Calculation rules when integrating solar and storage

From Electrical Installation Guide

When solar electricity production and storage are integrated into buildings, the electrical installations evolve from single-source to multi-source, from generator-based generation to inverter-based generation, and from a single operating mode to multiple operating modes.

Consequently, the way to calculate electrical installations must also be adapted to take into account some complementary points, as detailed below.

Identify all operating modes

One peculiarity of installations with solar production and storage is that they have multiple operating modes. This means that different operating conditions will take place depending on the available power sources and connected loads.

Typically, the integration of solar production leads to at least two operating modes. In the first case, during the hours of solar production, the installation is supplied by the grid and the photovoltaic system in parallel. On the other hand, when there is no PV production, for example during the night, the installation is supplied only by the grid.

In addition, if the PEI is islandable, calculation shall be done in island mode as well. See Islandable PEI

Fig. K54 – Examples of different calculation scenario for an islandable PEI

When storage is added to the system, the number of operating modes increases even more, and for two reasons:

  • there is a larger number of possible combinations of energy sources producing or not, and
  • the storage must be considered both as a source and as a load: when the battery is charging, it is a load for the electrical installation, but when the battery is discharging, it provides electrical energy to the installation and becomes a source.

Calculate the electrical installation for each operating mode

Consequently, a calculation should be done for each operating mode, because calculations can result in significantly different power and current values from one operating mode to another. Once the calculation is done for each mode, the equipment must be sized considering the worst constraints. The protection plan must also be set to be valid whatever the operating mode.

As an example, a simple installation with grid connection and storage has at least three operating modes:

  • Supplied by the grid only – when the electrical energy storage is in standby
  • Supplied by the grid with the electrical energy storage as a source (battery is discharging)
  • Supplied by the grid with the electrical energy storage as load (battery is charging)

Consider solar and storage inverters as current sources

Another peculiarity of installations with local energy production is that storage and solar inverters do not have the same characteristics and behaviors as traditional sources, such as the grid supply or generators. For example, in the case of a short circuit, solar and storage inverters limit their current output to a value not much higher than the nominal current, and so behave as constant current sources.

Fig. K56 – In case of electrical fault, solar and storage inverters behave as current sources

In practice, this means that when calculating an electrical installation by the impedance method, photovoltaic and electrical energy storage systems are not represented by impedances in the same way as all other components of the electrical installation.

When making short circuit current calculations, the short circuit contribution from the inverters, as provided by the inverter manufacturer, may simply be added to the other short-circuit currents in the installation.