The variable speed power generation brings higher efficiency and other economy benefits namely in low power range sources and isolated systems with local net supply. Typically it is in mobile power generation based on fuel engine drive, but the results are also useful in the area of wind power generation and maybe in few years in automobile industry for car board-net supply as well. To keep high efficiency, the cheap constant-geometry wind turbine must be operated at variable rotation speed in dependence on wind speed. Permanent magnet synchronous generator was first to used in VSCF generation, but its flux cannot be decreased at reduced load, therefore its excellent rated efficiency is not so optimal in competition with much cheaper and robust asynchronous generator (AG) at partial load. The mathematical model for AG optimal slip estimation from efficiency point of view is created. Some model results are presented to show the AG efficiency at varying load and its other advantages.