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

For many inverter and battery systems, a separate backup box is required for backup functionality (think of Growatt backup box, Huawei SmartGuard and Enphase System controller). This is usually installed directly after the main connection and manages the switching in case of a power outage.

However: 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 points to consider.

Want to learn more about separate backup boxes? Also check out this article

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

With an inverter with built-in backup, you can usually configure 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)

A clear example of this can be seen in the connection diagram of the Growatt WIT-HU inverters. In this 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 groups that fall 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 regularly done via a separate kWh meter/energy meter.

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

The bypass breaker: why it is important

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

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

• the inverter is off,

• you are performing maintenance,

• or there is a fault in the backup circuit.

Without a bypass, everything always runs through the inverter! Unless explicitly stated otherwise. And this poses a risk:

Beware of available power

If you continuously feed 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.

• In most cases (with 3 phase), this is also only 1/3 of the power. An inverter of 9kW often delivers a maximum of just 3kW per phase! See also how this is often represented: (example from the MOD-HU by Growatt).

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

Best practice: use an ATS (Automatic Transfer Switch)

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

This achieves two things:

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

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

Conclusion:
Install an ATS before the bypass, so that the Primary Load is only supplied 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)