Isolated large communities in the Amazon region of Brazil are power by Islanded grids powered by diesel generator systems. The costs of these systems are high due to the complex and unreliable fuel supply chain. Including large-scale photovoltaic installations in these isolated grids have the potential to increase the reliability and reduce the cost of electricity. This paper investigates the impact of PV generation insertion on the specific consumption of isolated thermoelectric power plants. A diesel thermoelectric power plant (DTPP) of 4 MW, supplying electric energy to an isolated grid, was used as a case study. Five Photovoltaic penetration levels are evaluated, considering the operational regulatory constraints, three diesel generators (A, B and C), two daily solar irradiance profiles and annual solar irradiance profiles, and two dispatch strategies. The power plant loading range is bounded by the Brazilian regulatory framework to comply with fuel consumption efficiency measures and to guarantee the reliable operation. In the evaluated scenarios, PV penetration was not subject to power curtailment measures controlled by the DTPP. However, the power plant loading range considers both variation of photovoltaic generation and load. The results presented in this paper indicate that maintaining a higher number of machines in operation can make the DTPP more reliable to absorb the PV generation variations, enabling the growth of the PV penetration on the electric system, reducing the DTPP's fuel consumption and return on investment (payback) of the PV system without change significantly the DTPP's specific consumption. The results also show that the quality of the diesel generator is more important to keep specific fuel consumption low than the dispatch strategy used for a given PV penetration. The replacement of Generator A (less efficient) for B (more efficient) together the PV penetration of 50%, for example, can achieve a yearly fuel consumption reduction superior to 1 million liters. For the same PV penetration, an average annual reduction of 0.78 million liters in DTPP’s fuel consumption can be reached, with PayBacks that could come close to 1.4 years.