Switchboard, ATS, and Backup Wiring

The switchboard is the collection and protection hub of an entire hybrid system: the meeting point of grid, inverter, battery, loads, and solar panels. One component that often causes confusion is the automatic transfer switch (ATS): when is it needed and when is it not? This document explains switchboard layout, the role of the ATS and the backup port, and how to decide whether an ATS is required.

For: technicians designing switchboards and carrying out installations, sales staff explaining equipment line items in quotations, newcomers who need to understand hybrid switchboards.

Quick summary

A hybrid switchboard consolidates all switching and protection devices for both the DC and AC sides (including circuit breakers, surge protection devices, the backup port, terminals, and sometimes an ATS), connecting the five key points: grid, inverter, battery, loads, and solar panels.

An ATS (automatic transfer switch) automatically transfers loads between the grid and the inverter backup source, while blocking dangerous backfeed to the grid when a power outage occurs.

Many hybrid inverters have a dedicated built-in backup port (EPS port), so an external ATS may not be needed. However, there are important exceptions: the standard Growatt SPH still requires an external ATS-T module; only the SPH UP series integrates the function natively and therefore needs no external ATS.

Hybrid switchboard layout

The hybrid switchboard is the electrical hub of the system, where the five groups (utility grid, inverter, battery storage, household loads, and solar panels) connect and are protected. Inside, it typically contains circuit breakers for both DC and AC sides, surge protection devices (SPDs), terminals, a changeover switch for the backup circuit, and, in systems requiring full automation, an ATS.

This arrangement ensures that every branch has its own isolation and protection device, so that during maintenance or a fault the correct branch can be de-energized without affecting the rest of the system. Details on individual protection devices such as surge arresters, fuses, and residual-current devices are covered in Chapter 01.

What an ATS does

An ATS (automatic transfer switch) performs three main functions. First, it isolates the backup load group (lights, Wi-Fi, cameras, refrigerator, and fans) so that these loads continue to run during a power outage. Second, it blocks dangerous backfeed, preventing the inverter backup source from feeding back onto the grid and protecting both utility workers repairing the line and the system itself. Third, it handles automatic source transfer: switching to the backup source when the grid goes down and switching back when the grid is restored, with no manual intervention required.

When an ATS is needed and when it is not

The answer depends on the inverter type and the system design. Many hybrid inverters already have a dedicated backup port (known as the EPS port) that automatically islands from the grid and supplies the backup load group during an outage. For these inverters, correctly wiring the backup port and using appropriate circuit breakers is usually sufficient, with no external ATS required.

An ATS is appropriate when fully automatic source transfer for the entire house is desired, or when the backup loads are large, span multiple branches, and require more stable and manageable source changeover. An ATS should be avoided when the only option is a cheap, low-quality unit prone to overheating contacts and transfer faults, or when the backup port is wired incorrectly in conjunction with an ATS in a way that causes backfeed and inverter errors.

Sales takeaway

When a customer asks why a quotation does or does not include an ATS, the answer depends on the inverter. If the inverter has a dedicated backup port and the requirements are straightforward, an ATS may not be needed, which reduces cost. If the customer wants fully automatic source transfer for the whole house with no manual intervention, or has large backup loads, a quality ATS is a worthwhile investment. What must be avoided is promising a customer that a given inverter needs no ATS when the specific product line still requires one (as is the case with the standard Growatt SPH), because this results in missing equipment at installation.