Application Note
©2014 OutBack Power Technologies, Arlington, WA 98223 Revision 2/FINAL Page 7 of 14
the associated control and coupling hardware plus adding a more expensive inverter/charger often means
skimping on energy-storage with lower-grade batteries and enclosures, lowering overall system performance and
utility.
OutBack’s AC-Coupling solution: another approach is to keep things elegantly simple by outlining some basic
sizing and operational guidelines by which a simple BB inverter, a battery bank, and a remote-operated relay can
be added to an existing GT inverter system to tie in the building’s available PV power to the critical loads during
grid loss. The rest of this article discusses an advanced electro-mechanical solution from OutBack Power with an
option for automatic generator control if desired. Unlike most frequency dithering solutions, our control circuit
consists of the two OutBack inverter Auxiliary (AUX) ports and two OutBack relays. This cleaner, more compact
component design allows the safe lock-out of the GT inverter when the batteries are full, and also keeps the GT
inverter locked out if an optional generator is started and running in the system. Integrated with a more
advanced, smarter inverter/charger such as OutBack’s Radian series with dual AC inputs and advanced generator
features, the result is a system that achieves higher performance at lower effective cost.
Notable features of the OutBack GSLC175-AC-120/240 AC-Coupling solution include:
UL-1741 end-to-end—when used with an OutBack battery rack, the entire system is certified specifically for
this application, ensuring fully-compliant operation.
Split-Phase design—more easily integrated into standard household wiring without costly, inefficient
transformers
Dynamic Stability—more stable output provides a cleaner signal to the system’s grid-tied (GT) inverter during
load spikes and variations, ensuring it remains on-line producing electricity.
Universal design—works well with other brands and models of GT inverters.
Grid instability, extreme weather and earthquakes are all factors to consider, and geographic and seasonal
vulnerabilities will affect the choice of which loads are most critical. Following are a few guidelines on how to size
the system and interact with the two types of inverter systems.
Guideline Number One: The daily critical load watt-hours shouldn’t exceed 80% of the watt-hours available from
the battery bank. An off-grid system is typically designed to discharge the batteries no more than 50% per day to
extend the life of the batteries. However, this assumes that the backup system will only be used a few days or
perhaps a week or two per year, so discharging batteries to an 80% depth will not significantly reduce their life
below what is considered the normal life cycle for a battery. Cycle-testing of OutBack’s Energy Cell batteries has
shown that 600 cycles are possible for 80% depth of discharge (DOD) on sealed AGM batteries, which translates to
600 days of backup power if sized to provide a day’s worth of power.
The rate or speed of which the batteries are both charged and discharged will affect their overall capacity. The
slower the rate of charge or discharge, the more capacity in the battery. The following table shows the typical
capacity for one to six strings of batteries. One would take 80% of these numbers to estimate how much power is
available to the loads for a 24-hour period. This is reflected on the 12-hour discharge rate in the next sizing table.
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