The decline in specific power reduces wind power density and system efficiency

US wind power generation has grown significantly over the last decades, both in number and average size of operating turbines. A lower specific power, i.e. larger rotor blades relative to wind turbine capacities, allows to increase capacity factors and to reduce cost. However, this development also reduces system efficiency, i.e. the share of power in the wind flowing through rotor swept areas which is converted to electricity. At the same time, this may also decrease output power density, the amount of electric power generated per unit of rotor swept area. In this study, we present a decomposition of historical US wind power generation data for the period 2001--2021 to examine to which extent the decrease in specific power affected system efficiency and output power density. Furthermore, we decompose the wind power available to turbines into changes due to new locations and the effect of changes in average hub heights.