The monthly survey Form EIA-860M, Monthly Update to Annual Electric Generator Report supplements the annual survey form EIA-860 data with monthly information that monitors the current status of existing and proposed generating units at electric power plants with 1 megawatt or greater of combined nameplate capacity. EIA estimates the current and near-term unit inventory of electric power generating capacity by integrating the information on these surveys along with ongoing EIA research of new units. However, creating this monthly estimate sometimes requires the use of data submitted on the annual EIA-860 before it has been fully verified by EIA. For this reason, reported capacities are EIA's preliminary estimates of capacity for that month. Estimates will be corrected without specific acknowledgment or explanation in subsequent month's inventory.

Starting with March 2017 data, the Preliminary Monthly Electric Generator Inventory includes a comprehensive list of generators which retired since 2002. The list is available on the Retired tab of the datafile.

Starting with March 2018 data, the Preliminary Monthly Electric Generator Inventory also includes data from Puerto Rico for existing, retired, and proposed generating units at electric power plants with combined nameplate capacity of 1 megawatt or greater.

Capacities reported in this preliminary inventory are best estimates of current generating capacity, but are not meant to be capacity commitments by the associated facilities.

The final inventory of generators is published each year with the annual EIA-860.

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