Add battery back-up power option to existing grid-tied PV and solar systems: Page 10 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.
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 many conditions, especially on cloudy days, will require supplemental power to meet load and battery charging demand.
 
Most of North America has 3-5 average sun hours per day which translates to 18 kWh to 30 kWh of available PV power with a 6 kW array, notwithstanding various losses that could reduce the daily production significantly. However, sometimes PV production will meet or exceed the PV module nameplate so for estimating purpose 18

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