Application Note
©2014 OutBack Power Technologies, Arlington, WA 98223 Revision 2/FINAL Page 9 of 14
Table 2 – Matching PV power and load demand with the OutBack Energy Cell 200RE AGM battery at the 12 hour
discharge rate and 80% depth of discharge. Based on following calculations: 2000W array x 75% efficiency =
1500W. Charging amps implied: (1500W ÷ 48V) x 0.96 (eff of FM60) = 30 amps.
Charging Amps
(based on array size)
Number of 200RE
Battery Strings
Available Capacity at
C/24 Rate 80% DOD
The load kWh demand should be matched to equal one of the values in the table above to the actual PV array size
and the loads that will be moved to the critical load panel. Typical load profiles can be used as illustrated in Table
3 for some typical critical loads for assistance in making this estimate. Note that the refrigerator cycles for
typically 15 minutes out of every hour.
Table 3 – typical critical load profile.
So now when matching the 12.4 kWh of daily usage to the battery capacity table, we can see that one string of
batteries will not be enough. While two strings will be sufficient, the array size would have to be 4 kW or less. If
the array is 6 kW then three strings would be required, as the battery bank must be sized large enough to handle
the maximum unregulated charge rate in a worst-case scenario. The alternative would be to go with the
“diversion load solution.” However, by the time the cost, complexity and invasiveness of the diversion loads and
diversion load protection and control are added together, eliminating a string of batteries does not really actualize
any savings.
Guideline Number Two: The OutBack inverter power rating should be 1.25% of the GT inverter power rating. This
guideline ensures that the GT inverter does not overwhelm the charging circuitry in the OutBack inverter if the
load demand goes to zero and all available GT inverter power is flowing to the OutBack inverter. While
admittedly an unlikely scenario, for safety and equipment protection it’s best to follow this guideline. For
example, the 8 kW rating of the Radian inverter would dictate a GT inverter no bigger than 6 kW.
Guideline Number Three: This ensures that either the daily load demand or battery charging don’t exceed the
power from the PV array, or adds an optional generator to the backup system. A previous section of this
application note described a scenario when available PV power exceeds load demand, requiring that the GT
inverter is disconnected with an OutBack remote operated circuit breaker if the excess power begins to
overcharge the batteries. In reality, the condition in a backup system whereby the available PV power is out-
producing the load and battery charging demand is unlikely. Critical loads will rarely turn off completely and
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