Projects proposed under an ESPC or UC are usually retrofits to an existing building. The existing equipment continues to consume energy and require maintenance until it is replaced by the energy conservation measures, sometimes over a period of several years. The service period in this case begins at the base date, and the base date and service date coincide. Energy and non-fuel costs have to be adjusted to account for the changes in usage as new equipment is installed. This is accomplished by "indexing" the energy and OM&R costs, which allows you to specify a change in the annual energy usage or OM&R pattern.