Add battery back-up power option to existing grid-tied PV and solar systems: Page 9 of 14

August 05, 2013 //By Mark Mays
Add battery back-up power option to existing grid-tied PV and solar systems
Mark Mays of OutBack Power explains how to add a battery back-up power option to existing grid-tied PV and solar systems.
based on load demand. Since the relationship between available PV power and load demand is highly unpredictable, on sunny days the battery bank may get charged quickly and even turn off the GT inverter while the sun is still shining; it does so to prevent the batteries from being overcharged. On cloudy days, the battery bank may not get fully charged from the PV array and a generator may need to finish the charge on the batteries. If a generator is not part of the system, the premises occupants will face some hard decisions about which loads to keep up-and-running and which ones to divest or “shed” until more sunny days arrive.
 
The PV power column is the maximum amount of power that can be back-fed through the inverter to charge the batteries. The calculation for this guideline presumes some losses on the array and in the GT inverter.

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.

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

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