Mrica Generation Unit (UP) is located in Banjarnegara, Jawa Tengah. Utilize water that accommodated in the reservoir to move the turbine so that the generator can spin and produce electrical energy. It has 3 synchronous generator units, each generator has 60 MW or 180 MW capacity if they fully operational. Synchronous generator function is to convert mechanic energy to electrical energy. Electrical energy which is produced by this generator is expected can fill the load of the consumers. When the load fluctuate, it will affect the value of the generator output voltage. Therefore, to find out the reliability of a power plant an analysis is carried out on one of the factors, namely the stability of the output voltage on the generator. The method is taking the data during 3x24 hours, within take the daily generator operation Mrica Generation Unit especially unit 2 in form of generator output voltage (Vout), excitation current (If), power active (MVA) and water debit. The fluctuate change of consumers affect the stability of output voltage generator in Mrica Generator Unit, unit 2. Water debit that effect the speed of the generator rotation and excitation current are used to stabilize the output voltage. The percentage of regulation voltage that already noted are +2,2% and -3%, so that the reliability of the system must be improved.