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In the Presizing process, the proposed battery pack capacity is determined according to the required autonomy of the system, given in days.

The autonomy is defined as the time during which the load can be met with the battery alone, without any solar inputs, starting of course from a "full charged" battery state.  With non-constant loads (seasonal or monthly definition, weekly use), this is accounted as the worst case over the year.

The calculation takes the minimum SOC disconnecting threshold, and the battery "energy efficiency" into account. It also has to apply a capacity correction, as this mode of use usually occurs at a rather slow discharge rate (therefore corresponding about to the C100 capacity definition, i.e. with discharge in about 100 hours); when the nominal capacity of the batteries in the PVsyst database is defined as the C10 value.  The C100/C10 ratio is usually of the order of 120% to 130%.

These 3 sizing parameters are adjustable in the "Hidden parameter".

NB: When defining a very large autonomy, the global system optimization process will choose the "minimum" PV size which just meets the required "LOL". This may lead to a very low average State of Charge value over large periods of the year, which is quite damaging for the battery.

In these cases, the actual PV-array sizing should be slightly increased with respect to the PVsyst proposal, in the definitive "Project" study.