Abstract One of the most important issues in planning Small Hydro-Power Plants (SHPPs) of the “run-off river” type is to determine the optimal installation capacity of the SHPP and estimate its optimal annual energy value. In this paper, a method to calculate the annual energy is presented, as is the program developed using Excel software. This program analyzes and estimates the most important economic indices of an SHPP using the sensitivity analysis method. Another program, developed by Matlab software, calculates the reliability indices for a number of units of an SHPP with a specified load duration curve using the Monte Carlo method. Ultimately, comparing the technical, economic and reliability indices will determine the optimal installation capacity of an SHPP. By applying the above-mentioned algorithm to a sample SHPP named “Nari” (located in the northern part of Iran), the optimal capacity of 3.75 MW is obtained.