FERC reports on the US electricity infrastructure monthly here, identifying new power capacity by type, and total operational capacity by type. It does not delineate decommissioned capacity.
It is particularly helpful in tracking renewable capacity. FERC reports for April 2014 the following capacity. Notice the steady growth in Solar, the recent increase in wind, and the declines in both coal and natural gas. Coal capacity should be dropping precipitously in the next 24 months.