In this paper, safe techno-economic sizing methodology of a standalone photovoltaic system is investigated. It is proposed to size the system on the basis of solar radiation measurements. Then, Markov transition matrices (MTM) are used to simulate stochastic cloud cover sequences. Each sizing is then tested on a large number of scenarios. It is thus possible to use a statistical approach of the sizing results, in particular on the Loss of Power Supply Probability (LPSP), which is highly sensitive to the stochastic nature of cloud cover. A case study (Saint-Martin en Haut, in France) is given to illustrate this methodology.