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How to add smart control to a hot water heater
Electric hot water systems are one of the largest energy users in many Australian homes. They often run quietly in the background, using power at times that may not be ideal for your electricity plan, solar production or home energy goals.
By adding smart control to your hot water circuit, you can make your system much more flexible. Instead of simply letting the heater run whenever power is available, you can control when it heats, monitor how much energy it uses, and automate it around solar generation, off-peak tariffs or household demand.
In this SmartHome exclusive blog, we will show you how you can easily add smart control to your electric hot water heater using inexpensive Shelly devices – but first…
Why control your hot water heater?
A standard electric storage hot water system is often a simple resistive heating load. It turns electrical energy into heat and stores that heat in the tank for later use. Because the tank acts like a thermal battery, there is often flexibility around when the water is heated.
That makes hot water a great candidate for smart control.
Key benefits include:
1. Use more of your solar power
If your home has solar, your hot water system can be used to absorb excess solar generation during the day. Instead of exporting unused solar back to the grid, you can heat your water while the sun is producing.
For example, you could create an automation that turns the hot water heater on when solar export is above a certain level.
2. Reduce grid usage during expensive periods
If you are on a time-of-use tariff, you may want to avoid heating water during peak-rate periods. A smart control setup can help you schedule hot water heating during cheaper times.
3. Add remote boost control
Sometimes you may need extra hot water outside the normal schedule. With Shelly, you can create a simple “boost” button in the Shelly app, Home Assistant, Google Home, Alexa or another smart home platform (like Home Assistant).
4. Monitor energy usage
With the right Shelly device, you can track how much energy your hot water system is using. This can help identify whether the system is running longer than expected or using more energy than it should.
5. Build better home energy automations
Once hot water is connected to your smart home platform, it can become part of a broader energy strategy. You can combine it with solar monitoring, battery state of charge, electricity pricing, occupancy or manual override buttons… and more!
Important safety note
Hot water systems are fixed-wired mains appliances. In Australia, this type of work must be carried out by a licensed electrician.
Before choosing a control method, your electrician should confirm:
- The hot water system’s element size and current draw
- Circuit breaker rating
- Cable size
- Whether the system is single-phase or three-phase
- Whether the system is connected to controlled load/off-peak supply
- Installation location and enclosure requirements
- Device approvals and suitability for the application
This blog is intended as a general overview of what is possible, not a DIY guide. ALWAYS use a licensed electrician. NEVER attempt your own mains electrical work… it is illegal and very dangerous.
We have options...
We are going to cover two very popular solutions for controlling your electric hot water heater using different Shelly devices. The solutions I will outline below will suit most applications – but not all. Please feel free to contact us if your setup is slightly different and you need help identifying the best solution for your setup – we are always happy to help.
Option 1: Ogemray Shelly 25A Wi-Fi Relay
The Ogemray Shelly 25A Wi-Fi Relay is designed for larger single-phase appliance control. It is marketed for loads such as hot water heaters, pool pumps and other electrical devices up to 25A / 6000W, and it includes built-in power metering.
How it works
In this setup, the Ogemray 25A relay is installed in-line with the hot water heater circuit. The relay directly switches power to the hot water system.
Basic concept:
When the Shelly relay is on, the heater receives power.
When the Shelly relay is off, the heater is disconnected.
Because the device includes power measurement, it can also report energy consumption through the Shelly app and compatible smart home platforms.
Why this option is appealing
The main advantage of the Ogemray 25A relay is simplicity. It combines switching and energy monitoring in a single device.
For many standard single-phase electric storage hot water systems, especially those with a 2.4kW, 3.6kW or 4.8kW element, the 25A rating may be suitable on paper. A 4.8kW element at 240V draws around 20A, which is within the 25A rating.
Pros
- All-in-one solution
Switching and power monitoring are built into one device. - Rated for larger loads
The device is listed for up to 25A / 6000W, making it more suitable for heavy loads than smaller smart relays. - Built-in energy monitoring
Useful for tracking hot water energy usage over time. - Shelly ecosystem support
Can be used with Shelly Cloud, local control, Home Assistant and other supported integrations.
- All-in-one solution
Cons
- The full heater load passes through the smart relay
Even though the relay is rated for high current, it is still doing the heavy switching. - Less traditional than a contactor-based switchboard setup
Some electricians may prefer a contactor for long-term serviceability and separation of control/load switching. - Needs careful installation location
The device must be installed in a suitable enclosure with correct cable management, ventilation and access. - Not ideal close to the maximum rating
If the heater is near 25A, or if the environment is warm or enclosed, a contactor-based approach may be preferred. - Not suitable for every hot water system
Heat pump hot water systems, three-phase systems or systems with electronics may need a different approach.
- The full heater load passes through the smart relay
Best suited for
The Ogemray Shelly 25A Relay is a good option when:
- The heater is single-phase
- The load is comfortably under 25A
- Built-in power monitoring is desired
- There is a suitable installation location
- The electrician is satisfied with the device rating, approvals and installation method
Option 2: Shelly Pro 1 with a Contactor
The second option is to use a Shelly Pro 1 to control a contactor. In this setup, the Shelly does not directly switch the hot water load. Instead, it switches the coil of a contactor, and the contactor switches power to the hot water heater.
The Shelly Pro 1 is a DIN-rail mounted, single-channel relay with dry contacts, Wi-Fi, LAN and Bluetooth connectivity. It supports up to 16A, but in this application it is only controlling the low-current contactor coil rather than the full heater load.
Note: Although rated at 16A, it is never a good idea to run these modules near their maximum load rating for extended periods.
How it works
The contactor is installed in the switchboard or suitable enclosure. The hot water circuit passes through the contactor’s power terminals.
Basic load side concept:
When the Shelly turns on, it energises the contactor coil.
The contactor closes and supplies power to the hot water heater.
When the Shelly turns off, the contactor opens and the heater turns off.
Why this option is popular with electricians
This is a more traditional electrical control method. The Shelly handles the smart control, while the contactor handles the heavy electrical switching.
Contactors are specifically designed for switching larger electrical loads, and they are available in different current ratings to suit the circuit. This makes the setup flexible and scalable.
Pros
- The Shelly does not carry the hot water load
It only controls the contactor coil. - More electrician-friendly
A contactor-based setup is familiar in switchboard and control applications. - Better for higher loads or margin of safety
The contactor can be selected to suit the heater and circuit. - DIN-rail installation
Shelly Pro 1 is designed for professional DIN-rail installation inside a breaker box. - LAN connectivity
Shelly Pro devices include Ethernet, which can be preferred for reliability in switchboard installations. - Dry contact flexibility
The Shelly Pro 1 has potential-free outputs, giving more flexibility in control wiring.
- The Shelly does not carry the hot water load
Cons
- More components required
You need both the Shelly Pro 1 and a suitably rated contactor. - Power monitoring is not built into the Shelly Pro 1
The Shelly Pro 1 provides control, not hot water energy monitoring. If monitoring is required, you would need an additional metering device such as a Shelly Pro 3EM, Shelly EM, or another suitable meter. - Usually more switchboard space required
The Shelly Pro 1 and contactor both need to be mounted safely. - May cost more overall
The hardware and installation time may be higher than a single relay solution.
- More components required
Best suited for
A Shelly Pro 1 with contactor is a good option when:
- You want a professional switchboard-based solution
- The heater load is high
- You want the smart relay separated from the heavy switching load
- The electrician prefers using a contactor
- You want LAN connectivity
- The installation needs to be robust and serviceable
Which option should you choose?
For many standard home electric storage hot water systems, the Ogemray Shelly 25A Wi-Fi Relay is a very attractive option. It is compact, powerful, includes energy monitoring, and is designed for larger appliances such as hot water heaters.
However, for installations where long-term robustness, switchboard serviceability and electrical separation are priorities, the Shelly Pro 1 with a contactor is often the cleaner professional approach. The Shelly handles the smart control, while the contactor handles the high-current switching.
As a general guide:
Choose the Ogemray Shelly 25A Relay if:
- You want a simpler all-in-one solution
- You want built-in power monitoring
- The heater is comfortably within the 25A rating
- The installation location is suitable
You can get your Shelly Ogemray 25A module here.
Choose the Shelly Pro 1 + contactor if:
- You want a switchboard-based control solution
- You prefer the contactor to handle the heater load
- The heater is closer to the relay’s upper limit
- You want a more traditional trade-friendly installation
- You may add separate circuit-level monitoring later
“Where do I get a contactor” I hear you ask… you don’t! Leave this to your electrician, they will know the right contactor to get for your particular setup and will combine it with the Shelly 1 Pro if this is the direction you choose.
Final thoughts
Smart control of an electric hot water heater can be one of the most practical energy-saving upgrades in a modern smart home. It allows you to shift a large energy load to better times of the day, make better use of solar, reduce peak-rate usage, and gain more visibility over your home’s energy consumption.
For a simple, compact solution with built-in monitoring, the Ogemray Shelly 25A Wi-Fi Relay is a strong option.
For a more traditional, electrician-friendly switchboard setup, a Shelly Pro 1 controlling a suitably rated contactor is often the preferred professional method.
Whichever option is chosen, the installation should always be assessed and completed by a licensed electrician to ensure the hot water system, circuit, device rating and installation method are suitable.
As always, if you need help choosing the right smart home product for your requirements, do not hesitate to contact us, we are always happy to help.
