Renewable energy is energy that is collected from renewable resources, which are naturally replenished on a human timescale, such as utilizing wind speed, solar power, and hydropower. However, in Indonesia renewable energy power generation has grown as a result of clean energy policies. One of the fastest growing of these green energy sources are wind power and solar power. One of the main challenges that PLN faces in terms of generating power in insular isolated system is the cost of electricity. This problem is caused by its supply mainly comes from diesel generators which use very high price fuel due to difficulty in transportation. In order to overcome this challenge, UPC Renewables initiate to develop Hybrid Power Plants in several insular isolated system. These Hybrid Power Plant consist of Wind Farm or PV Farm

as its primary energy source and will be equipped with battery for smoothing and load shifting functions to ensure safe, reliable, and economic operation of the system. Bigger battery can do more, but it gets expensive, which will increase energy selling price. In the other hand, too small battery is cheap but maybe will not solve the problem in the system. Thus, In order to be beneficial technically and economically for both PLN and UPC sides, the optimum battery sizing must be calculated carefully. The optimum battery size balance between cost and benefit.