Commissioning Document
©2019 OutBack | an EnerSys company
FA-KF-10/11/2019
Page 1 of 12
Paralleling SkyBox
This commissioning document covers the proper set-up of a system with two SkyBoxes connected in
parallel and applies for firmware versions 1.3.27 and greater. Parallel connection of multiple inverters
is referred to as “stacking”; the combined system is referred to as a “stack”. Paralleling two SkyBoxes
enables system capacities of up to 10kW of power output. At this time stacks with more than two
SkyBoxes are not supported. For a system power requirement that exceeds 10kW, please contact
OutBack Sales.
Theory of Operation
Table 1 – Connected and Separate Components for Stacked Systems
System Component
Independent and
Separate for Each Unit
Co-dependent and
Connected for Each Unit
PV
Battery
Grid
Load
A SkyBox stack operates differently when on-grid and off-grid. On-grid operation is straightforward;
each unit acts independently to perform its programmed function. The master SkyBox maintains
communication with the slave unit and propagates any grid setting changes. The master does not
direct the operation of the slave unit. Each unit independently maintains its own battery bank and
harvests power from its own PV array unless the stack is set up to limit power sold back to the grid.
In off-grid operation, the master unit controls only the AC output of the slave unit. The slave unit
maintains autonomy over its solar production and battery charging.
This is the reason separate
battery banks and separate solar arrays are required for each SkyBox in the stack.
During a grid outage, the stack will power the loads on the protected loads panel by drawing power
from the batteries and inverting it to produce the needed 120V/240VAC power. The master unit
takes control of the output voltage and modifies the current it is sending to the loads to maintain a
120V/240VAC output. The master then communicates this value to the slave unit so it contributes to
maintaining the 120V/240VAC power. If either unit is not harvesting enough PV to cover the power
demanded by the loads, it will begin to discharge its battery. Once EITHER unit reaches the minimum
state-of-charge (SOC) set point, the entire stack will stop providing power to the loads. The smallest
of the independent battery banks determines the backup capacity for the stack.
NOTE
: The minimum state-of-charge set point must allow enough remaining battery power to
maintain SkyBox tare loss until the next solar day or until the grid returns.
Once the grid returns, the master unit will synchronize with the grid. Once the grid timer has expired,
the master will close its grid relay and command the slave unit to do the same. This returns the system
to on-grid operation.
