Over the past thirty years, Sunpower, Inc., the originator of the linear free-piston Stirling engine (FPSE), has developed core Stirling engine modules for a variety of applications. These range from domestic to military, terrestrial to space, and stationary to portable applications, with outputs from tens of Watts to 7.5 kW. This paper will present the recent results of a thermodynamic performance characterization study for three models of current Sunpower free-piston Stirling engines: 35 We, 80 We and 1.1 kWe. The data was measured over a wide range of acceptor and rejector temperatures, and power output levels.