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Grid-Connected Solar Plus Batteries – DC Coupling

February, 2014


DC Coupling is the standard method to combine a grid-tied solar system with batteries to provide power to your house when the grid is down.  In this approach, DC electricity is fed from solar panels to a charge controller and then to batteries.  The batteries connect to a battery-based inverter that converts power to AC electricity to use in your home.


As opposed to a fully off-grid home, grid-tied battery based systems power a critical load panel when the grid is down.  By choosing to power only specific loads you can decrease your costs since the battery bank will be much smaller.  In addition, since the grid will be down relatively infrequently, low maintenance batteries that discharge only when the grid is down can be utilized.  Several manufacturers are now providing systems with packaged batteries and control mechanisms that streamline the design and installation of these systems.


It’s important to recognize that while battery grid-tied solar arrays provide the advantages of secure power, the batteries do have downsides.  In addition to extra cost, the batteries decrease the system production by approximately 10%.


When the grid is up, the utility-interactive battery-based inverter converts DC to AC electricity to match the power coming from the grid.  In this case, the inverter sends AC power to the critical load panel and the rest of the house.  Excess power can flow into the grid and the meter turns backwards (like a standard grid-tied system without batteries).


When the grid is down, the battery-based inverter energizes only the selected circuits.  When the grid is down and the sun is down, the batteries will provide the power to the critical loads.  Because a switch is thrown in the battery-based inverter, no power will be sent to the main panel or the grid, preventing power flow to the grid and preventing harm to utility workers.