When sizing a PV stand-alone system, the basic constraints are the availability of solar energy during 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, in order to meet the energy requirements, eventually accepting a specified "Loss-of-load" probability that the energy will be missing.
The first requirement is therefore the evaluation of the user's needs along the year, and the choice of an autonomy period for the battery bank (which should cover the worst sequences of days without sun). A usual choice is 4 days.
PVsyst pre-sizing tool : a random process
In the PVsyst pre-sizing tool, the optimization of the required PV power is performed by a fast simulation of the whole system for different PV array sizes. This simulation uses an approximate calculation based on monthly meteo values, and a series of 365 days constructed by the Synthetic Generation of the Collares-Pereira model. The Loss-of-load evaluation is of course dependent on the day's series: it may be different if you construct another weather series. And there is no clear limit for determining the "Fully safe" limit (in battery capacity or PV array), i.e. which will contain the worst weather time-series possible for this site.
The system sizing highly depends on the time-series, and therefore will be different from one execution to another one of the presizing tool, as well as for the detailed simulation.
NB: In a future version, we will provide a more refined statistical évaluation over several random years.
PV array size and back-up
The PV array size is dependent on the specified risk of "loss-of-load". Especially in middle-latitudes, implementing a back-up genset for covering - say - 5% of the time drastically reduces the PV array. This is less the case for sub-tropical climates where the insolation is rather well distributed over the year.
An oversized array will of course lead to overload losses (i.e. unuseable available energy when the battery is full). But with the decrease of the price of the modules, this may be now a good option.