Inverters and batteries with integrated backup: how to connect them?

For many inverter and battery systems, you need a separate backup box for backup functionality (think of Growatt backup box, Huawei SmartGuard and Enphase System controller). This is usually placed directly after the main connection and controls the switching during a power outage.

But: there are also systems where this functionality is already built into the inverter or battery. In this article, we explain how to set up such an installation and what extra things to watch out for.

Want to know more about separate backup boxes? Then also check out this article

Inverter/battery with built-in backup: the basic setup

With an inverter with built-in backup, you can usually set up the installation in two ways:

• Only part of the house on backup (critical loads)

• The entire house on backup (if the system's capacity allows this)

A clear example of this can be seen in the wiring diagram of the Growatt WIT-HU inverters. In that series, you can also connect a generator to the inverter — handy, but note: this is not possible with every brand or type.

What do you see in the diagram?

In the diagram, the backup side is often shown as a separate output (for example “Load” or “Backup”).

• The circuits under backup are often called Primary Load (or “critical load”).

• The total consumption is, in this case, measured with CT coils. In other systems, this is often done via a separate kWh meter/energy meter.

In short: you measure what is happening in the house, and you control a selected part (or all) via the backup output.

The bypass breaker: why it is important

In many diagrams, you will also see a “bypass breaker” (bypass switch).

The bypass is not just for show: it ensures that the Primary Load can always receive power, even when:

• the inverter is turned off,

• you are performing maintenance,

• or there is a fault in the backup circuit.

Without a bypass, everything always runs through the inverter. And that carries a risk:

Watch out for available capacity

If you continuously supply the Primary Load via the backup output of the inverter, it may be that:

• the maximum output of the inverter/battery is lower than the capacity of the main connection,

• and you therefore have less power available than you are used to.

You will especially notice this during peak loads (cooking, heat pump, charging station, etc.).

Best practice: use an ATS (Automatic Transfer Switch)

The most practical solution is to not perform the bypass “manually”, but with an ATS (Automatic Transfer Switch).

With this, you achieve two things:

1. During normal grid operation, the Primary Load runs directly on the grid (not unnecessarily via the inverter).

2. During a power outage, the ATS automatically switches to the backup output (“Load”/“Backup”) of the inverter.

Conclusion:
Place an ATS before the bypass, so that the Primary Load is only fed via the backup output during a power outage. In practice, this usually provides the most usable power and makes the installation more reliable.

Schematic (with ATS)