Installing new solar & new hot water cylinder - query about dual element setup

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fritzman

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#323408 29-Nov-2025 16:05

Hi all.

So, we are about to pull the trigger on a 22 or 24 panel 465W (JAM54D40-465/LB Black Frame) or 505kW (Trina Vertex S+) system, likely paired with a Deye SUN-8K-SG05LP1-AU Single Phase LV Hybrid Inverter.

Our HWC is old and looking at a new 250L dual 3kW element setup.

I am completely ignorant of how these new dual-element cylinders work, so maybe my idea is completely redundant... I am thinking that it would be good to have the ability to (via wifi) turn off the 2nd heater element when there are only my wife and I in the house. When we have guests (often 2 to 4) and need more hot water and better recovery, turn the 2nd heater element back on.

Hoping someone has been down this route and come up with the best solution (if indeed there is a problem lol).

As an aside... if you have strong opinions about the solar components, maybe PM those to me, but for my money, in the thread is fine, as others might be interested.

Thanks GZ!

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timmmay

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#3438546 29-Nov-2025 16:15

With solar you would often benefit from a lower power element, say 1-2kw, for a longer period, as you're more likely to be able to generate that in winter. 3kw is fine in summer. With solar you may want 6kw occasionally, but being able to switch each element on individually would be handy. You can get a diverter system that puts whatever spare power you have available through the solar system, but I use a simple Shelly on / off switch with a contactor controlled by Home Assistant over WiFi.

Alternately, consider a heat pump hot water system. I'm not sure about their longevity though.

fritzman

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#3438549 29-Nov-2025 16:28

Thanks.
One of the potential suppliers has mentioned those Shelly add-ons.. about $100 each iir.
I grabbed a simple wifi-controlled on-off connector from Bunnings and dropped it in to manage the heated towel rail. So much better than the one that was there and worked on set lengths of time, managed only by power-cycling it to extend. Never knew where it was at, whereas now, it’s on decent timers plus fully controllable.
I wondered about doing the same with each element.

Sons Rig: Asus TUF Gaming X-570, Ryzen 9 3900X, G.Skill neo 2x16Gb 3600's, Sabrent Rocket 1Tb M.2, Win10 Pro, Phanteks case, EVGA G5 850W.

NAS: DS1819+ - 52Tb in Raid6

My rig: HP Elitebook X360 Lappy with a 2Tb SN850.. woohoo.. I've retired!

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timmmay

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#3438617 29-Nov-2025 20:20

You need something made to handle very high loads for a long time for a hot water cylinder - a contactor. Anything compatible can drive it, but the Shelly devices often can. Be wary of devices that say they can switch 10A or more if they don't have a contactor built in, many fail - there's a thread somewhere around here about it. I have a contactor on my HWC, when it switches you know about it, you can hear it clearly through a wall or door.

You can get a Shelly with two circuits, two contactors, and do things that way. There's more on this thread.

tweake

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#3438627 29-Nov-2025 21:15

fritzman:

Hi all.

So, we are about to pull the trigger on a 22 or 24 panel 465W (JAM54D40-465/LB Black Frame) or 505kW (Trina Vertex S+) system, likely paired with a Deye SUN-8K-SG05LP1-AU Single Phase LV Hybrid Inverter.

Our HWC is old and looking at a new 250L dual 3kW element setup.

I am completely ignorant of how these new dual-element cylinders work, so maybe my idea is completely redundant...

i looked at this a few years back when i replaced mine.

they have two different element set ups. some are two elements in parallel, other are changeover ie only runs one element at a time.

the idea was to have solar to feed one of the elements and to have the other element as back up. couple of ways, either run a DC element as a regular thermostat controlled element, or use a diverter to send excess solar to the hot water. depends on if you want to solar to do most of the water heating or just part time. ideally you want to get a stainless cylinder as the ceramic coated ones (standard mains) can crack the coating when heated extremely hot if your trying to do energy storage ie use the solar to heat the water as hot as possible.

but watch the cost, it may end up very similar cost to a heat pump cylinder.

sibel

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#3438752 30-Nov-2025 14:27

You may want to look at something like a Myenergi eddi - it’ll divert only excess power to the hot water cylinder. Any relay set up is all 3kw (element size) or nothing; vs the Eddi’s ‘whatever is excess goes to hot water’.

Our old neighbour has one - and swears by it.

We went with a HWHP instead.

fritzman

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#3438755 30-Nov-2025 14:45

Thanks for the inputs do far..

so:

going with a single 2-3kW element seems to be way simpler, and if my memory serves me correctly, it can be managed entirely by/through the Deye inverter (which I understand comes with a WiFi dongle).
time the hwc to be heating during the off peak daylight hours if that’s doable given showers at night, number of people, etc.
I’ve found a Shelly Ogemray 25 amp smart switch that seems like it has solved all the issues people have been having with the lighter units overheating.

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NAS: DS1819+ - 52Tb in Raid6

My rig: HP Elitebook X360 Lappy with a 2Tb SN850.. woohoo.. I've retired!

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Mighty Ape

Shop now at Mighty Ape (affiliate link).

timmmay

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#3438757 30-Nov-2025 15:26

The Ogemray is on sale right now. Looks good. I'd still use a contactor personally but it's meant to be fine. Alternately you can get a diverter that uses whatever power is available, but the payback time is quite high - it depends how much you're paid for your power vs the power cost.

Re #2 I use actual power export numbers to turn HWC on and off, but if I didn't have that I'd have it start heating at about midday given our solar system is facing slightly west of north, and we get peak power around then. We have a 300L tank from memory, it gets heated in about 1.5h in summer and 3h in winter, once it's heated we don't need to heat it again for 24h. When it's below 5 degrees overnight I turn it on for 30 mins before we have our morning showers but I'm pretty sure I don't actually need to do that.

FineWine

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#3438760 30-Nov-2025 15:49

sibel:

You may want to look at something like a Myenergi eddi - it’ll divert only excess power to the hot water cylinder. Any relay set up is all 3kw (element size) or nothing; vs the Eddi’s ‘whatever is excess goes to hot water’.

Our old neighbour has one - and swears by it.

We went with a HWHP instead.

Absolutely. Love our Myenergi eddi. A Small Mains Boost at 0500 for work showers and a small solar boost at 1600 for, if required, evening showers. We can also control it from phone app. Plus when we get an EV we will run the Zippi car charger off it.

Whilst the difficult we can do immediately, the impossible takes a bit longer. However, miracles you will have to wait for.

fritzman

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#3438763 30-Nov-2025 16:15

Thanks all.. looks like Mighty Ape & DSE both that that Ogemray for NZ$86.. I think the AU site that has them on BF sale might attract gst and work out very similar cost.

Sons Rig: Asus TUF Gaming X-570, Ryzen 9 3900X, G.Skill neo 2x16Gb 3600's, Sabrent Rocket 1Tb M.2, Win10 Pro, Phanteks case, EVGA G5 850W.

NAS: DS1819+ - 52Tb in Raid6

My rig: HP Elitebook X360 Lappy with a 2Tb SN850.. woohoo.. I've retired!

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