Autonomy and battery sizing (stand-alone)

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

Autonomy is defined as the period during which the load can be met by the battery alone, without any solar input, starting from a fully charged battery state. For non-constant loads (seasonal, monthly, or weekly variations), the worst-case scenario over the year is used for sizing.

The calculation accounts for the minimum SOC (state of charge) disconnection threshold and battery energy efficiency. A capacity correction is also applied, as this mode of operation typically occurs at a slow discharge rate (corresponding approximately to the C100 capacity definition—discharge over about 100 hours), while nominal battery capacity in the PVsyst database is defined as the C10 value. The C100/C10 ratio is typically 120% to 130%.

These three sizing parameters can be adjusted in the "Hidden parameters" section.

Note: when specifying a very long autonomy period, the overall system optimization will select the minimum PV size that just meets the required LOL (loss of load probability). This can result in a very low average state of charge over extended periods of the year, which can be damaging to the battery.

In such cases, the actual PV array sizing should be slightly larger than PVsyst's proposal in the final "Project" design study.