The New Catch Power Green Hot Water Diverter Reviewed: Smaller, Cheaper, Cuter

Catch Power Green Catch PV diverter review

Hot water diversion is now more affordable with the new Catch Power ‘Green Catch’

Hi everyone.  Happy New Year!  This is a short article about a new hot water diverter developed by Australia’s Catch Power.  I’m just letting you know it’s available and because I don’t really have anything bad to say about it this may seem like an advertisement. 

Regrettably, I have little choice other than to write articles that read like ad copy now that Finn has fired me for writing the word “megawatt-hour” when I should have written “megawatt”.1  Little choice, that is, if I want to score a new job in advertising!  Specifically, on Madison Avenue in New York.  I saw a TV show and there’s an agency there where everyone is totally retro and they all dress like it’s the 1950s.  I could totally see myself working there.

If I shaved this is exactly what I would look like in a suit.

Advertising companies need people who enjoy pointing out flaws in products, right?

What’s A Hot Water Diverter?

A hot water diverter is a device that sends surplus electricity from your solar power system to your hot water system.  This saves you having to use expensive grid electricity to heat water, although it will cost you the feed-in tariff you would have received if the solar energy had instead been sent into the grid.  This means they are no good to you if you are still receiving an old high feed-in tariff.  This article goes into more detail on diverters, this article compares different types, and here is our Diverter Comparison Table.

If a home has a solar power system of 5 kilowatts or more it is possible for a diverter to allow over 90% of the electrical energy the hot water system uses to come from rooftop solar, although the exact percentage will depend on location and hot water consumption habits.

If

  • your solar feed-in tariff is considerably lower than what you pay for the grid electricity your hot water system uses (which could be a controlled load or off-peak rate) and
  • your hot water consumption is high enough,

then a diverter can be an effective investment.

The New Green Catch Diverter

Catch Power produces a Blue Catch diverter, but their new product is a more compact and lower cost version of their Green Catch diverter that used to look like this:

Previous Green Catch model

Oh god! So ugly! Avert your eyes!

But now it looks like this:

Green Catch new model

Yeah, that looks a bit better.

However, the above picture doesn’t give a good impression of the true size of the device:

Green Catch PV diverter size

Oh my ghawd! It’s so cute!

Tech Specs

Below are some technical specifications taken from a brochure you can see in full here:

Green Catch technical specifications

The maximum hot water system heating element you can use the Green Catch diverter with is 4.8 kilowatts.  Most hot water systems have elements that size or smaller, although there are exceptions.

The warranty is 5 years, which is a lot better than the 2 years some diverters have.

Price

The new Green Catch diverter has a Recommended Retail Price of $680.  This is a considerable improvement as Catch Power previously told us the installed cost of the ugly old Green Catch was $1,000.  Getting a qualified electrician to install a cute little new one is likely to cost over $200 and Catch Power has told me the typical installed price is $850 to $900.  Hopefully, as this diverter becomes more popular, the installation price will come down or at least be at the lower end of that range, as practice make perfect.  Or at least faster.

Western Australia Is The Best Location

The best place to install a hot water diverter is Western Australia because:

  • there are no longer controlled loads available to reduce the cost of operating electric hot water systems and
  • WA’s solar feed-in tariff is only 7.1 cents.2

But the following conditions are important:

  • An electric hot water system is required instead of the gas hot water most WA homes have.
  • A home or business should be on a standard tariff that charges a fixed amount for every kilowatt-hour of grid electricity used rather than a time-of-use tariff that lets hot water systems use lower cost off-peak electricity late at night.  A standard tariff is usually better for solar homes.

Somewhat offsetting the value of a hot water diverter is the simple fact Western Australia doesn’t get that cold, limiting the amount of hot water people use.  But even a household with modest hot water use can benefit from a diverter.  If a two person household has the following characteristics:

  • They use an average of 4 kilowatt-hours a day for heating water.
  • They have the Synergy standard tariff that charges 28.3 cents per kilowatt-hour.
  • Their solar feed-in tariff is 7.1 cents.
  • They have a solar power system of 5 kilowatts capacity or more.
  • A diverter will allow them to get 90% of the electricity their hot water system uses from their solar system.

In this scenario the household would save:

  • 76 cents a day.
  • $279 a year.
  • $1,393 over the 5 year warranty period of the Catch Power Green .

So even if the Green Catch diverter died the day after the end of its warranty — which it hopefully won’t do — they would still be almost $500 ahead even if the installed cost was $900.  If it keeps working just a couple of years past the end of its warranty the household will be over $1,000 ahead.

It seems like a no-brainer.  Especially for larger households with higher hot water consumption, which can have far higher savings.  However, having a diverter installed needs to be compared to the cheaper alternative of putting the hot water system on a timer that switches it on during the day.  The effectiveness of this approach varies with the size of the heating element.  Smaller elements are able to use a higher portion of solar energy as they are less likely to draw more than the available amount of surplus solar power.

If we assume the following:

  • Having an electrician install a hot water system timer costs $200.
  • The timer enables the hot water system to get 50% of its electricity consumption from rooftop solar.
  • Everything else is the same as in the example above.

Then the household will save:

  • 42 cents a day.
  • $155 a year
  • $774 after 5 years.

In this example, thanks to its low cost, the household can be better off with a timer after 5 years.  But a diverter doesn’t need to operate much longer for it to clearly pull ahead.  While it will depend on the individual circumstances, a Green Catch diverter will often be the best choice by a wide margin.

A Queensland Example

Hot water diverters can pay for themselves outside of Western Australia, but a household may need to be a large user of hot water for it to be worthwhile.  I’ll give an example of what kind of savings can be expected for a two-person household in rural Queensland since I happen to have the electricity bills for this kind of home in front of me.  This household has the following characteristics:

  • It uses only 3 kilowatt-hours a day for hot water because the climate is even warmer than Perth’s.
  • The hot water system is on a controlled load called Tariff 31 that is 19.2 cents per kilowatt-hour.
  • Grid electricity is 27.8 cents per kilowatt-hour.
  • The solar feed-in tariff is 9.4 cents.
  • It is assumed 90% of the hot water system’s electricity consumption will be solar after the diverter is installed.

Under these circumstances a hot water diverter will save the household:

  • 18 cents a day.
  • $66 a year.
  • $331 over the 5 year warranty of the Green Catch diverter.

If the Green Catch diverter lasted for 10 years — twice as long as its warranty —  it still wouldn’t save this household money.  If the household was larger and used twice as much hot water while keeping everything else somehow stayed the same, the diverter’s simple payback period would be 6 to 7 years.

Because the cheaper tariff 31 is available for hot water systems, using a timer often won’t be a better option than using a diverter.

The Future Of Diverters Looks Brighter

While the advantages of PV diverters aren’t nearly as clear outside Western Australia, under the right circumstances they can still save money.  The decrease in the price of the Green Catch is likely to improve its popularity and Catch Power should be able to make plenty of sales in WA.  So I’m thinking the future of hot water diverters is brighter than it was.

Unfortunately, now that I’m unemployed, my own future is looking pretty bleak.  With no way to pay rent to my ex-wife, my children will be soon be thrown out into the snow.  (Finn is having some snow specially imported for them to be thrown into.)  My only hope is for some kind of New Year miracle — such as my being sacked turning out to be just a low grade running joke I’m refusing to let go of.  I sure hope this turns out to be the case because I don’t actually want to work in advertising.  Who would want to go to a drab, soul destroying, advertising office like this everyday?

Footnotes

  1. It was a fair cop.
  2. And you can only receive that feed-in tariff if your solar inverter is 5 kilowatts or less.  If it’s larger they just steal your excess solar electricity and give you nothing.  They don’t even buy you flowers and take you to the movies first.
About Ronald Brakels

Many years ago now, Ronald Brakels was born in Toowoomba. He first rose to international prominence when his township took up a collection to send him to Japan, which was the furthest they could manage with the money they raised. He became passionately interested in environmental matters upon his return to Australia when the local Mayor met him at the airport and explained it was far too dangerous for him to return to Toowoomba on account of climate change and mutant attack goats. Ronald then moved to a property in the Adelaide Hills where he now lives with his horse, Tonto 23.

Comments

  1. Mike Gaskin says:

    Or, you can go with a pure solar hot water heater. I have one here in Mexico where they are very popular. Cost about 400 u.s. dollar equivalent installed. They have two types, one for pressurized systems or like mine that is gravity from a roof tank higher than the heater. It has way hotter water than a gas heater and the insulated tank is way larger. Four people can take a hot shower day or night. It produces acceptable hot water even on overcast days.
    I have had electric, gas, instant on gas heaters and by far this one beats them all hands down. Saves me 35 bucks a month on gas, paid for itself in one year and still rocking on.

    • Donald Firth says:

      I think the cost of relocating to Mexico every time I want a shower would somewhat negate the savings!

    • Pure solar HWS are reasonably popular here, but relatively expensive. If I had a conventional electric HWS, installing this diverter would likely be a lot cheaper than a pure solar HWS – even allowing for some extra PV panels.

      However… I already have a pure solar HWS, and a premium feed-in tariff, so no diverter for me.

      I have a friend with a new solar PV system (no premium tariff) and an electric HWS, so this diverter would likely be a good investment in that situation.

      • Not being argumentative at all, but it seems a one or two year pay back schedule beats a five to seven hands down. One thing they do here that seems not to be fair is you are allowed 500 kw in a TWO month period, then your electric bill quadruples. And, to top it off you have to stay below the 500 mark for one year for your bill to drop down to the lower rate. I have 4200 watts of panels, grid tied and a battery back up system with inverter. What they do here is you can accumulate kwhrs if you generate more than you use, BUT, on the first of January they erase any accumulation and you start over. I am losing 1100 kwhrs. this round. Reading comments from folks in other countries seems to indicate that no matter where you live they write the rules to profit the power companies to the extent they can get away with.
        They tried (CFE) to eliminate the accumulation of extra kw’s generated and zero them out every month, but the large solar companies took them to court and got a ruling forcing the power company to restore or rather just honor the contracts we all had to enter into when we installed panels, had to buy the special meter, get approved yada, yada.
        Nothing like changing the rules after you sign a contract. Ah, the thrills of living in the third world.

        • OK… if I could buy a pure solar HWS for $400 it would be an utter no-brainer. Here in Western Australia you can tack another zero onto the price (unless you know a guy who knows a guy…).

        • (Note I am serial commenter “Scott” and not someone from Catch Power who is also commenting on this thread under the name “Scott” – I have no vested interests in HWS or diverters)

  2. Hi,
    I have been in favor of using a ‘standard’ HWS powered by solar for some time now. I built 2 of units in Lismore (NSW) with 250L HWS, powered by a simple time switch set to turn on at 10AM, & off at 14:00. These 2 units are using at last bill, 6KwH/day grid import for the total unit! I am still exporting power that gives me a credit overall. (Using Enova Energy 16c Grid feed: Great plan for non-Metropolitan NSW)
    I fitted 2.4Kw elements to the HWS, then plugged the HWS into a power point (Wired at the build time). This allowed me to use a simple timer that is plugged in. (Make sure it is a solid state switch, not a relay contact for reliability)
    The advantage of using ordinary HWS is they can be installed close to the Hot water users. It is even possible to have separate systems for the Kitchen & the rest of the house if the pipe runs are long to reduce water wastage. The heat-pump style HWS are much more efficient, but are installed further away from the users as well as being ‘mechanical’ so eventually they will fail. In my case the TCO of a cheap, easily replaced HWS that is reliable makes much more economic sense.

    regards, Doug

    • I want to use a wifi timer so that i can boost it easily but is hard to find the right one.

      Any suggestions.

  3. Tony Tucker says:

    Have been thinking of this, but it’s not sensible in Canberra (20 cents/kWh from grid & 8 cents/kWh FiT). But am looking at upgrading my Plugin hybrid EV to full EV. came across a Zappi EV charger from Myenergi which can be set to use surplus solar to charge car & the topup with off-peak overnight. But They also market a diverter for under $1000 that apparently does the same thing (using surplus solar topped up with off-peak) for 2 heavy draw appliances: EV, hot water, pool pump, etc. I think the warranty is only 3 years though. Would be good to see this reviewed if you have time.

    • Ronald Brakels says:

      The Zappi EV charger looks interesting.

      Note the Eddi diverter, like diverters in general, will only be able to send current to heating elements such as those in hot water systems or perhaps under floor heating and can’t be for devices with motors or electronics. It has a warranty of 3 years. It can handle two heating loads, which will be useful for some applications.

      • Marc Talloen says:

        Hi Ronald,
        Is there any of the currently available solar energy diverters that’s appropriate for EV charging? Any suggestions?

        • Ronald Brakels says:

          Hi Marc

          The power supplied by hot water diverters is only suitable for heating elements such as those found in hot water systems or perhaps under floor heating. It can’t be used for anything with a motor or electronics and so they aren’t suitable for electric cars.

          What you can get is a smart electric car charger that uses a sensor to detect when your home is exporting solar power and charges your electric car only with surplus solar energy.

          There are other ways to do it such as using a Fronius or other relay:

          https://www.solarquotes.com.au/blog/using-fronius-inverter-smart-meter-relay-make-solar-electric-hot-water/

          But if you already have a solar system and are getting an electric charger buying a smart one may be the best way to go about it.

      • Hi Ronald….The Fronius Smart Meter allows diversion of excess power to Hot Water also. How does this compare? Also if retailers get wind of any such diverters I understand they disconnect off peak circuits anyway.

    • Electricity in Canberra is only 20c/KWh??
      Why am I not surprised?

      • Yeah, it should be cheaper than that. Most of Canberra power is from Snowy Hydro, whose production costs have hardly changed in recent years. But they can now sell at great profit into the eastern grid with their reliable base load power, so we pay a lot more.

        [Btw I think I’m paying about 20c/kWh – but the advertised consumption price here is just over 25c/kWh plus a 97.5c/day supply charge. With 25% discount on consumption (but not supply) to complicate matters, so it took me ages to do comparisons with other suppliers with different consumption charges, supply charges and discounts]

  4. Yes Minister says:

    Maybe worth considering with off-grid PV installations which generate considerably more power than is used during the middle of the day. A question that would need to be addressed is just how efficient a storage hot water system is in terms of keeping the water hot for hours. I’m currently using an instantaneous gas arrangement which goes through a single cylinder per year, so I’m not exactly under pressure to change anything, but then ‘free’ water heating which doesn’t subsidize the likes of Origin is always attractive. The ability to tell the establishment / big business to go do something physically impossible has value to me that transcends a normal cost-benefit calculation.

  5. My feed in reduced to 10 cents and the off peak water heating increased to 29 cents so I decided to determine the optimum period of solar generation from my system and divert that to water heating by having the off peak disconnected and placed on the main meter. A timer was installed on the water heater circuit breaker. The cost of a timer was $250 and installation one hour by the sparky.

    So for under $400 my water heating is covered by solar generation. The timer has a control dial so I can alter the on and off times when the seasons change – a simple one minute job.

  6. Victor Bivell says:

    Let’s hope this new model actually works and diverts “surplus” solar energy because I have had a Blue Catch Power system for over six months and it does not divert ‘surplus’ electricity. Instead, it continually diverts until the thermostat cuts in, meanwhile I am importing more power from the grid than I did without the diverter. I believe it is costing me money. The technology either does not work, the unit is faulty, or it has been incorrectly installed. The unit was installed last June. Within days I brought the funny numbers to Catch Power’s attention and seven months later I am still waiting to have it fixed. It has wasted so much of my time that I would be happy if they just remove it. At this stage of my experience, I would NOT recommend a Catch Power diverter.

    • Yes Minister says:

      I appreciate the heads-up regarding the lack of interest in support. That is an immediate deal killer for me. Far too many companies want to flog gear and immediately lose interest in the moment it walks out the door. Best response to that is to blacklist the company. A certain mob on the Sunshine Coast that sells re-branded Chinese gear at a big markup and isn’t interested in after sales support belongs in the same category.

    • Victor, I have an email from you dated 11th December. I responded a few days later that I would get back to you once I had time to digest the numbers that you sent through to me, just to clarify the “seven months of no action” comment.. It is true, you’ve had questions about the operation of the device since it’s install, and with each query I have been in contact with you.

      At CATCH Power we pride ourselves on backing up the product and following through issues until the owner is satisfied.

      Your post is not an accurate account of the course of your ownership and definitely not reflective of the experience of the vast majority of our customers. If a full refund is the solution that works for you, then we would take your lead, but you have never offered this as an option. Very unfair of you to voice it here without suggesting it to us directly.

  7. Here is a thought!

    Whilst every household wont have a pool, could this be used for pool heating?
    Or am i overthinking this and running the pool filter on Off Peak (Control Load in Endeavour Energy) sufficiently offset by the NSW Feed in Tariff?

    Should I

    • Ronald Brakels says:

      You should be able to run a pool heater off this diverter provided it is an electric resistance heater and not a heat pump. The pool heating element will also have to be 4.8 kilowatts or less. Whether or not it makes economic sense will depend on your how much your controlled load grid electricity costs, the size of the heating element and the size of your solar system. Heap pumps seems to have taken over from resistance heating for pools so in many cases it won’t be possible to use a diverter but a timer or a relay that only provides power when there is sufficient surplus solar power can still be used.

  8. Jim Dixon says:

    Hi Ronald,
    i am in west Australia with a flat rate for imports of 28.75 cents per Kw and a FIT of 7.135 cents per Kw. There are only two of us in the house and we are home most of the day as we are both retired.we have a 80lt storage electric hot water system, which we have just had fitted with a 1200 watt element, it was 3600 watts before, on our roof we have a 6.625 Kw solar system with a 5 Kw Fronius inverter with data manager and a relay for the hot water system.the house load during the day is 500 watts. I have just set the data manager to turn on the hot water relay at 1800 watts and of at 1600 watts with a minimum time of 1 minute run time.i use pvoutput.org to track our usage, we did it this way as a smart meter and a power diverter would have cost about 8 times as much and therefore had a payback time longer than the warranty on the diverter, we are very happy with the setup and look forward to see how much it saves us in the next 12 months.after the first full month of having the new small element installed and using my Fronius inverters load management system to control it,energy imported from the grid has dropped by over 40% and my self consumption from the solar has gone up by the same amount,so we are importing on average now 2.8 Kw per day less for a total out lay for the relay and new element with fitting of $280 so the new system for heating our hot water my well pay for itself in about 12 months

  9. If you have an electric heat pump HWS, then the ambient air temperature is also a significant factor in the heating efficiency.

    For a heat pump HWS, you could just use a mechanical switchboard timer to disable the heat pump overnight and enable it during the peak hours for solar PV and ambient temperature – say 11am to 5pm.

  10. Ian Thompson says:

    Hi Ronald
    I haven’t heard much in the way of analysis of Solar HWS.
    Here in Perth I very, very seldom need to electrically boost the system, between about mid-September, through to late April. In fact the problem for us during summer is to pevent the system from boiling over – achieved by covering 50-60% of the panels. The booster is permanently switched off.
    For maybe 3-4 months of the year, our timer can be enabled when occasionally required, to use excess PV during the day. I guess we will (very occasionally) need to import power – as will a diverter – on really gloomy days, or when hot water is needed early.
    I know Solar HWSs cost more than electrical HWS, but they typically last for 20-25 years (with minor service) – and I understand their efficiency for thermal energy recovery is vastly greater than using PV.

    • Ronald Brakels says:

      Hi Ian

      The efficiency of a solar hot water system is higher than using solar panels and an electric hot water system, but it is actually possible to get higher efficiency by using high efficiency PV panels and a heat pump hot water system, as I mention here:

      https://www.solarquotes.com.au/blog/efficient-hot-water-system-solar-pv-powered-heat-pump/

      Although if there is room on the roof solar panels plus and standard electric hot water system can be cheaper.

      But given that people in WA people are much more limited in how much solar PV they can install than in other states a solar hot water system can be a good idea for people who want to make more use of solar power than they are permitted to.

      • One important issue (I think Finn mentions this in his book) is that the cost of a solar thermal HWS is, and has long been, surprisingly high – considering the lack of rocket science that goes into making one.

        Mike in Mexico (commenting above) suggests that they are an order of magnitude cheaper over there.

        Any idea if this is correct, and why? I expect a few factors like economies of scale and more competition come into play, but should these account for such a large difference?

        • Ronald Brakels says:

          There are a number of reasons why solar hot water systems are more expensive in Australia than other countries. I’m not familiar with Mexico, but I know they are much cheaper in China where they are very popular. There are several reasons for the price difference.

          First there’s is the cost of labour. It’s more expensive to get the installation and plumbing done in Australia than most countries. Australian standards prevent many low cost solar hot water systems from being sold here. (Some people say Australian standards are too restrictive.) And then there is the cost of transport. Even if a foreign made solar hot water system meets Australian standards an empty water tank takes up a lot of room and costs more to transport than its weight would suggest.

  11. Tim Lawson says:

    I am currently in analysis paralysis on this one.
    I got into this mess because I have an ailing Gas Hot Water system that needs replacement. I have spent weeks back flipping between on-demand gas or a Solar HWS
    Gas requires a large bore 25mm line to feed the beast.
    Solar is pricey, has a relatively short warranty and winter electric boost expenses in the order of $250 pa.
    I got caught in an up-sell and have decided to get a PV system. which sways the economics to an Electric Boost Solar HWS.

    Much like the article says, the cheap and cheerful solution is a timer – Let the Thermal cut-out do the thinking.

    It isn’t perfect, and I think I can do better, and cheaply but I need to get it right:

    The Energy management System built into the Fronius inverter and Solar Quotes recommendation to spend extra on a quality make it a very attractive, and possibly obvious solution to my requirement.
    https://www.solarquotes.com.au/blog/using-fronius-inverter-smart-meter-relay-make-solar-electric-hot-water/

    But here is the catch, I need to know how (if) it will work to give me a hot shower when I need it.
    There is a mention of it in the diverters chapter. It is all about how you can guarantee a hot shower without wasting valuable daytime PV generation. It feels like a game of chicken.

    Advice so far: Know my usage, Keep it simple.

    Summer is easy, In WA there is so much sun, the tank will be bubbling away like a kettle, boost will be rare and will be covered by PV.
    Winter is where the clever thinking needs to be done.

    Showers in my house happen between 5 and 7:30 pm, so my goal is to have a full tank of 60 degree water by 5pm each day.

    My thoughts:
    1. Use the Sun
    2. Use the Fronius to control heating water whenever there is PV energy available, up until 3pm.
    3. Run a Timed ($ Grid $) boil From 3 until 5:00pm to guarantee hot water.
    4. Thermal cutout will stop power draw when the temperature has been reached
    5. I think the Fronius is smarter and can track total active time, managing a shorter Grid Boil. This is a bonus, but I won’t count any chickens.

    Case 1: (no Sun) 2 hour grid boil: Heating element gets tank to temperature in 2 Hours.
    Case 2: (Mild sun) PV and the Sun Team Up to give me >60 Degree water by 3pm.
    Case 3: (Full Sun) PV gets the tank to 60 Degrees quite early, Sun then excess -boils the tank. Some grid regen loss. Not ideal but not too bad.
    Case 4: (Cold Morning shower) How much heat do I lose overnight?
    Case 5: (Out of Hot water) – Log into the Fronious and switch on boost, wait.

    My remaining challenge is to scale the Boost element in the Solar HWS
    I am planning a 6.6 kW PV system, but being realistic, I am budgeting for 3kW Winter production.

    Too High, and I will be pulling grid power, a slow boiling 1.8 kW is a good economic solution, but I don’t want to have to wait for my shower. My gut is telling me to match a 2.4 kW element to the available power

    Any thoughts or experiences anyone can add to this?

    Is a 3rd party diverter going to do much better? A tank temperature sensor and weather forecast are the missing parts to close the control loop.

    • I’ve owned two solar HWS on two different houses – the first close-coupled (thermosiphon, tank on roof) and the second split (pumped, tank on ground). Experience with both has been generally positive. One thing I’d suggest is that if you have a decently sized system then cold showers in the morning should not really be an issue. The insulation in the tank works much better than I expected it would.

      I’d favour morning showers since that leaves you with a colder tank to maximise energy collection through the day. A good boost at night (if there hasn’t been enough sun) ensures hot showers in the morning.

      Both systems pump out plenty of hot water, though I think the roof tank is a little more reliable (I needed the pump serviced after a few years on the split).

      Good luck!

      • Tim Lawson says:

        Scott, Thanks for your insight.
        Yes, I have steered away from the split system. Less parts = Less parts to break down.
        Your morning shower logic is sound, but if you’d met my family, you would understand why afternoon showers are a must. Active Teenage boys need to wash, and wash often.

        In my scenario, I will be using my hot water in the afternoon and be left with a tepid tank until the following day, when there is sunlight to heat the collector or activate the PV.
        I am curious to know if there will be enough heat left in the tank for one last morning shower before it is untenable.
        There is a legonella danger in not boosting the tank. I figure that running a late afternoon boost is the solution – if it is already hot enough, it won’t draw any power

        How do you know when a good boost is needed? Is it a manual process?
        This is where some of the diverters excel, they look at the weather forecast and apart from being able to run an element at fractional power, can look at the weather forecast. Some also take a temperature sensor input from the tank.

        • The timer is a fairly simple mechanical device, installed in a switchboard slot, allowing selection in 15 minute increments. If you were concerned about having a cold shower in the morning, you could set a shortish boost in the morning?

          Using the majority of the water in the evening (though understandable) probably isn’t the ideal use case – if you fully boost the tank after that, and use only a little in the morning, then you start the day with a hot tank. Of course you have readily available hot water, but you are not making as much use od the free solar heat.

          Water needs to get to 60 deg C to kill legionella, a timed boost avoids this by enduring that this happens even on cloudy or cold days. On hot days, the sun should raise the tank temp well above this, in which case the thermostat will switch off tthe booster and avoid wasting electricity.

          As long as the power to the booster is on, the thermostat will also start boosting any time the tank is below 60. This is why a timer is preferred – why do this at 8am when the sun is about to do the job?

          Having typed all of that I can start to see the appeal of a smart device that does the thinking – rather than me trying to run game theory on boost optimisation!

          • Tim Lawson says:

            Yeah, I was originally going to use a timer too, But when I found out that the Fronius has an intelligent timer built in and it is also clever enough to boost only when energy is available. I need to buy an inverter for my PV anyway and I was planing to buy a quality one, This is a bonus.

            The one drawback is that, unlike a timer, it only has 1 timing range, and won’t be able to do a morning pre-heat.

            Evaluating my expected usage has made be think that I might be sacrificing convenience for cost. I want to be sure that I can make it work before I go ahead.

            It is this reason that I am considering gas boost solar or maybe even throwing my toys out of the pram and being an environmental vandal just going for an instantaneous Gas. I can run it for $100 per year more than the solar and it is 1/2 the price.

  12. Alex Simmons says:

    My FIT is higher than my off-peak hot water tariff. No further assessment required in my case.

    But for those where the tariffs are the other way round I still struggle to see many scenarios where the high cost of a smart switch will be worth it compared with a far cheaper and simple “dumb” timer set to heat water during hours where the household *typically* has the most excess PV production. Use a HW unit with a lower power heating element for homes with modest sized PV systems.

    Only have to calculate the upper limit of potential savings using solar PV to heat HW to see the problem:
    Upper limit of savings = annual HW energy usage x (HW tariff less Feed-in tariff)

    e.g. if you use 2500kWh/year to heat water and the difference between your HW tariff and FIT is $0.06/kWh, then the upper limit of savings from using excess solar PV is $150/yr.

    A smart switch might perhaps capture 10-15% more of those savings compared with a simple timer. That means a smart switch will save perhaps $15-20/yr more than a simple timer. But if a smart switch costs $600 more than a simple timer, well it’s just not worth it. $600 / $20/year = 30 year payback.

    It’s the same scenario for my pool pump, which is set to operate during times where typically I’ll have sufficient excess solar PV output. Yep, sometimes I won’t have that excess PV power available and it’ll need to draw from the grid as well (cloud cover and/or when we happen to use another high power device at the same time) but such differences in grid energy draw are quite small such that a smart switch won’t be able to justify its high cost.

    • Ian Thompson says:

      Hi Alex

      Ronald did say that things would have to be fairly ideal, for the option to prove economic.

      However – in Perth – and Ronald did give this as an example – I pay 28.3272 cents per unit tariff, and have a 7.135 FIT – difference is ~ 21 c/unit – so in your example the upper limit in savings is (21/6)*$150 = $525/year, so less than a 2 year payback.

      However in my case, we already have a solar HWS, so most of the year we pay $0 for electrical water heating. In winter, we also have a “dumb” switch – me – we leave the booster switch OFF, until our more recent PV is developing enough to cover the extra load. In actual fact, our booster switch also has an “AUTO” mode, which allows a timer to switch the booster as you have suggested – but we very seldom have occassion to use this.
      Same goes for the pool pump – which has a variable speed drive and only draws ~ 700W when running – the timer for this, and for our lawn irrigation system (from a 3 kW bore pump), are also set to only run when other house power draw is low, ans the normal PV generation is high.
      The only time I really pay for imports is overnight – to run lights, entertainment, and refrigerators – plus boil the odd kettle of water.
      Where we some have difficulty – for only a small part of the year, is running the bore pump during the shoulder seasons – our water restriction rules require irrigation to be completed before 9am. In summer we are usually producing 4kW when needed – but less than 3kW in the shoulder seasons. However, even a diverter couldn’t help much with this – instead I shorten the watering time (less water needed, anyway), and set the timer to switch on in time to time out by exactly 9am. Only happens 3 times a week. I have contacted our Water Utility about relaxing the 9am cut-off in the shoulder seasons – as evaporation is less and occurs later – and they have forwarded my email to a higher authority.
      So, yes Alex, I think a simple timer can deal with, and provide near optimum savings – if you are willing to make adjustments – mine works off an irrigation timer, driving 24Vac coil contactors.

  13. Hi Ronald,
    I am now in the process of getting quotes for a sanden heat pump. I have a question though.
    I am considering keeping my 315L 4.8kw heater, replacing the element with a smaller 2.4kw and using something like this diverter to pre heat water before the it enters the sanden. I live in the hawkesbury NSW, (yet to install the solar mind you, getting quotes on that also).
    Aesthetics don’t bother me but do you think this is worthwhile?
    Good luck job hunting, love your articles!
    Many thanks,
    Tim

    • Ronald Brakels says:

      Hi Tim

      Just considering the economic return, if your current hot water system is on a controlled load then you may be better off keeping it on that and using the money you save from not altering it or getting heat pump hot water to invest in a larger solar system. Later, when your current hot water system needs replacing, you look into getting a heat pump.

      But if you are getting a Sanden heat pump hot water system, because it is so energy efficient there will be no benefit to preheating the water. The Sanden should use around one fourth the amount of electricity to heat an identical amount of water to the same temperature as a standard electric hot water system. So any electricity you use preheating water in the conventional way will cost you more than letting the heat pump do all the work, whether that energy came from your solar panels or the grid.

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