We present the process followed to create a decision-aid tool for use in renewable energy and energy efficiency investment decisions. Our tool is targeted at home and small business owners in the Pioneer Valley. We begin with the development of two prototype tools. The first was created for the Hitchcock Center for the Environment, and is an Excel-based tool that allows users to select various combinations of technologies and instantly see the financial, environmental, and educational impacts of their choice. The second examines only two technologies, solar photovoltaics and combined heat and power, and uses a cost minimization approach. These prototype tools inform the development of the Pioneer Valley Sustainability Network (PVSN) decision-aid tool. The PVSN tool allows users to compare a building’s current energy consumption with the expected performance given the implementation of one or several renewable energy or energy efficient technologies. The PVSN tool evaluates financial costs along with externalities like emissions damages and health impacts. It also provides modeling of decision making under uncertain costs of damages from carbon emissions.