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When sizing a PV stand-alone system, the basic constraints are the availability of solar energy throughout the year, and the satisfaction of the user's needs. The problem to be solved is the optimisation of the size of the photovoltaic generator and the storage capacity, subjected to criteria which may take on different weights depending on the use:

-        Reliability of the supply, which is very important, for example in decentralised telecommunication installations. But in a domestic installation, this may be overcome with a small back-up generator. This reliability is measured as the "Loss of Load" Probability ("P LOL").

-        Investment and maintenance costs, which should take into consideration the cost of the PV generator, the initial cost of the batteries, as well as that of their maintenance and replacement. The high price of the kWh used necessitates a highly detailed study of the real user's needs, and the use of specific appliances that are highly economical regarding its energy consumption.

-        Durability: the cost of the batteries is closely related to the quality of the batteries chosen, as well as their longevity, which is itself dependent on the conditions of use (average state of charge, cycling, depth of discharge, temperature).

It is therefore observed that the optimisation of a PV system is a complex problem with several criteria, which has no ready-made solution. The preliminary design tool allows for "playing" with the main parameters, and rapidly observe their effects on the system performance, to optimise the desired characteristics by successive approximations.

But the accuracy of these monthly results is not guaranteed and the behaviour of the chosen system will have to be verified by a complete simulation in hourly values, with real available components.