1. Introduction
There are several methods for sizing off-grid photo voltaic (pv) systems. The more simplistic are called intuitive methods. They are used as a first approach, but they are very inaccurate. Another group of methods are denominated numerical methods. These methods usc system simulations. These ones are more accurate than the previous ones but present the problem that they need to have long time series of solar radiation for the simulations. In a third group there are methods which use equations to describe the pv system size as a function of reliability. These ones arc called numerical methods.