You've correctly identified that a standard shunt trip breaker won't work for your application. They're designed to be tripped when power is applied, not to disconnect when power is lost.Here's a breakdown of the problem and some potential solutions, ranked from simplest to more complex

The Core Problem:

You need a device that automatically cuts power to specific circuits when grid power fails, and restores power to those circuits when grid power returns. It needs to be fail-safe and simple.

Solutions

1.

Normally Closed (NC) Relay with a Power Supply

This is likely the simplest and most reliable solution.

Components



Normally Closed (NC) Relay

Choose a relay with a coil voltage that matches a readily available low-voltage DC power supply (e.g., 12VDC or 24VDC). The relay contacts need to be rated for the voltage and current of the circuits you want to disconnect (120VAC, at least 50A for the AC and 30A for the Water Heater would be best, but you could parallel multiple relays if needed for the current rating).

Low-Voltage DC Power Supply

A small, reliable power supply that outputs the DC voltage required by the relay coil. Choose one that is UL-listed.

Enclosure

A suitable enclosure to house the relay and power supply for safety and weather protection.

Wiring

Connect the DC power supply to a grid-tied circuit (the main panel, before any battery backup system). This is crucial. Wire the AC input of the water heater and AC compressor through the NC contacts of the relay. The relay is designed to pass power when it is NOT energized. If the grid fails, the power supply will loose power, de-energizing the coil of the relay and the circuit will open, cutting power to the AC and water heater circuits. If the DC power supply can handle it (usually the case with low-draw controllers), use the 120VAC input signal on the SolarEdge Load Controller as a signal to turn on the DC power supply. This would disable both loads during a backup, but might work as an interim measure.

How it Works

When grid power is present, the DC power supply provides power to the relay coil. The relay is ENERGIZED, keeping its NC contacts OPEN (which passes power through to the circuits connected to the AC and Water Heater). When grid power fails, the DC power supply loses power. The relay DE-ENERGIZES, and its NC contacts CLOSE, cutting power to the AC and Water Heater circuits. When grid power returns, the DC power supply powers back up, energizing the relay, re-opening its NC contacts, and restoring power to the AC and Water Heater circuits.

Advantages



Simple and Reliable

Minimal components, easy to understand, inherently fail-safe. If the power supply fails, the relay will drop out and disconnect the circuits.

Cost-Effective

Relays and power supplies are relatively inexpensive.

Fail-Safe

If the power supply or relay fails, the circuits are automatically disconnected.

Disadvantages

Requires a bit of wiring. Need to select the correct relay (voltage, current rating, NC configuration). Requires an enclosure.2.

NO Relay, Manually Switched



Components

A normal AC relay A 120VAC 100A breaker.

Wiring

When grid power is present, the power to the water heater and compressor is routed from the breaker through the NO contacts of the relay. When the grid goes down, someone manually deactivates the breaker. When grid power returns, they manually reactivate the breaker.

Advantages

Simple and Cost-Effective

Disadvantages

Requires someone to be on-site during the power outage and to remember to reactivate the breaker Not Fail-Safe3.

Motorized Breaker (with reverse logic)

A motorized breaker that closes on loss of power. You would need to make sure that it would always open on a return of grid power.

Components

A Motorized Breaker.

Wiring

When grid power is present, the circuit is open and the water heater and AC will not work.

Advantages

Automated and reasonably simple.

Disadvantages

Requires an enclosure. Complex implementation Can easily become dangerous with wrong wiring. May not be very cost effective.

Why not the Load Controller + Shunt Trip? While technically feasible, it's overly complex. The Load Controller is designed for more sophisticated load management, and using it to just trip a shunt breaker for these two circuits is overkill. It's adding unnecessary points of failure and complexity.

Important Safety Considerations



Licensed Electrician

All electrical work MUST be performed by a qualified and licensed electrician in accordance with local codes. This is not a DIY project.

Proper Wire Sizing

Ensure all wiring is correctly sized for the amperage of the circuits.

Enclosures

Use properly rated enclosures for all components to protect against electrical hazards and environmental conditions.*

Testing

Thoroughly test the system after installation to ensure it functions correctly and safely.

Recommendation

I strongly recommend the

Normally Closed Relay with a Power Supply solution. It's the simplest, most reliable, and inherently fail-safe option for your needs. Work with a qualified electrician to select the appropriate components and perform the installation. Flag for review