Do I Need an AC Transfer Switch for my Off-Grid System?

Many people assume they need a transfer switch for an off-grid system when using a generator to charge the batteries or power loads. If you are using an inverter/charger there is an internal transfer switch to select between the inverter’s output and an incoming AC source. Most inverter/chargers will remain in “Invert” mode unless they see an acceptable AC source coming through (Shore / Grid Power or Generator Power). Once the inverter/charger accepts the AC voltage, it transfers over to supply power to loads and dump excess power into the batteries. Once this AC source is disconnected, the inverter will transfer, and use battery power to power the loads. Some inverter/chargers will accept two AC sources and transfer between either of those and the inverter’s output. If this is how your system would be set up, then there is no need for an external transfer switch.

You may need an external transfer switch if you have an inverter/charger with only one AC input and you want to switch between two AC sources like a generator and shore/grid power. This is the case in most mobile applications. Another scenario is when you have a generator that can deliver more power than the inverter/charger can pass through and you want to power some heavy loads. The inverter charger is limited by the rating of the internal transfer switch. For example: Let’s say we have a Schneider 3.8kW inverter/charger with an internal 30A transfer switch with a split phase 120/240VAC output. The max load we can run by passing the generator power through is 7.2kW (30A x 240VAC = 7200 Watts). If we have a load that exceeded this 7.2 kW, and we have a generator that is able to satisfy this load, we would want the inverter/charger to be bypassed, allowing the generator to power the load directly. In this event we could use an external transfer switch to select between the inverter’s output and the generator’s output. When the inverter/charger is properly sized, this scenario does not occur very often.

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