Add battery back-up power option to existing grid-tied PV and solar systems: Page 4 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.
the grid. This keeps the inverter from trying to power other homes on the grid, as well as keeping energy off the power lines so utility workers don’t get shocked. The BB inverter also provides a power source to the GT inverter that keeps it online and “inverting” the DC power to AC power for the critical loads (see Figure 4).
 

Figure 4 – Current paths when grid power is lost.

When the sun goes down and the PV power is no longer flowing through the GT inverter to the loads, the power stored in the batteries will now provide power to the critical loads panel until morning (see Figure 5).

 

Figure 5 – Current path from the BB inverter with both PV and Grid loss.

 
When the sun comes out the next day, the system reverts to the power flow as shown in Figure 3, with any extra energy used for recharging the batteries. If no excess is available then some manual load-shedding (divesting of less critical loads to prioritize the most critical, such as lighting and refrigeration) may be necessary through the turning-off of the critical load panel connected devices until the batteries are charged. If the critical loads are absolutely essential and load shedding is not an option, then adding a generator to the backup system can satisfy the critical load demand while charging the batteries.
 
One might ask: “why not just add the generator and forget about the BB based inverter? “That might be a viable option for some, but there are some important things to consider before making that decision.

1. The generator will need to run the entire time that electrical energy is demanded by the building’s loads, perhaps 12-18 hours per day. Aside from the noise factor, many low-cost generator motors require frequent maintenance, and are very inefficient at low power output.

2. As an example, higher efficiency can

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