Solar used to be simple; but in todays frenetic world, energy rates change by the hour, so managing your consumption is more important than ever.
Read on while we explain how you can make best use of different options for energy storage, and the smart technology to automate it.
Efficiency Is King
The cheapest kilowatt hour of energy is the one you never needed to buy in the first place. Making improvements such as better home insulation can slash your energy bills.
Self Consumption Is Simplest
I can’t stress it enough, the more solar you have on the roof, the more likely it is you’ll have your own energy available to use behind the retail meter.
At my house there’s 15kW of panels on the roof and about a third of them face what’s currently the brilliant afternoon sun. My electricity retailer has no idea the hot water service is energised and the dishwasher is running because at 3:45pm we still have around 5kW of solar yield.
While I could be exporting 5kW of solar, it’ll only pay 2cents/kWh, which is a small fraction of what you save from self-consumption instead of importing energy from the grid.
Many appliances have delay start timers built in. For instance, one keystroke on our Fisher and Paykel dishwasher turns it on, holding the button down enters delay start mode, pressing again advances the counter one hour at a time.
Hot Water As A Battery
Take these two different methods for storing energy using a resistive hot water service.
Using a variable type diverter, you can harvest every watt of available solar because it will follow yield dynamically. It’s even more refined when Fronius use a temperature sensor to give you and the system feedback from the tank.
Yellow shows electricity consumed in the house while the “excess” solar (in orange) is poured into the hot water. Once the tank is up to temperature, electricity is exported to the grid (in grey).
A more basic approach uses a contactor to simply switch the hot water service on and off. It’s not as refined, but not as expensive either.
The best results come from using a small element in the tank because you need a lot of solar to run a standard 3.6kW element. There will be more hours in the day with a surplus of 1.2kW or 2.4kW available, so you can switch the hot water on without needing to import electricity.
You can use a CatchControl device for the required signal and it will automate the most efficient operation.
Or utilise a relay output/dry contact which is built into better quality solar inverters as standard, the only catch is you may have to watch the monitoring and learn how to fettle the controls to best suit your situation.
Here we have a hot water tank which is switched on when export rises above 3kW and switched off when it drops below the nominal 2.4kW element rating. There is a rule for 25minutes minimum run time, so the contactor isn’t clunking on and off with every passing cloud, and in this instance it’s caused a small amount of grid import (shown in red).
Heat Pumps Are A Winner Too
It might be a deeply held belief, bordering on religious fanaticism if the comments section is anything to go by, but heat pump hot water is incredibly efficient. Though it’s mechanically and electrically complex compared to a standard hot water tank, the management is really simple.
A basic solar switching system is outright most efficient but all you really need to do is set a timer.
Stick With A Single Manufacturer
As you can see in the header image, there’s 7 different components for your electrician to connect up with power and control wiring.
This is where having a single manufacturer ecosystem, a compatible CatchControl solution, or an integrated software system like ChargeHQ is essential. Once all these components can talk to each other, it becomes far simpler for the end user to set and forget.
This chart shows the full Fronius ecosystem at work, with the household loads being met with yellow solar energy, then hot water system soaking up solar in purple, plus an underlying green graph of energy being stored in a battery. Grey is electricity exported to the network, which could alternately be stored in an EV battery.
Choose Your Storage Options
Default programming will see your solar battery system begin charging up at sunrise, but this doesn’t always produce the best outcome. There are as many scenarios as there are customers but here’s how it could work:
- Self consumption makes most sense, so morning toast and coffee powered with solar should be prioritised.
- After that, you might charge the EV for whatever trip you’re making that day;
- The hot water gets topped up after the car leaves.
- Lastly, get the house battery charged before expensive evening peak period prices, whether using solar or cheap daytime electricity.
Your electrician may need to program which of these three storage devices to prioritise for which part of the day and which part of your retail plan.
However if your behaviour, retailer or seasonal yield changes, you’ll want to know how to adjust it yourself, so be sure the installer can explain it to you.
Home Assistants
If you have an appetite for much tighter control, and an aptitude for some basic computer coding, then using Home Assistant is a way to control any number of things in your house, without being forced to use the internet or surrendering your every private habit to Google, Apple or Amazon.
Bluetooth for white teeth? Why on earth would an electric toothbrush need data connectivity I hear you ask. Well one savvy user has written some rules because he knows a) when he’s leaving the house b) on a weekday, c) he picks up his toothbrush, and d) this activates his car’s climate control system. Hooray!
Image credit Chris Cathcart
Petrol Is Expensive
One of the major costs Australian households can minimise is fuel for transport. Given enough rooftop solar, many will be able to completely cover their energy needs without needing to import expensive fuels.
As federally incentivised home batteries are now much cheaper, they’re proving very popular. However they don’t suit every household, especially when a similar cost per kWh of capacity will buy you a very large battery and a free car.
If you can charge your car at work during the day, then bring it home and siphon a little electricity off overnight, it’s likely an economic no-brainer.
You can do it right now, using current V2L systems on many EVs. without any special network permission or solar compatibility. And it’s cheap.
If you’re looking to invest in a higher capacity, fully automatic, bi-directional charging solution, then the technology is now available, almost.
Makers like Sigenergy have a solution, only for Ausgrid customers who own a Ford F150 Lightning, but other networks and other car models are still coming down the pipeline, as are other V2G inverters.
This will be an interesting space to watch but full V2G will be made complex by network approvals and car warranty conditions. A simple V2L system may still be an elegant solution for those who just want a little backup and some nifty savings.
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About Anthony Bennett
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Much like the HWS in the article, I charge the BEV at full whack (7.2 kW), instead of using the “Surplus Solar Only” dubious smartness. In today’s intermittent cloud that is pulling 3 kW from the house battery as a monstrous dirty grey cloud lumbers over, but soon I can zoom into town on a full BEV charge, and the house battery can recoup while I’m out.
*** That’s as good as 50 panels on the car roof, charging at 100 km/h on the highway, just with no drag. ***
Off-grid here, so no risk of grid import, but it can be done on-grid too – just max the battery subsidy for a bigger battery.
Using battery support to consume flat-stick, regardless of clouds, I’ve managed a daily peak of 52.4 kWh useful consumption, with the battery at 100% SoC at the end of the day, BEV & HWS also.
The cloud went – now 11 kW into house battery, 19 kW solar yield. Not bad for winter – enough for opportunistic catch-up. SoC dipped to 98%, but 28.1 kWh pushed through in the morning. This stuff is superb!
Interesting Erik. Who designed your setup? I have ‘phase 1’ up and running, but before the addition of ‘ph 2’ wish to get input from a designer who is all over smart offgrid / hybrid setups : Cheers)
Hi John, designed it myself (retired engineer), but at 71 have too much else to do to gain certification in the field – and keep up with all the market offerings. But bouncing ideas with an informed adopter, exploring options, is fun, and scratches my itch to help at least a few adapt to what’s coming – it will bite, harder each year.
I went Victron/Fronius balanced DC/AC coupled, DIY 48V battery banks, but Selectronic is Aussie, & subsidies make DIY redundant now. My system smarts extend only to the Linux-based Victron controller, and local & cloud monitoring – no Home Assistant, yet. (Have to plant fruit trees, build tractor shed, …) The 27 kW PV, 24 kW of inverters, 46 kWh battery system is ample even in winter, e.g. produced 103 kWh in last 48 hrs, so smaller would have sufficed.
My brother used a really good designer/troubleshooter in NSW, but he’s flat out with battery work, so you’d have to queue.
I’m not much into fora, but start a Whirlpool thread & post the link?
I live in a cold climate and I’m considering getting one of those old-style thermal storage heaters. They have gone out of fashion now, but are still available, eg modern units by Heatpac.
I could store solar during the day, and release it when the house cools down overnight. Or I could switch to an energy company that gives 3 hours free electricity in the middle of the day, eg Ovo. I could store up heat for no cost and release it slowly over the rest of the day and night.
Most people don’t have enough supply capacity for such heat banks.
Ie tank @300L looses 3kWh a day just standing there and typical element is 3.6kW so almost an hour to put that back and another 7kWh into the water which doesnt go far
There is no water involved. The storage heater I’m looking at uses a series of elements inside a dense ceramic core.
https://heatpac.com.au/
My HWS is only 250L, but still great for dumping excess off-grid energy. It is indoors, so the 2.5 kW(?) heat loss provides free room heating. As a result, the indoor winter temperature is now 22°C at 5 p.m., without other heating, 13°C outside. An outdoor HWS seems quite wasteful. (1.1 MWh/year!?)
Granted, off-grid is more likely to afford ample winter PV yield for a simple resistive element, but if on-grid, a heatpump HWS would greatly boost a limited PV array, left over from the old days of costly scarcity – and if there’s three free hours of grid energy, then 3h x 3.6 kW heats the whole tank from 20 to 57°C, so the solar doesn’t have much work to do after that.
On the page I have linked, you can see a calculator that shows you how much energy is stored in water. You just need to enter, the temperature increase or decrease and the volume.
As immersed resistive heating elements are virtually 100% efficient, you can use this to calculate how much energy you are storing in a resistive HWS or how much you will use to heat it back up again.
I am sure some of you will find it of interest. I mainly use it to work out how long the spa will take to heat after a refill or very cold night.
Thanks Matt,
That’s really useful.
Cheers
Now for the actual link mentioned above!
https://www.engineeringtoolbox.com/energy-storage-water-d_1463.html
Petrol is expensive? Compared to what? At $2/litre and 10 litres per 100 km, you’re looking at 20c per km. At twice that performance – and many vehicles do better than 100 km per 5 litres, and $1.50/litre – slightly lower than current prices in assorted parts of Australia, you’d be looking at 7.5c per km. Note this includes excise and GST. If excise is removed …
Depending which city, or region, you’re in, the cost of public transport may be much much higher per km, assuming it’s even available.
Yes if you charge an EV at home the cost per km will be lower, but only so long as you have sufficient range to not need to refuel on the way. According to Canstar Blue, a standard public charger averages 32.5 c/kWh. Assuming 10 km per kw that translates to 3.25c/km. Cheaper, sure, but it’ll take an hour or 4 for every 100 km you need. Level 3 chargers are faster, if you can find them, but also cost much more – basically the cost of fuel without excise!
Hi John,
The majority of EV owners charge at home with an ordinary power point, they don’t need fast changing because the average commute is sub 20km & average journey is even less.
Feel free to check but I’m pretty sure there’s highway chargers to get you from Cairns to Port Lincoln… and on to Perth.
In fact it’s not difficult to drive 13,500km around Australia in 10 days in a Tesla.
Time to get with the program.
https://www.rewiringaustralia.org/report/factsheet-for-electric-vehicles
A home level 2 charger, at 7.2 kW, is ideal if the BEV is the only car. Between clouds, mine put 36.07 kWh into the MG4 in 5:08 hrs today, ready for a 65 km pizza run in a few minutes. I could have done it with the prior remaining 37% range, but I’d only have 12% on return. (And it’s less efficient to charge the BEV from the off-grid battery after dark.) Make miles while the sun shines, I figure.
A granny charger is a bit slow, if you do frequent mid-range trips, e.g. 65 km to town.
Fast chargers are a moving feast, adequate now, I think, but much faster coming.
A relative’s work colleagues were terrified their EV would run out of ‘gas’ when recharging ‘failed’ & they had a long drive back to base.
Recently I had an appointment to get to, a fair amount of travel, and no time to waste. Petrol was reliable, an EV not an option for me.
Yes if you can recharge off solar overnight, or you don’t drive much so can trickle charge while the sun’s out, an EV may work. For everyone else, petrol may be a better option, or diesel, or even LPG.
I can’t comment about the ability to drive around Australia in a Tesla, let alone in 10 days. A travel writer who planned to do Britain in an EV in a month gave up after a week, switching to an ICEV because as EV friendly as southern England is, it’s still not good enough, and the rest of Britain is worse.
I did a quick look at recharging options and some are asking 75c/kwh! Between multiple sites, being hard to find, unclear pricing, high charges … Most of those I know will stick to ICEVs. But you do you.
Hi John,
EVs don’t suit every purpose, but they do suit nearly everyone who cares to look rationally.
Australians love to imagine they’re Ned Kelly living in the bush.
85% are in fact living within an hour of the coast, like a suburban Ned Flanders.
I notice you haven’t provided any links to your sources but it’s worth noting that there’s a lot of vested interests who are perfectly happy to fund or promote Fear, Uncertainty & Doubt.
So your third hand tale of terror really suits the likes of 7 West media because they like FUD.
However inaccurate, it creates “contoversey” to get clicks.
A ten day “big lap” was news in 2023 and chargers are only getting more reliable & more numerous.
https://thedriven.io/2023/12/15/record-broken-as-ev-enthusiast-rocks-around-australia-in-just-10-days/
Having mentioned links, you might be interested in these;
https://thedriven.io/2022/03/24/sydney-to-perth-in-five-days-in-a-short-range-byd-t3-electric-van/
https://bydt3campervan.wordpress.com/
It is sad and regrettable that BYD, with the T3 (only 100 supplied to Australia, where, at least 100 would likely have easily sold in WA alone, and, no passenger versions available in Australia, but, available outside Australia), like too many other vehicle manufacturers, simply treated Australians with contempt, with undersupply or no supply at all (apparently, while it is available in NZ, the Riddarra RD6 electric ute will not be available in Australia – 6kW of V2L capacity, and, the Chery E5 has V2L outside Australia, but, not in Australia – just a couple of examples…).
Roughly 64% of Australians live in 1 of 5 capital cities. Most of the few other (small) cities, and many Australian towns are coastal, but that still leaves plenty of Aussies, residents, and visitors, an hour or more from the coast.
My experience and the relative’s colleague obviously aren’t sourced. For the British EV story try uh …https://www.thetimes.com/travel/inspiration/comment-inspiration/electric-vw-id-buzz-campervan-circumnavigate-the-uk-chris-haslam-vjdr5mtn9
Call me skeptical when it comes to that 10 day lap. The driving time alone would be about 14.5 hours per day. That doesn’t include any time for toilet breaks or getting food – I’m assuming eating was done while driving to save time. The 60 charging events means roughly 4 charges every 24 hour period, as opposed to what would be one petrol stop a day. Overnight charging every day, plus 1 fast charging every day, plus additional charging when grabbing meals? And what was the point, bragging rights about speed?
Hi John,
I read your link and it had lots of feelings & few facts.
ie column inches of codswallop for clicks.
“Unluckily the van’s plug did not fit the site’s socket. Probably because I’d failed to salute a magpie earlier in the day.”
It’s like pulling into a motel, complaining they have no hay to feed your horse, then despite supposedly having all the information to hand via apps and route planners… blaming your lack of foresight on the rain; in England no less.
If you’re sceptical you could try the links in the article I posted or search for “atto gal ev” doing a big lap towing a caravan.
Better still try this one:
https://www.abc.net.au/news/2024-06-10/regional-rural-queensland-electric-vehicle-uptake/103914408
The point is proving it can be done, to rebutt the fear, uncertainty & doubt that is sown by those with vested interests in the status quo.
Whilst no provision is available, to reply to my post timestamped “August 29, 2025 at 9:58 pm”, in addition to the content in that, are these;
1) the BYD T3 van – a BEV Light Cargo Van, came with V2L, and,
2) the Geely Riddarra RD6 BEV ute, which, I said “while it is available in NZ, the Riddarra RD6 electric ute will not be available in Australia – 6kW of V2L capacity,” has, from a video I have just watched, up to 15kW of V2L capacity – “Charge Port (DC discharge via adaptor)”, which is the full capacity of a single phase electricity grid connection; therefore, effectively (with appropriate connection), V2H. But, this is Australia. We are not allowed that in Australia. Only countries outside Australia, are allowed that.
John Alba: – “Most of those I know will stick to ICEVs.”
Burning more carbon-based substances is ‘civilisation suicide’!
https://www.lithgowenvironment.au/docs/road-to-climate-ruin-geoff-miell-4jun25.pdf
In less than 50 years planet Earth could be largely unlivable.
https://globaia.org/habitability
We/humanity are all in the same lifeboat (i.e. planet Earth) together. If we/humanity don’t all work together to change course (i.e. rapidly reduce our GHG emissions, remove some atmospheric carbon, cool down the planet similar to pre-industrial Holocene climate) then we all sink together (i.e. planet Earth becomes incompatible for human civilisation well before the end of this century). No one and no country is immune from this!
https://actuaries.org.uk/news-and-media-releases/news-articles/2025/jan/16-jan-25-planetary-solvency-finding-our-balance-with-nature/
The problem is that economies are based on consumerism and to grow the economy you need more customers = more population = more manufacturing = more pollution and more consumption of the earth’s natural resources – soybean crops instead of rainforests, etc etc…
Difficult to see how that’s ever going to be reversed globally.
Here’s how it works for me. I installed solar and battery in 2018. Back then it cost about $20k for solar and battery (a lot cheaper now). I also bought an EV in 2019.
Before solar and battery, electricity was costing me $2600 PA. This dropped to about $665 PA streight away. A couple of years ago I moved to Amber Electric and it dropped to $0 PA with spot trading.
See here: https://drive.google.com/file/d/1Ic5zeqls8gRdIwzvuEPmysxj4W2lNydl/view?usp=sharing
Gas was costing me about $1500 PA until I installed HVAC and got rid of the gas heating in 2021 when it droped to about $600 PA (still have gas hot water and cooking), saving me $900 PA.
see here:
https://drive.google.com/file/d/1ZNgdMund6Bjfly_miYDpqkpO02_B3qIW/view?usp=sharing
Now before I got an EV in 2019 I was paying about $1,500 PA for petrol for 15,000km PA. Now I pay nothing.
So I’m saving $5000 PA not to mention I haven’t paid for a car service for the past 6 years.
“I haven’t paid for a car service for the past 6 years.”
Do electric cars not need servicing at all? I didn’t know that.
Hi Alan,
My EV service consists of pumping up the tyres, changing the windscreen wiper blades & topping up the washer fluid.
At 5 years & 100,000km it should get ¾ of a litre of gear oil.
At 20 years it’ll need 4 litres of coolant.
https://www.solarquotes.com.au/blog/australia-cheapest-electric-car-2/
Yep, we jokingly refer to filling up the washer fluid as ‘due for its major service’.
Hi Glen,
It probably costs 10 cents for the washer additive yet timing belts on a Subaru is over a thousand dollars. O.o
I did the kids Tesla gear oil at 300,000lms needs 2.3L of fancy ATF9 at $60 a bottle and a $15 filter from AliExpress
Model S I paid Tesla to do and it was $600 for both drive units
Brake fluid change on my model 3 was beyond easy as you can literally push a button and it uses the ABS pump to push fluid out.
Brake pads I changed on the model S at 200,000kms just because they left brown dust on the wheels and that car has 450KW and much Regen braking (60kW max ) than a model 3 (90kW) and it’s 400kgs heavier.
Brake pads on the kids 300,000km.car are worn 1mm -2mm out of the 8mm.new pad friction material.
They will likely therefore need replacement at 1.5million kms
We drive our cars like we’ve robbed a bank everywhere it’s great fun.
Last year we spent $65 charging 3 EVs to do 65,000kms between OVO free period and 3c/kW export rate and my old Model S which has free lifetime supercharging.
Some do I believe but I drive a Tesla, and they don’t. Wiper blades, tyres, cabin air filters need changing. But even the brake pads and rotors last for much longer due to regenerative braking.
So, no more inconvenient trips to the mechanic for oil change or to the service station for fuel.
Alan, it depends on the definition of “need”. Anthony has clearly bought a good’n. My MG4 dealer would like to walk around the BEV after 2 years to see if anything has fallen off … and charge a very small fee. After 4 years, they want to change the battery coolant. (Perhaps their brew isn’t as strong as Anthony’s.) And yet there’s a 10 year warranty!
What most biased me against buying a BYD was their aggressive service regime, wildly unnecessary on a good BEV, as amply demonstrated by Anthony’s example. OK, fuel savings will (currently) cover the unnecessary service cost, but that’s better spent on chocolate, I believe.
No oil changes, no radiator, no clutch, no spark plugs, and minimal brake wear as regen braking can do most of the work wear-free. There will be windscreen wiper blades in a few years, though – and there’s the wiper fluid bottle to top up with water every now and then. 😉
More than petrol stations will go the way of ostleries, that’s inevitable.
Hi John
Our previous diesel vehicles, while fuel efficient, were definitely expensive to run compared with our EVs. Three to four times more expensive.
Typical fuel cost of our diesel cars: $9.00 – $12.00 per 100 km.
Typical fuel cost of our electric cars: $0.10 – $3.50 per 100 km.
I have nearly 2 years now with our MG4 and this includes all driving including highway trips requiring use of highway fast charge stations.
Recently added a “used” EV for my wife (“used” meaning unsold dealer with 5 km on the odo) and she absolutely loves it. Replaced the old Golf. She is happy to plug in at home and never have to use a bowser again.
But the vast majority of charge comes from our home and is either using our own solar PV, or super off-peak or free electricity.
You can see the energy stats here:
https://i.imgur.com/NJVgKHm.png
And costs here:
https://i.imgur.com/BvWFmL1.png
Charging from home is an order of magnitude cheaper than any petrol or diesel car is to refuel.
I maintain a SoC 40% to 70%. Home energy mngt system charges depending on price of electricity. Don’t go over 80%. I’m with Amber so feed-in & supply tariffs change every 5 mins.
Forecast for today (White line is the charging forecast):
https://drive.google.com/file/d/1ebB0RqK1L_MzT100vOSHiFO0qi8Dir2c/view?usp=sharing
Supply tariff forecast at 5 cents (that’s conservative) 4c is more likely ‘sunny day’. So, a full charge is $3.04 for 500km range. In summer it’s $1.20. However, I spot trade electricity so pay nothing for electricity.
Car plans long trips. e.g. trip to family’s cattle station:
https://drive.google.com/file/d/1UwctrxCbFfd82HYE0L3_KScQmF5x3PVr/view?usp=drive_link
15 to 20 minutes stops every 2 or 3 hours, car manages everything. Even availability of chargers etc. Fast chargers are ~60c. Full tank costs $45.60 for 500 km range (probably more like 450km at 110km/h).
People who use their EV like they use an ICE car have not made the transition to modern driving
John, you seem detemined to not like EVs, and that’s fine.
Everyone’s situation is different but our usage made it a no brainer to get an EV. We do a few hundred km a week, mainly within suburbia, or weekend day trips at most. We have a 7kW charger at home and can charge at 8 cents per kWh overnight. We have decent enough solar and battery storage that we can often avoid using even this cheap grid power, even during winter.
In our 6 months of owning the car, we have needed a public charger exactly once. This was a planned charge on a longer trip, not being caught short. The running cost for that trip was maybe half the cost for a pertol car, which still seemed expensive.
We have an ICE SUV for longer trips.
We’re saving at least a couple thousand per year by not buying petrol. Servicing is cheap and trivial.
EVs make plenty of sense for certain types of use.
Will be interesting to see if appliances like dishwashers and washing machines will migrate to integrated heat pumps (like water heaters and dryers are slowly doing) – i.e. when will the economics stack up.
What do people think will happen with ovens as I not aware of a likely solution?
The added complexity and cost of a heat pump for washing dishes and clothes does not warrant it.
Modern appliances are already pretty energy and water efficient and as a result do not need a lot of energy to heat water for a wash cycle. Dishwashers require water much hotter than a heat pump can supply, while clothes washing machines also have cycles with very hot water (not that everyone uses that option).
A heat pump for drying clothes makes sense, although even that has some complicating factors to consider.
Over 12 months at our last home these appliances consumed:
Washing machine: 235 kWh (0.64 kWh/day)
Clothes dryer (heat pump): 231 kWh (0.63 kWh/day)
Dishwasher: 145 kWh (0.40 kWh/day)
That’s for two people, plus typical family/friends visits/stays.
ARH, proliferating heat pumps into multiple appliances seems rather dubious up-front economics and a lurking costly maintenance burden to boot. When snooping the aisles of the local whitegoods store for a washing machine, I noticed some machines had only one water inlet, so on-grid would incur the penalty of costing ToU electricity instead of just drawing hot water from the HWS. That seems nuts, but off-grid it would, in heavy overcast, perversely deplete the house battery, limiting other energy use options.
The HWS is a much cheaper energy store, and I’m yet to find a house with no hot water plumbing. Perhaps I’m missing something, but water heating washing machines seem to be all downside. Are there any benefits?
By all means spend the extra money on a heat pump HWS, but one is more than enough maintenance burden, I figure. A washing machine heat pump would need an external compressor unit and through-wall refrigerant plumbing, adding several thousand dollars to the cost.
“What do people think will happen with ovens as I not aware of a likely solution?
”
Are you aware of this…
https://copperhome.com/products/charlie
I set my green catch diverter on solar only and even if it’s cloudy will send as low as 200 watts to my resistive hot water so heats up until the thermostat turns it off !! I don’t do anything !! All I miss out on is the lousy 1 cent a KW i would have got going back to the grid !!
What is better – a green catch diverter or a catch control for a 3.6kW resistive hot water system? Not sure which is the better solution. (I have solar, battery and EV).
Hi PF,
With a 3.6kW element, you’ll need a lot of excess solar to cover the HWS consumption when just switching on and off with catch control.
Better to have a 1.8kW element and then you’ll have more hours of the day available where it won’t be forced to top up with grid power.
(Catch Control does have some other tricks, like monitoring and Solar Analytics)
Green Catch will be much more refined though. Keep the 3.6kW element and the green unit will simply follow solar yield dynamically… it’s as efficient as a resistive system will ever get.
Silicon Chip magazine has designed a diverted which uses pwm control and queries your inverter using modbus to get export data.
It ramps up the powered delivered to your resistive water heater based on amount of excess solar.
It also has temperature monitoring.
I’m old and no good with apps and modern technology so like the green catch sending every skerrick of excess solar to the hot water service even as low as 200 watts instead of getting bugger all for it with fit !! I’M NOT SURE BUT THINK YOU HAVE TO BE MAKING OVER 1000 WATTS WITH THE RELAY !! I don’t think it matters how big the element is because it’s only sending EXCESS even as low as 200 w to the hot water hence only using what otherwise would’ve went to the grid !!
If visitors come and you need extra hot water there’s 2 boost modes !!
I’m NOT a expert !!
Hi Peter,
With most devices you need go over a threshold to start the diversion but they’ll run down quite a bit lower before disconnecting.
Just as a point of etiquette, you can *emphasise* things with some punctuation, or use italics if you’re being fancybut when the young whipper snappers see you writing with all capital letters they’ll assume you’re YELLING at them… and it’s not always well received.
😉
It is also very easy to store cold,… using a plug in power point timer to your freezer.
For those who buy in bulk or store a bit of their own meat or freeze their surplus veggies or fruit instead of the bottling method can run the timer from 8 am to 8pm, which will keep the freezer properly frozen. Even those less efficient freezers will keep your goods frozen for the princely FIT loss of 2c per day.
While there is no sun is at 8pm in winter, the freezer will already have turned off well before sundown, and summertime gives greater runtimes for greater need.
Typically a freezer only runs 1/3 of the time, and a manual setting if you have just killed 3 sheep will freeze them in 1 1/2 days before change back to timer.
We run three freezers and don’t even worry of the 5c per day “lost/cost” for all 3.
Of course running your electric heater during the day to preheat or pre cool your home does wonders for ecomomy and comfort too.
Shelly have some neat units for switching hot water systems on and off, with temperature monitoring.
I have setup where my Fronius Inverter turns a Shelly relay on and off based on production, while ensuring a minimum runtime. In addition a temperature probe also ensures that when the grandkids are visiting and they empty the hot water service it will run and additional heating cycle.
Re using a smaller power element in your HWS, I considered that because, in winter we get days where my 6Kw system doesn’t produce 3.6Kw excess. But a 1.8Kw element would take twice as long to heat our tank and. then, if it’s a patchy cloud day, there might not be enough excess production hours to heat it fully.
So I compromised by setting my Catch relay to turn on when there is an excess of 2.8Kw and am happy to take the other 0.8Kw from the grid. I’m finding that most sunny days the excess is 4Kw or higher.
Shower thought!
If hot water systems are like a battery, we should be able to CHARGE them like a battery. None of this diverter, relay, dry contact, Home Assistant stuff.
We do have a very well-understood, established battery charging system: CCS. Or technically J1772. You know, the EV charging standard.
Imagine if adding a hot water system was as easy as plugging in an EV. (You wouldn’t actually plug it in, it’d be hard-wired. But conceptually…) CCS has protocols that tell the HWS “you can charge now, max 8A” or whatever. It’s got the smarts built-in. It can detect you’re exporting solar and instruct your HWS to consume it.
Actual batteries? Make batteries use this standard too! Finn’s idea of home batteries for renters? Plug in via the EV standard. Wait, you want to discharge your battery? CCS is bi-directional!
It can do load balancing/management. It can do metering/monitoring/accounting. And… when you get home, you can charge your EV off it too.
John Alba,
Yes, some of the adventurous souls making the first BEV circumnavigations of Oz are likely to push the pace, if doing it for bragging rights, rather than to stop and go fishing. And yes, almost every BEVer will eat & refuel concurrently, in a most natural rhythm.
Taking umbrage at the irreversible electrification of motoring, which presages the end of climate-wrecking fossil-burning travel, does look like foreboding grief over the imminent loss of old ways. Your post seems heavy with nostalgia for the status quo ante, now well on the way out in advanced countries like Norway and China – lagging a bit here, as so often is the case.
Xpeng BEVs now drive themselves to and onto the export ship, and leave ICE cars for dead wrt features, price, & ownership cost. Obsolete ICE cars will no longer be globally mass produced in a decade, it seems.
“the irreversible electrification of motoring, which presages the end of climate-wrecking fossil-burning travel,”
You say “climate wrecking”, I say murdering people.
https://www.abc.net.au/news/science/2025-08-22/traffic-pollution-study-estimates-1800-premature-deaths-per-year/105677620
– “The new research, led by the University of Tasmania’s Centre for Safe Air, estimates particulate and nitrogen dioxide pollution from traffic leads to more than 1,800 earlier than expected deaths a year, or more than the 2024 road toll of 1,300 fatalities.
The study found cars were responsible for about half of the estimated total annual premature deaths from air pollution, which is Australia’s leading environmental cause of premature death and disease. ”
So, 1) The annual “road toll” should more accurately include the number of people deliberately killed by pollution from ICE vehicles, and, 2) the killing of people using pollution from ICE vehicles, should cause proper emissions regulation.