Energy Innovation LLC chose to develop a web-based interface to improve the accessibility and reach of the Energy Policy Simulator (EPS). Much of the power of the EPS comes from the way it allows users to explore many different policy packages in real time, learning about the available options and their effects on emissions, costs/savings, and other outputs. Without a web-based interface, this would only be possible for users who download and install Vensim Model Reader, the free software used to run the EPS on one’s own computer, as well as the EPS itself. We wished to provide the opportunity to interact with a running copy of the model even to users who might not have the time, inclination, or technical expertise to install and learn how to use Vensim Model Reader.

The other purpose of developing the web application is to provide an improved user interface for model users who are not performing demanding, technical tasks. For example, policies have helpful descriptions and guidance text in the online interface, and switching between graphs of different output variables is fast and easy. There are also some features not available in the downloadable version of the EPS. CO2e emissions reduction targets can be shown on the graphs, to give users relevant benchmarks or targets to aim for. The web-based interface allows for the sharing of scenarios by email (or anywhere a URL may be posted), and it allows quick and convenient downloads of data for output graphs and associated policy settings. For most users, we believe using the EPS through the web-based interface will be faster, easier, and more intuitive than it would be using a local copy of the model.


The online interface does not contain nearly as many features and options as Vensim Model Reader. This limits the ways in which users can interact with the model, if they do not download the local version of the EPS. Some of the most important limitations of the online interface are:

None of these limitations exist in the free and open-source version of the EPS that may be downloaded and run locally. If you expect to do any of the tasks above, you are encouraged to run the EPS on your own computer. Please see the Download and Installation Instructions for details.

Web App Architecture

The online interface is a web application (web app) developed in the Ruby programming language. It interfaces with a 64-bit, multi-context (multi-user) version of Vensim that is running on the server. Each time a policy lever is changed, the web app sends the relevant changes to Vensim, instructs Vensim to run the model, obtains data for relevant output variables, and displays the data onscreen. User scenarios are saved as combinations of policy lever settings in a database, under a particular user’s account (if that user has signed up for an account). The web app reads settings from the database and requests new model runs from the Vensim library as needed.

The following diagram shows the high-level, technical architecture of the web application and associated components:

architectural diagram of the web application


The online interface and the technologies necessary to communicate with a copy of Vensim running on a webserver were developed by Todd Fincannon.