AC Coupling In An Off Grid Solar Installation

In the off grid world AC Coupling may be a wonderful addition to expand your systems capabilities by integrating a less expensive grid tie inverter instead of adding an expensive off grid inverter charger. If designed right you can also ease up on your batteries, prolonging the battery lifespan.

How it works:

A typical DC coupled system has a charge controller that conditions the PV (Solar panel) voltage into the battery voltage. The inverter uses the DC power from the batteries to convert it into AC household power.

 If you decide to upgrade to an AC coupled system you can add let’s say 6000 W in PV modules and a 5000 W grid tie string inverter that is in its regular purpose not turning on until it senses 120/240 V 60 Hertz grid power. However, tied in to your off grid inverter the string inverter is made to believe the grid power is present. It has to qualify the stable grid power of 60 Hertz before it will turn on. Now it can supplement your off grid inverter for all your household appliances and also help charge your batteries. The mechanism used that prevents the batteries from over charging is called frequency shifting. Your grid tie inverter will start shifting the frequency to 61.5 Hertz. That is outside the operating parameters of the string inverter, so it will automatically go off line.

A very elegant solution to upgrade an off grid system, provided your existing off grid inverter has the intelligence to perform the frequency shifting.

PS: Should your home be grid tied and you are using a battery bank as an emergency power back up, you can potentially export surplus electricity to the grid if your utility will offer you a Net-Metering contract and also recharge your batteries from the grid when there is not enough sunshine available.

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