Grid Tie Inverters with Battery Backup

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Grid-tied solar PV installation with an inverter and battery backupThis image shows the required components for a grid-interconnected system with a battery backup; this is called grid-interactive operation. The arrangement provides power from the battery during grid outages. The primary difference between the dependent and interactive modes of the grid system is battery storage and another electrical supply panel to service the home's essential loads.

With the utility system, the electricity generated is exported and imported like that in a grid-connected system. When power is interrupted on the grid, the inverter disconnects itself and goes into standby mode, awaiting grid reconnection. But a grid-interactive configuration disconnects the inverter from the grid and connects it to another available "electrical load supply panel" which could work from the battery bank or any renewable source. This allows the inverter to produce enough power to continue servicing connected loads. All essential electrical loads in the home must have the highest efficiency and the lowest wattage possible in order to prolong battery life and allow power generation to continue during blackouts.

No Battery Grid-Tie Inverters

Grid-tie inverters are different from their interactive-grid cousins. Since low-voltage batteries are not needed, the inverter has no step-up transformer, transfer switch, battery charging system or generator control.

Inverters must comply with safety and regulatory standards when they're connected with your utility, which include anti-islanding, ground-fault protection and frequency control, but these requirements do not apply to off-grid systems. The explosion in global sales of such inverters has caused their capabilities to be enhanced dramatically. Choosing the right model may therefore be quite daunting; get assistance from either a reputable dealer or a licensed electrician/installer to pick a model which is reliable and has plenty of features, such as:

  • Protection from ground-fault
  • Maximum tracking of power point
  • Direct-to-computer metering
  • Weatherproof containers

For batteryless inverters:

  • Confirm that the inverter is certified according to UL1741 or CSA C22.2 No. 107.1-01 and IEEE 1547.
  • The PV array "watts input DC" should match the inverter's rating. This is a manufacturer value which can be calculated using Standard Test Condition (STC) PV rating measured in watts multiplied by the efficiency factor of the inverter.
  • Check the operating voltage window of the PV array with that of the inverter. Almost all grid-tie inverters function at up to 600V DC, which is very high. Multiple PV array panels would have to be wired in series to reach this voltage, although in reality the output voltage rises and falls depending on the amount of sun on the PV panels. The input voltage must not be more than the inverter operating window at any time or in any circumstance. (Just like the adage that it is impossible to return the smoke to the capacitors when the unit blows.)

 

Off-Grid Inverters 

Grid-tied systems with a backup battery apply the same inverter technology that off-grid systems use, with sine wave inverters as described below.

Inverters with off-grid systems can be from 12V DC for a simple cottage to 120V AC or a square wave model to grid-interactive battery backup units as already described. The primary consideration for off-grid systems is the