Proposed hybrid electric power generation system is constituted from Free Piston Stirling engine generator and solar photovoltaic system. Free Piston Stirling engine electric generator uses biomass, such as wood pellets as the heat source. This system produces maximum output of 1.6kW and $45^{\circ} C$ 200 L hot water with linear generator. This paper studied the control of output voltage of Free Piston Stirling Engine. ILQ (Inverse Linear Quadratic) control theory is used to keep output active power between 400W and 500W. In long range time constant of burning system and in short range time constant of electrical circuit, experiment yields temperature of FPSEG head between upper limit and lower limit.