Billing analysis is widely used to evaluate the impact of energy efficiency programs. The performance of a billing analysis is attractive as it allows the analysis of a large group of customers and it makes use of existing data sources, such as billing data and program tracking data. This existing information can be combined with primary data collection from surveys or on-site visits to provide complete customer-specific information for analysis. Fundamentally, a billing analysis uses customer billing records to measure changes in consumption attributable to program actions. This approach is sometimes referred to as retrospective in nature because customers are typically evaluated a year after participation.