The database is a compilation of existing cost data for wind, solar photovoltaic (PV), solar thermal (CSP) and geothermal energy technologies, including historical costs and projected costs for each. The data sources and references used to compile this information are included, as well as the experts that were consulted in identifying sources. The database separates information collected from primary and secondary sources. Often, data collected for one study is recycled into another, which is further referenced elsewhere, and so on. Identifying which sources are based on original data collection and which relied on previous work should be useful information.