Big Ladder Software


EnergyPlus is the US Department of Energy’s flagship program for building energy modeling. Developed by a team of researchers, engineers, and programmers at US national labs, universities, and private companies, EnergyPlus offers the state of the art in modeling capabilities and accuracy.

EnergyPlus is also an open-source project. The code was released under an open-source license in January 2012. The latest code is now hosted on a public repository on GitHub.

Big Ladder officially joined the EnergyPlus Development Team as a contractor to the National Renewable Energy Laboratory (NREL) in June 2014. We have a productive collaboration with the team as we continue our efforts to improve the EnergyPlus engine, make it easier to use, and develop tools and resources around it.

Some of our current efforts include:

And also see our related projects that you might find useful:

Further Reading

Early Work
Work for Department of Energy
Work for Other Clients

Early Work

We have a long history of working on the EnergyPlus engine from our careers before Big Ladder. Peter Ellis, president and founder of Big Ladder Software, joined the EnergyPlus Development Team in graduate school where he authored component models for Trombe walls, tubular daylighting devices, and light shelves. Before founding Big Ladder, he worked at NREL for nearly six years. While at NREL he developed more features for EnergyPlus including solar hot water systems, hot water heaters, and energy management systems. Peter was also the original creator of the OpenStudio “legacy” plugin that couples SketchUp to EnergyPlus.

Neal Kruis worked for five years at NREL before joining Big Ladder in June 2012. While at NREL, Neal maintained the Slab and Basement ground heat transfer preprocessor programs for EnergyPlus.

Work for Department of Energy

FY14 Tasks

Command-Line Interface

This task is to implement a POSIX-style command-line interface (CLI) for EnergyPlus. This effort should be of interest to anyone who has ever tried to run EnergyPlus via a batch file or scripting. The CLI makes it easy to launch a simulation with a single line that specifies input file, weather file, and various run options. No more copying and renaming files to a temporary directory just to run the program. For example:

 energyplus.exe --weather denver.epw --readvars --design-day inputfile.idf

Development is now completed and the new feature should be available in the next major release of the program.

FY15 Tasks

Usability Study

In our experience, usability is a critical factor limiting the adoption of EnergyPlus. Usability spans many aspects of the software: input-output interface, robustness, documentation—-even how you download and install the program. For this task we completed a scoping study of the usability issues in EnergyPlus. We performed research to identify and document usability problems in all aspects of the program but especially to target the input object definitions and relationships.

Work for Other Clients

In addition to our work for the Department of Energy, we also do EnergyPlus development for others. Clients have included nonprofit organizations and product manufacturers.


We are working on ASHRAE research project 1588-RP, “Representative Layer-by-Layer Descriptions for Fenestration Systems with Specified Bulk Properties Such as U-factor and SHGC” in collaboration with White Box Technologies and Peter Lyons & Associates. The objective of the project is to develop a new window modeling algorithm that can generate a good representation from minimal data (U-factor and SHGC) when that is all you have, or a better representation when combined with commonly observable characteristics such as number of panes, frame material, glazing treatment (coated, tinted, low-e), etc. The algorithm will also be integrated into EnergyPlus for use as a new window object.

Phase Change Materials

We are contracted by a phase change material (PCM) manufacturer to validate the correctness and accuracy of the PCM algorithm in EnergyPlus. This on-going project includes both a field study and laboratory experiment. The field study is a multi-year data logging and analysis effort of a PCM product installed in a small office building in Colorado. The field data are analyzed to evaluate the performance of the PCM product. The data are also compared to an EnergyPlus model of the building to determine if the simulation matches reality. The laboratory experiment is a small-scale study of the PCM product under highly-controlled conditions. Data are collected for various conditions and compared to an EnergyPlus model in order to diagnose and debug any discrepancies. Results are still pending for both the field study and laboratory experiment.