Energy Analysis Tools

by Richard Paradis, P.E., BSCP
National Institute of Building Sciences

Last updated: 06-10-2010


Building thermal performance calculations are made for two primary reasons. They are made to size and select mechanical equipment or to predict the annual energy consumption of a structure. While these two tasks are not mutually exclusive, and some programs can handle both tasks, they do tend to be conducted in isolation from each other.

  • Sizing programs are primarily designed to calculate peak hourly loads during the heating and cooling seasons. Almost all buildings of any complexity have a sizing analysis of some kind run by an architect, engineer, or mechanical contractor. Most sizing programs are based on consensus procedures and algorithms established by ASHRAE, but many are proprietary products distributed or sold by equipment manufacturers.
  • Energy programs are primarily designed to predict the annual energy consumed by a structure in terms of BTUs, dollars, or pollution avoidance. In the past, few buildings benefited from energy analysis. Today energy analysis tools are becoming more common and are being applied earlier in the design process.

To decide what computer-based, energy analysis tool is best for your project, it is important to have a basic understanding of how these tools operate.


A. Calculating Annual Energy Consumption

The flowchart diagrammed in Fig. A indicates the steps that must be followed to fully estimate project energy costs.

Energy costs flow chart. Step One, determine thermal zones; Step Two, calculate loads for each zone; Step Three, Select HVAC system(s); Step Four, Calculate hourly energy consumption; Step Five, Input utility & energy rate information; Step Six, calculate energy costs.

Fig. A. Flowchart to determine energy costs

Step One. Determine the Number of Thermal Zones

A "zone" is a segment of a building with similar thermal requirements serviced by the same mechanical equipment and controls. The number of thermal zones will vary depending on many factors including the building use, size, and shape. For example, a single family house or free-standing branch bank may have only one or two zones while a large office building may have over one hundred.

Step Two. Calculate Loads for Each Zone

A "load" is the required, hourly rate of heat removal in summer (or heat supply in winter) necessary to keep a building comfortable. In step two, the annual, peak hourly heating and cooling loads for each zone must be calculated.

Step Three. Select HVAC Systems

Based on the peak loads calculated in step two, size and select building mechanical equipment. For comprehensive simulations of multi-zone structures, thermal interactions between zones must be taken into account (such as the mixing that occurs in water-loop heat pump systems).

Step Four. Calculate Hourly Energy Consumption

Calculate the loads placed on the selected equipment for each hour of a Typical Meteorological Year (TMY) and determine the amount of energy required by the equipment—based on system efficiencies and part load curves—to meet these loads.

Step Five. Input Electric Utility and Fuel Rate Information

For the specific building construction site, input energy rate information including electric peak demand charges.

Step Six. Calculate Energy Costs

Calculate the cost of the fuel consumed for each hour of the year. Annual performance is calculated by summing the hourly results for all 8,760 hours of the year.

Some software programs are designed to excel at one or two steps in this process while others tackle the whole problem comprehensively. Other tools use simplified methods to expedite input requirements or minimize run time, while still others are more detailed and precise.

For example, HVAC equipment manufacturers have emphasized the development of software that addresses Steps Two and Three quite well. But these same programs often do not handle the interaction between strategies (such as daylighting and energy efficient lighting) necessary to accurately model energy consumption in Step Four.

Boundaries between energy analysis tools are beginning to blur as developers in different industries are converging to produce software that is more graphic, easier to use, and capable of greater accuracy. Nevertheless, important distinctions between software programs still remain and will do so for the foreseeable future. What is unquestionably true is that with today's powerful personal computers it is no longer necessary to compromise—we can have both speed and accuracy.

B. Match Tool to Task

Not only do energy analysis software programs have varying levels of accuracy; they are also intended to be used at different phases of the design process; and require very different levels of effort and cost. For example, some tools have been designed to provide immediate feedback to the designer or project manager during the earliest phases of a project while others such as DOE-2 or BLAST, require more input time and detail. Consequently, they are generally reserved for later in the design process when many architectural decisions have already been finalized. EnergyPlus is a newer building energy simulation program for modeling building heating, cooling, lighting, ventilating, and other energy flows—building on the most popular features and capabilities of BLAST and DOE-2. Most energy analysis tools can be classified as being one of four generic types. Note: The software examples listed are meant to be indicative, not exhaustive.

C. Screening Tools

Screening Tools are designed to evaluate project viability during the earliest stages of programming and often include some economic analysis capability. They also tend to be correlations, rather than full hourly simulations.

In a correlation-based program, daily, monthly, or seasonal building performance is computed by comparing, or correlating, the performance of the building in question against predetermined equations (or curves) that predict the performance of the building based on key thermal characteristics and climate information. Correlation programs generally run quickly because they demand a minimum of computation, but this speed is at the expense of some accuracy. Also because of their relative simplicity, correlation programs are not able to evaluate the important trade-offs between certain interactive energy strategies such as daylighting and heating or thermal mass and cooling. The following are some examples of screening tools.

FEDS (The Facility Energy Decision System)
Provides a comprehensive method for quickly and objectively identifying energy improvements that offer maximum savings. It is an easy to use tool for identifying retrofits, selecting minimum life cycle costs, determining payback, and enabling users to prioritize options. The FEDS system allows data input to range from minimal to extremely detailed.

Table 2: FEDS Version 5.0.1 Key Characteristics

Key StrengthsKey Weaknesses
  • Technology and Fuel Independence
  • Life Cycle Cost Optimization
  • Peak Tracking
  • Alternate Financing Analysis
  • Optimizes retrofit opportunities
  • Cannot evaluate interaction of some strategies
  • Not a buildings design tool

D. Architectural Design Tools

Architectural Design Tools are intended to evaluate the relative importance of design decisions such as building orientation, glazing, and daylighting.

Residential and Small Commercial

Energy Scheming
A design tool to help the user create an energy-efficient building; provides loads analysis for 24 hours for each of 4 seasonal evaluation days. Input is graphical and intuitive and is designed to support the earliest phases of design, where energy considerations can have the most impact.

Table 4: Energy Scheming Key Characteristics

Key StrengthsKey Weaknesses
  • Graphic input
  • Supports visual thinking
  • Educational tool
  • Apple only
  • Uses highly simplified algorithms; single zone; does not size or calculate HVAC

Building Design Advisor
The BDA is a computer program that supports the concurrent, integrated use of multiple simulation tools and databases, through a single, object-based representation of building components and systems. BDA (Building Design Advisor) acts as a data manager and process controller, allowing building designers to benefit from the capabilities of multiple analysis and visualization tools throughout the building design process. BDA is implemented as a Windows-based application and is linked to a Schematic Graphic Editor and two simplified simulation tools, one for daylight and one for energy analyses.

Table 5: Building Design Advisor Version 3.1 Key Characteristics

Key StrengthsKey Weaknesses
  • Graphic input
  • Does not require in depth knowledge to use linked tools for energy and daylighting
  • Limited database of options for building components and systems

E. Engineering Design Tools/Load Calculation & HVAC Sizing Tools

Engineering Design Tools/Load Calculation and HVAC Sizing Tools are designed primarily to size and help select equipment such as boilers, furnaces, or chillers. Many load calculation and HVAC sizing tools also include the ability to perform annual energy simulations. Some of the sizing tools are proprietary software products created and distributed by equipment manufacturers.

TRACE Load 700
Combines the power of the building and load design portions of TRACE 600 with the simplicity of a Windows-based operating environment. TRACE Load 700 uses ASHRAE-standard algorithms and enables non-sequential data entry that encourages "what if" analysis. You can edit building construction details in any order and easily change the building model as the design progresses. The extensive predefined (but editable) libraries and templates of construction materials and building load information increase the speed and accuracy of the modeling process. You can export the completed project file to TRACE 700 for a detailed energy analysis.

Table 6: Trace Load 700 Key Characteristics

Key StrengthsKey Weaknesses
  • Intuitive Windows interface
  • Simplified input methods
  • Models more than 25 types of air distribution systems
  • Requires TRACE 700 to perform energy and cost analyses

HAP (Hourly Analysis Program)
A system design tool and an energy simulation tool in one package. Uses a Windows-based graphical user interface and 32-bit software. HAP's design module uses a system-based approach that tailors sizing procedures and reports to the specific type of system being considered. Central AHUs, packaged rooftop units, split systems, fan coils, and PTACs can be designed, as can CAV, VAV, single- and multiple-zone systems. The ASHRAE-endorsed Transfer Function Method is used to calculate building heat flow.

HAP's energy simulation module performs a true 8,760 hour energy simulation of building heat flow and equipment performance. It uses TMY weather data and the Transfer Function Method. Many types of air handling systems, packaged equipment, and plant equipment can be simulated. Costs can be computed using complex utility rates. Extensive reports and graphs document hourly, daily, monthly, and annual energy and cost performance.

Table 7: HAP Version 4.2 Key Characteristics

Key StrengthsKey Weaknesses
  • Bestested to DOE-2
  • Compares energy consumption and operating costs of design alternatives
  • Limited ability to calculate interactions between some strategies

Hourly, whole-building energy analysis program calculating energy performance and life-cycle cost of operation. Can be used to analyze energy efficiency of given designs or efficiency of new technologies. Other uses include utility demand-side management and rebate programs, development and implementation of energy efficiency standards and compliance certification, and training new corps of energy efficiency conscious building professionals in architecture and engineering schools.

Table 8: DOE-2 Version 2.1E Key Characteristics

Key StrengthsKey Weaknesses
  • Detailed, hourly, whole-building energy analysis of multiple zones in buildings of complex design
  • Widely recognized, the de-facto standard
  • High level of user knowledge and computer literacy required

Windows interface to the DOE-2.1E energy simulation program. Through the graphical interface, users construct a model of the building's geometry using standard block shapes or using a built-in drawing tool. Building systems are defined through a point-and-click interface. A library of constructions, fenestrations, systems, and operating schedules is included, and the user can add custom elements as well. If desired, the program assigns default values for parameters based on the vintage and size of the building.

VisualDOE is especially useful for studies of envelope and HVAC design alternatives. Up to 20 alternatives can be defined for a single project. Summary reports and graphs may be printed directly from the program. Hourly reports of building parameters may also be viewed.

A graphical front-end to the DOE-2 building energy analysis software (see below). Includes graphical editing and scheduling capabilities, and flexible output options. There is online help in addition to a user manual. Designed for U.S. and international users. Weather data is available for U.S. and some international locations; custom data may be entered by the user. A free demo is available for download.

Table 9: VisualDOE 2.6 Key Characteristics

Key StrengthsKey Weaknesses
  • Dramatically reduces the time necessary to build a DOE-2 model
  • Uses DOE-2 as a simulation engine
  • Displays a 3-D model to help verify accuracy
  • Implements DOE-2's daylighting calculations
  • imports CADD data to define thermal zones
  • Relatively expensive
  • Passive solar features poorly modeled

BLAST (Building Loads Analysis and System Thermodynamics)
Performs hourly simulations of buildings, air handling systems, and central plant equipment in order to provide mechanical, energy and architectural engineers with accurate estimates of a building's energy needs. The zone models of BLAST that are based on the fundamental heat balance method, are the industry standard for heating and cooling load calculations.

Evaluation of high-potential, cost-effective energy efficiency projects in existing Federal buildings; calculates results that are within 4-5% of DOE-2 annual energy results; using quick input routines, permits evaluation of a 10,000 sf. building in about ten minutes. ASEAM (A Simplified Energy Analysis Method) Version 5.0 automatically creates DOE-2 input files.

Table 10: BLAST Key Characteristics

Key StrengthsKey Weaknesses
  • Uses detailed heat balance algorithms that allow for the analysis of thermal comfort and other factors that cannot be analyzed in programs with less rigorous zone models
  • High level of expertise required to operate

is a building energy simulation program that builds on the most popular features and capabilities of BLAST and DOE-2. EnergyPlus includes innovative simulation capabilities including time steps of less than an hour, modular systems simulation modules that are integrated with a heat balance-based zone simulation, and input and output data structures tailored to facilitate third party interface development (see Table 11). A few of the new features in EnergyPlus Version 8.2 include: development transitioned to GitHub, where the source code is to be made fully public in the spirit of the open source license; building envelope integrated slab calculation; improved sizing options for HVAC, Plant and Refrigeration; and ice storage curve-fit tool. Several interfaces and utilities for EnergyPlus are available, including EP-Quick which creates an EnergyPlus input file based on a broad range of zone templates. For up-to-date information on interfaces click here.

Table 11: EnergyPlus Version 1.2.2 Key Characteristics

Key StrengthsKey Weaknesses
  • Accurate, detailed simulation capabilities through complex modeling capabilities
  • Input is geared to the 'object' model way of thinking
  • Successful interfacing using IFC standard architectural model available for obtaining geometry from CAD programs
  • Weather data for more than 550 locations worldwide available on the website
  • Difficult to use without graphical interfaces
EnergyPlus screen capture

Fig. 3. EnergyPlus screen capture

F. Economic Assessment Tools

BLCC (Building Life-Cycle Cost)
Provides comprehensive economic analysis of proposed building capital investments. BLCC is especially useful for evaluating energy and water conservation projects in buildings. Up to 99 alternative designs can be evaluated simultaneously to determine which has the lowest life-cycle cost. Economic measures, including net savings, savings-to-investment ratio, adjusted internal rate of return, and payback period are calculated for any design alternative relative to the designated base case. It contains modules to evaluate agency-funded projects according to 10 CFR 436A and projects that are financed through ESPC or utility contracts as directed by Executive Order 13123. The remaining modules, now in BLCC4 (for DoD military construction projects, OMB projects, and private-sector projects including taxes and financing) will be programmed into BLCC5 in the next few years. It complies with ASTM International standards related to building economics and NIST Handbook 135, Life-Cycle Costing Manual for the Federal Energy Management Program (95 ed.).

Table 12: BLCC Version 5.3-05 Key Characteristics

Key StrengthsKey Weaknesses
  • Updated annually for discount rates and energy prices
  • Performs high quality LCC analysis
  • User's Guide included as file
  • Results are not particularly graphic
  • User-requested improvements for alternative financing are still being incorporated

QuickBLCC (Quick Building Life-Cycle Cost)
Used to set up multiple project alternatives for life-cycle costing analysis in a single input file. The Quick BLCC (Quick Building Life-Cycle Cost) program provides a convenient method for solving relatively simple LCC problems that require finding the lowest LCC design alternative among many mutually exclusive alternatives for the same project. Input data files are transferable to BLCC for more detailed analysis.

Table 13. QuickBLCC Version 2.9-05 Key Characteristics

Key StrengthsKey Weaknesses
  • Ideal for preliminary economic evaluation of multiple design alternatives
  • Users guide included as file with program
  • No private-sector tax analysis included

G. The Limits and Benefits of Energy Analysis Tools

Users of energy analysis tools should be aware that energy calculations, regardless of their sophistication, cannot precisely predict actual energy consumption. Factors such as construction quality, occupancy schedules, and maintenance procedures may vary markedly from assumptions contained in the analysis and skew results. However, this does not mean that energy analyses are not important tools.

It is also important for users of energy analysis tools to understand the interrelationships among all aspects of building design. Employing an integrated 'whole building' design approach to site selection, orientation, building envelope and high-performance HVAC system choices, while considering life cycle cost analysis, is critical to achieving a truly successful building design.

Conducting an energy study of a new building or a major retrofit project is an excellent means by which to evaluate the relative energy performance of alternative designs. In particular, the effect of low-energy strategies such as moving windows from one façade to another for passive solar heating or improved daylighting, optimizing glazing selection or installing dimmable ballasts can be carefully evaluated on a comparative basis.

H. Other Thermal Simulation Software

In addition to providing energy analysis, programs are available that analyze other building thermal issues, including:

Mold and Moisture Dynamics in building assemblies such as wall and roofs

Window Performance including the effects of frame area on net window performance

  • An example of such a program is WINDOW developed at the Lawrence Berkeley National Laboratory (LBNL); it calculates Uvalues, SHGF, and Tvis of window systems constructed from glass and frames of known properties.

Natural Ventilation including the effects of complex airflow patterns in atriums and multi-zone spaces

  • Programs based on the principles of Computational Fluid Dynamics, (CFD) can calculate three-dimensional airflow effects.

Emerging Issues

Software developers call the core set of instructions (or algorithms) that determine what calculations are to be performed, a program's "simulation engine." These engines are usually developed over long periods of time by experienced building researchers and programmers invariably with the benefit of government funding. In sophisticated computer models, these engines are written in basic programming languages such as Fortran or C++ with input and output formats that are not easily understood by the average professional user.

To address this problem, third party vendors have emerged who have created highly graphic input (called front ends) and output screens to sandwich around public domain simulation engines. For example, VisualDOE and DOE 2.2 are proprietary programs developed around DOE-2.


Some Federal agencies may require use of energy analysis tools to determine a project's annual energy consumption or to verify a project's compliance with agency energy criteria. However, before selecting a tool, check with your agency's project manager for approved computer-based, energy analysis tools.

Relevant Codes and Standards

Additional Resources


Products and Systems

Atria Systems
Federal Green Construction Guide for Specifiers: