Q&A: Taking grid-connected system off grid

I was very interested in the off-grid issue but it didn’t answer my question about converting from a grid-connected system to off-grid. What would I need to do if I want to convert my existing grid-interactive solar system to off-grid?

—Neil Garret

You could just sell the grid-interactive inverter and use the solar array as-is to charge an appropriately sized battery bank via an MPPT (maximum power point tracking) charge controller. This controller will match the high voltage output of the array to the lower voltage battery bank. You then add a standalone inverter and anything else required for the stand-alone system. Sometimes the array is reconfigured to produce a lower voltage and higher current, but it’s not necessary nowadays with MPPT charge controllers, some of which can handle very high array voltages, such as the MorningStar Tristar (www.morningstarcorp.com/products/tristarmppt- 600v/). This is called a DC-coupled system and is how most off-grid systems were traditionally done until recent times.

The other method is called AC coupling. You keep the current system and add the battery bank and a suitable inverter/charger. This new inverter creates a microgrid that the grid-interactive inverter can feed into. When there is more energy coming from the panels than the house is using, the inverter/ charger feeds it back into the battery bank. AC-coupled systems seem to be becoming the norm for larger domestic off-grid systems, not just those that have been converted.

There are pros and cons to either method. Personally I prefer the DC-coupled system as it doesn’t all hinge on the inverter/charger—if that fails, you have no energy available at all. With DC-coupled systems, if the inverter fails you still have the array keeping the batteries charged and you can use a low-cost backup inverter for mains power until the main inverter is repaired, or you can use DC power directly. Of course, you can do the latter on an AC-coupled system as well, but AC-coupled homes almost never have any provision for direct DC use, whereas DC-coupled systems often do, even if just for lighting.

We will be looking more closely at how AC-coupled systems work in a future issue of ReNew.

—Lance Turner