I actually didn’t realise that had changed (in 2000) though it seems to have taken some time for the effect to ripple through. It might explain why Essential Energy apparently re-tapped my transformer a couple of months ago:
Oh, but we do! You can calculate it (using Ohms law.) If the nominal wattage is known at a nominal voltage then the resistance is given by R=V²/P, 14.7 ohms in this case.
Since the possible voltage ranges up to 253 (as per the spec quoted by Tom) the maximum current that will flow with an in-spec supply will be 17.2A.
This is relevant because the question was whether a 16A rated switch is suitable for this application and you quoted the nominal case, not the worst case. So the 16A rating is not “rather close but technically ok” - it’s actually less than the requirement.
.
sparkydave is right, do not use the Shelly relay alone for switching a higher power resistive heating element. It will not end well. Use a contactor rated for higher current switching capacity and many thousands of switching cycles. Then use a smart device for controlling it.
Even so, you need to ensure you are not switching such a load on and off too often.
You might want to consider an alternative, such as a Catch Power Relay.
The reason I stated what I did was because we are only TOLD the wattage, rather than calculating the resistance. I know we can calculate it but we don’t KNOW that it is correct unless we TEST it. Where this matters is in the fact we don’t KNOW for a fact what voltage the manufacturer has worked out that wattage from. It’s an educated guess.
If I can add to that. Always make sure when working with low voltages to connect the red wire to the positive. Alah — magic smoke…
I hope you don’t mind me jumping in here. I apologize in advance for the length of this message.
I’d like to take a moment to share my perspective on hot water systems, as I believe understanding them can be quite important. While there may be different viewpoints, I think it’s beneficial to consider various perspectives before making a decision.
I prefer to stay positive and hope you find this information helpful.
Why Twin Elements? Twin elements are increasingly useful, especially with rising electricity costs. They don’t double the power but rather feature two separate heating elements in the hot water system, one at the top and one at the bottom.
Each element typically has its own power supply, and understanding why is crucial. The top element heats the water first, and only once it reaches a certain temperature does the bottom element activate. Consult with the manufacturer and your electrician for specific advice, and consider having your electrician install two separate power supplies for the hot water system. The top element heats about 50 litres, ensuring you never completely run out of hot water. The bottom element, powered by the second supply, heats the entire tank and can be used for more energy efficient operation, including smart options.
Switching Controlling
As @sparkydave recommended, it’s better to use a contactor rather than a Shelly device directly to switch the load, especially for something as important as hot water, where reliability is key. This approach is more robust, and I fully agree with his advice. You can use a Shelly to control the contactor… a Shelly 1 mini is sufficient since it only needs to handle the small load required to switch the contactor. If you’re planning to expand your system, consider a Shelly 3 or 4 Pro, which allows for additional controls like garden lighting.
For power consumption monitoring, as Dave suggested, consider using a different method, like a Shelly EM 50. Always consult with your electrician to ensure everything is set up correctly and safely.
It’s best to wire the top element to its own power supply, keeping it on 24/7, so you always have 50 litres of hot water available. The bottom element should be wired through a contactor controlled by a Shelly device, allowing you to turn it on and off as needed. There’s no need for an old school auto/manual/off switch…. you’ve got Home Assistant (HA) for that.
You might consider adding a manual push button in the switchboard. This button can control the contactor through the Shelly, acting as a boost if you can’t access HA or if WiFi is down…. It is a handy safeguard. Just wire it to the Shelly and configure it as a press button.
Below are 2 links as a reference.
Smart Automation Home Assistant
To maximize savings, you can set up a timer to turn the bottom element on and off when electricity is cheapest. This simple step can save you a lot of money. I use my own blueprint for this: 
Timer. Additionally, I use another blueprint, 
Trigger - Run ON Timer, which boosts the hot water for one hour when needed.
The manual button in the switchboard, when pressed, turns the hot water on until you manually turn it off. However, since I use my timer daily to heat the water, the system resets each time the timer blueprint runs. This prevents the hot water from being accidentally left on, avoiding any surprise bills. This is why using a press button instead of a switch is crucial.
This is why twin elements are so beneficial. The top element ensures you always have 50 litres of hot water for basic needs, while the bottom element handles longer showers. The boost button is there if you run out of hot water or need a safeguard. While twin element systems do cost more… requiring an additional power supply and a more expensive unit…. I believe the benefits are worth the investment.
What Size Hot Water Storage?
To save money, it’s best to heat your water once a day when electricity is cheaper. Ensure you have enough hot water to meet your needs. For example, I opted for a 400-litre twin element system. The size you choose will depend on your household’s requirements
What Power Hot Water Storage?
Most people choose a 3.6 kW system, but I opted for 4.8 kW. Why? To prepare for future solar power integration. You can feed in power ranging from 0.1 amps to 20 amps. Think of it like an electric stove, where you use the available power. This consideration ties into how solar power can be utilized.
Solar Power
Think of your hot water system like a battery. Instead of selling excess solar power back to the grid, you can direct it to your hot water system first. Once the hot water system is sufficiently heated, you can sell any remaining power. Since hot water is a major cost in the home, using your existing hot water system for storage can be more economical than buying a separate battery.
I chose a 400-litre system with a 4.8 kW power rating to store and utilize maximum power. However, consult with your electrician to ensure compatibility with your solar setup, as your solar system might only produce 3.6 kW. In this case, a 3.6 kW system might be more appropriate.
For solar integration, consider solutions like the Fronius Ohmpilot, which directs excess power to the bottom element of your hot water system. The top element runs 24/7, while the bottom element benefits from excess solar energy, saving you money. Although I don’t have solar power yet, I’m preparing my system for future use.
Hope this helps you in some way.
Blacky 
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If you size the water heater such that once a day heating cycle is ample to carry the hot water demand as well as cope with the occasional higher demand day, then a twin element is unnecessary. Water stratifies in a tank with hot sitting on top of cold and there is very little mixing.
If you really think you need a top element due to running out of hot water regularly, better to get a bigger tank than one with more complexity (and work on the demand side too by reducing hot water demand, e.g. use low flow shower heads).
If the largest tank that will fit is inadequate then sure, a second element can help extend its capacity a bit but it may turn out to be pretty expensive to operate given the heating may occur during times of peak electricity rates. In that case an alternative solution would be better, such as a heat pump water heater.
Choice on heating element power rating will depend on what is powering the water heater. If you are timing it to use cheapest grid electricity, then yes, provided the home wiring and supply/circuit board capacity is suitable.
But if you plan to use your own solar PV to heat water during the day then choice of heating element matters more and lower power elements may be a better choice.
I’m not sure what the OP is thinking as they did not mention why they want to have smart control of the water heater.
If using solar PV is desired and you are planning on using smart switching to turn on/off power to a water heating element, then you will want to size the heating element to suit two things:
- the likely available solar PV capacity
- the hot water demand and how long it takes to heat water.
The problem can be explained by this image:
The yellow represents solar PV output (which will obviously vary depending on a whole host of factors) while the red blocks represent the same amount of energy (7.2 kWh as a token example), just consumed at different power levels. In this case 1.8 kW, 2.4 kW and 3.6 kW. The lower power element is the shorter, wider block. In this case the solar PV system is modestly sized.
We can see in this instance that the higher power heating element would require a portion of energy to be imported from the grid. A 4.8 kW heating element would be even worse for this use case. This is where choosing a lower power heating element is often a better choice. but not too low as the time required to reheat is extended, so if HW consumption is high then some compromise is required.
However you may have a larger PV system, in which case you will have this scenario:
And here it doesn’t really matter as the water heating power demand will be covered. But then one needs to consider what happens on poor / variable solar PV days, like this:
You can time when a heating blocks starts/stops but fitting a high power heating element demand under the available solar PV output is much more difficult than with a lower power heating element. Inevitably more supplemental grid energy will be required with a higher power heating element.
This of course is only showing the water heater demand relative to PV output examples and not available excess PV. That is of course dependent on other household loads and there may be competing demands.
A relay can be programmed to track the available excess PV and switch relays on/off to match but too frequent switching under load is not a good idea.
So I think it will help to understand why the OP want smart control? Saving money is usually the answer but the methodology will vary depending on various factors. A standard resistive element water heater may not be the optimal choice.
We use a 315 litre resistive element tank but I have a variable power PV diverter which follows available excess PV. Over several days it looks like this:
The tank has ample capacity to ride through a few poor solar PV days.
What’s with all the ancient technology?
Resistive elements?
Why use 3.6kWh to heat water directly when you can use 1kWh to move 3.6kWh of heat from one place to another?
Heat pump hot water FTW.
Back when I was studying (a very long time ago) I did a price analysis of the 5 major water heater designs (at the time) and the results were like this: (if I can remember everything correctly)
| Type | Initial cost | running cost | Time to break even |
|---|---|---|---|
| Gas storage | 2 | 3 | 3 |
| Electric storage | 1 | 5 | 4 |
| Gas instantaneous | 4 | 2 | 1 |
| Electric instantaneous | 3 | 4 | 2 |
| Solar | 5 | 1 | 5 |
| 1= best | |||
| 5= worst |
I’ve probably got some wrong since it was over 20 years ago, plus things have changed now with the addition or solar power on homes. The main thing that I remember standing out at the time was that the initial cost of solar hot water (direct solar, not from electricity generation) was that it took 10 years to break even against the cost of instantaeous gas which was the cheapest overall.
Heat pump systems also didn’t exist back then.
I run instantaneous gas at my place. Never ending hot water and I only use energy when I want the hot water. Gas is cheap in Australia.
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Thanks for your insight.
Maybe I didn’t explain myself clearly.
A twin element?
The reason many manufacturers offer a twin element is to take advantage of off-peak power. This isn’t new technology… it’s been used successfully for over 30 years.
I have a 400 litre hot water system, but I want to stay on topic with @Opaquer’s question and his decision to go with a 315 litre 3.6 kW Rheem system. 400 litres is the maximum capacity you can get in that brand. While there are other options like heat pumps, which work differently, it’s important to consider what suits your needs. Since @Opaquer has already made his decision, I’ll focus on that.
In real life, I sometimes run out of hot water… not always, but it does happen. Even though I have water-saving shower heads, I also have three children, and as any parent knows, telling them to take a quick shower doesn’t always match reality. Honestly, I’m guilty of taking longer showers too… it’s just life.
A second element doesn’t increase the overall capacity of your hot water system. While it can operate 24/7, which means you’ll pay the current electricity rate, it is designed to function efficiently and correctly. The second element (top element) will only activate if the temperature at the top of the hot water system drops below its set point. In the rare event that you run out of hot water, the second element provides an additional 50 litres, which is typically sufficient for a shower (for me), washing dishes etc.
Note that with a twin-element system, there are two temperature sensors… one at the top and one at the bottom… for each element.
When using a hot water system with just a bottom element, it relies on convection to gradually heat the entire water storage. It slowly reaches your set temperature, typically around 60°C, starting from the bottom. With a twin element system, convection heats about 50 litres of water at the top to 60°C. Both elements never run simultaneously. In fact, you can run two power feeds from a single circuit breaker if your electrician approves.
As I mentioned, the top element must reach its set temperature before power switches to the bottom element. This design is intentional by manufacturers. Unfortunately, I’ve seen electricians mistakenly wire off-peak power to the top and 24/7 power to the bottom, which isn’t correct. The elements aren’t meant to run together to heat the water faster; they are designed so the top element satisfies its requirement first, then switches to the bottom element.
In a 3.6 kW unit, both the top and bottom elements are 3.6 kW. In my experience, the top element only activates occasionally, meaning you only heat 50 litres of water at peak rates, not the full 315 litres (or 400 litres in my case). If I choose to press the boost button, it will turn on the bottom element, running at the current rate, but that’s my decision at the time.
Yes and no. I did suggest consulting with your electrician, especially if you’re considering a 4.8 kW unit with only 3.6 kW of available power… it might not make sense. If you look into the Fronius Ohmpilot, you’ll see it only outputs 3 kW, so pairing it with a 4.8 kW unit might not be the best choice; a 3.6 kW unit could be more suitable. However, if quick recovery when using the boost button is a priority, you might still opt for the 4.8 kW. Again, consulting your electrician is key… they can provide the best advice. Personally, I chose a 4.8 kW unit and plan to write my own software for it using Home Assistant, which is one of the reasons we love it.
If you review your graphs, you’ll see the benefits of the Ohmpilot, which is exactly why Fronius developed it.
Let’s take a step back.
Just because you have a 4.8 kW element doesn’t mean you have to use its full capacity all the time. Think of it like charging a battery… there’s a limit to how much power you can send to it at any given moment. Here’s a simple analogy:
Imagine you have a pot of water on an electric stove with a temperature setting ranging from 1 to 10. If you set it to 1, it will slowly heat up the water, maybe reaching 60 degrees. You can adjust the heat up or down during the process, or crank it up to 10 to heat it faster. The Ohmpilot works similarly, automatically adjusting based on the power available.
As your solar panels generate electricity, the Ohmpilot will prioritize heating your hot water system before sending any excess power back to the grid. It won’t draw from the grid; it will only use the power your solar system generates. If your solar system can produce 4.8 kW and your hot water system can accept that amount, I plan to develop software that will direct the full capacity to the water heater before any surplus power is sold back to the grid.
However, if you have a smaller 3.6 kW unit, it will only take 3.6 kW, and any additional power, like the extra 1.2 kW in this example, will be sold back to the grid. By using the full capacity of what your system can generate, you can heat your water faster, keeping in mind that this value can fluctuate depending on the weather and your power needs throughout the day.
As I said I don’t have solar yet so every night when power is cheap I heat it up ready for the day. Heating your water at night when electricity rates are lower is an excellent way to save money. The 4.8 kW unit is particularly effective for quick recovery, which comes in handy when you need hot water quickly (boost button). It also sets a great example for my children about energy conservation and being mindful of water use. When the hot water runs out, it naturally leads to conversations about conservation. In fact, my children often get their siblings involved to address the situation, so you don’t have to step in 
Many people invest $10,000 to $15,000 in home batteries, but you already have a hot water system that can serve a similar purpose. My post aims to encourage people to consider this option. Heat pumps operate differently, and the choice ultimately depends on your preferences. I chose to use my hot water storage as a form of battery.
We all have different approaches, and that is a good thing. Thanks for sharing your methods!
Blacky
There are a lot of factors people consider when deciding if a heat pump is the right choice for them.
- Space: Heat pumps tend to take up more space. Inside or in apartment?
- Noise: Even minimal noise can be bothersome if you value peace and quiet.
- Storage Temperature:
- Heat Pump: 50°C to 60°C (122°F to 140°F)
- Traditional Electric Hot Water: 55°C to 75°C (131°F to 167°F)
- This difference can affect your storage capacity. For example, 315 liters at 59°C will yield less mixed hot water than 315 liters at 70°C, which can be beneficial during cloudy days with a solar system. Generally, heat pumps have lower storage capacities.
- Lifespan:
- Heat Pump: 10 to 15 years. They have more complex components like compressors and fans, which can wear out over time, especially in harsher climates.
- Traditional Electric Hot Water: 15 to 20 years depending on the quality of the tank.
- Climate: Heat pumps may perform differently in cold climates compared to warm ones.
- Maintenance:
- Heat Pump: Requires maintenance
- Traditional Electric Hot Water: Typically no maintenance
- Moving Parts and Risk of Breakdown:
- Heat Pump: Yes, which can lead to potential breakdowns
- Traditional Electric Hot Water: No, leading to potentially fewer issues
- A single breakdown in a heat pump could negate any savings. Take note of the warranty period and what they will provide. Do they break down out of warranty
- Running Costs:
- Heat Pump: Can be very efficient. Most modern heat pumps have a COP ranging from 2.5 to 4.5.
- Traditional Electric Hot Water: Less efficient, with a 1:1 energy usage ratio, but this depends on usage and setup. It can be very expensive to run, so smart controls, as suggested above, are recommended.
- Initial Cost:
- Heat Pump: Generally, more expensive upfront than traditional systems.
- Traditional Electric Hot Water: Typically, less expensive to purchase and install.
All the figures provided can be subjective, with different opinions to consider. There’s no right or wrong way… it all depends on what you prefer, much like choosing between red grapes or green grapes.
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Except for replacing the sacrificial anode before the tank rusts out. 
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Yeah but who does that 
. Normally it to hard to get at. It easier to just replace at around the 20 year mark. I replaced my one at the 23 year mark not because it wasn’t working or leaking it is a better way to do it than to have no hot water on Sunday. I planned it and got a good price something I would recommend people do.
The Rheem systems have the anode go in through the top so it’s easy to swap out in about 5 mins without losing any water. 
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Water heating is one of those things where the optimal technology choice varies by household given the multitude of factors in play.
Heat pumps are a good option and I did mention them as something to consider. But they are not always suitable.
The biggest factor is a heat pump (at least one of reasonable quality) is a very expensive item to install and depending on the home’s hot water consumption and the tariffs in play it often works out far more expensive over its lifetime than solar PV + resistive element water heater.
I had quotes for a decent quality heat pump water heater, it was going to cost at least $3.5k more. We consume, on average, 5 kWh/day for water heating using a standard electric resistive element.
So at best a heat pump is going to save us perhaps 3.5-4 kWh/day. We already had solar PV with ample capacity for either technology, meanwhile our solar PV feed in tariff is 1.4 c/kWh, so a heat pump was going to save us ~5c/day for an item that was going to cost several thousand dollars more to install. That makes zero sense.
An electric resistive element water heater may be old tech, but it is highly reliable technology, very cheap to maintain/service, very quiet and the few parts you may need over its life are all bog standard off-the shelf items. The water heater I replaced was 30 years old and still operational but on borrowed time which for an indoor installation I figured better to be ahead of the game.
If you have ample excess solar PV, and many homes in Australia do, then heating water this way makes perfect sense.
Heat pump water heaters on the other hand, unless you spend a LOT of money, have a terrible reputation for reliability, and of course add the noise of another compressor, some are better than others (the $$ again). The industry has shot itself in the foot by dumping low quality kit on the market chasing govt incentive programs.
Our feed in tariff is 1.4c/kWh, while some of our grid energy is free (12-2PM every day). Close enough to all of our water heating is covered by solar PV, just a little bit of supplemental grid power. At 5 kWh/day, our water heater costs less than $30/year to operate.
But if you have a large family home with high hot water demand, do not have a lot of solar PV (or none) and are reliant on grid tariffs for heating water, then a heat pump water heater may be an excellent choice. Just make sure it is a decent quality unit, else you’ll be replacing it in short order.
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Yeah, things have changed a LOT since then. Heat pumps are more common and the cost of rooftop solar PV is very low (in Australia at least). Tariffs have changed a LOT too with modern metering meaning time of use and demand tariffs making instant electric one to avoid unless it’s an infrequent use application. Meanwhile solar soaker tariffs are emerging making daytime electricity very cheap. Gas prices are not going to fall while domestic gas supply in Eastern Australia is heading towards a crunch.
You can use off-peak for heating water with single element tanks. There are hundreds of thousands of them in service in Australia. Including at my home. An extra upper element is just a way to provide a quick boost if you’ve run out of hot water or the tank is getting low on hot water supply.
These “controlled load” circuits, where the power supply is controlled by the utility, have stipulations on tank and heating element size as which type of controlled load supply you have. Those CL supplies which operate for fewer hours each day have a larger minimum tank capacity so as to avoid the scenario of running out of hot water between heating cycles.
There’s not a lot of downside to having a larger tank if it can fit. It really doesn’t cost any more to run as that is a function of how much hot water is consumed. There is a little bit more heat loss per day with a larger tank but that is minimal. But the larger the tank the less need for boosting when energy is more expensive.
Yes, it does. Indeed such tanks are sold as having both a volumetric capacity and an effective hot water supply capacity which is greater than the volumetric capacity. It’s the very fact that the upper element can switch on as the lower boundary of the hot water column moves up that enables the tank’s effective hot water delivery capacity to be greater.
There’s a nice video to explain it:
I have a variable power PV diverter. It works in a similar fashion to the OhmPilot, it just costs 1/3rd of what the overpriced Fronius option does. And I have a Fronius Symo inverter.
My unit is capable of diverting up to 4.8 kW if you have an element of that rating. Been using it for nearly three years.
As I explained, I use a variable power PV diverter which constantly monitors available excess solar PV capacity and diverts what is available to the water heater.
The OP however was not talking about such a device, rather they are discussing a relay control to turn the power supply on or off, so the power delivery in that case will be zero or the rating of the heating element, there would be no in-between.
Hence why consideration of the heating element power rating matters.
In much of Australia (perhaps not the more southern parts) if you have a decent sized solar PV system and consumption is modest (e.g. 1-3 people) then from a price, operational cost and effectiveness perspective it’s pretty hard to beat a basic programmable timer relay plus contactor to run your large tank water heater during the peak solar window. Adding more tech adds cost that is hard to recoup because our daytime energy costs are so low.
If the tank is large enough it will have the capacity to ride through 1-2 poor solar days, no need for boosting or a second element.
But if demand is high then alternatives are going to be required, as you note.
What time of day energy is cheapest is highly variable around the world. Much of Australia is moving to “solar soaker” tariff structures where the cheapest energy is the middle of the day. Several retailers are already offering free energy at this time of day. My 12-2PM tariff is 0c/kWh. When I use excess solar PV I am foregoing just 1.4c/kWh in feed-in credit.
Indeed and we do exactly that. I also have a battery but mine is an off-grid system I built for a fraction of the cost of those stupidly expensive systems like Powerwalls etc.
Yes, the optimal approach will vary for each individual home’s unique circumstances. You’ve noted many of the factors to consider.
We chose a stainless steel tank (our water supply is suitable) so have eliminated that concern. Our last tank using anodes was 30 years old. Changing an anode is a pretty simple task.
I went with the stainless steel option as it meant we could fit a larger tank. A regular larger tank would not have had sufficient head room for anode changes in our case.
Instantaneous Gas is what I run on all my houses. Is there any compatibility to control the temperature from Home Assistant? I run Rinnai with wall mount room controllers.
There is a wifi adaptor you can buy and use a custom integration. I haven’t done it myself as I almost never change the temperature so see no need. If I had the preheat system with looped hot water piping like in my last house I would as I could then automate that function which would be nice. If I add that system later I may do it.
I will have to look into it. Only changing of temps I do is 50 Degrees from the kitchen controller. Then when transferring to the bathroom setting the hot water to 42 so my 6 year old does not burn himself in the bath. For showers I tend to just set the temperature and use the hot tap only.
I do the same.
In case you don’t know, you can open up the unit and flick a few DIP switches to increase the max temp to 55C.