Home solar question, what upgrade path?

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WWHB

57 posts

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#3489198 9-May-2026 15:24

Dairusire:

So, we're sort of in the middle of the same as yourself.

We're in Hastings, 180m square ish and basically the exact same roof angle and direction.
We've got a wood burner for heating the house, but we're getting ducted heating/cooling done, as well as Hot water heatpump, which will then have us getting disconnected from Mains gas as we currently have infinity hot water.

All in for the ducted system, solar, and hotwater heatpump, we're looking around 45-47k.
We're lucky in that we can afford to do it all in one go.

We've found most Heatpump HWC quotes for a 250L-300L came back around $8-10k incl GST fully installed.
This is the one thing I've been wondering if I 'actually' need to do this. Our Infinity hot water is working perfectly fine, it's a lot of money to replace, for the only benefit being that we're no longer having lines charges and know that we're not getting a massive energy price increase at a providers or markets whim.

Ducted has been pretty variable in terms of costs. Averaging around $15.5k. That's for 4 bedrooms, a really large kitchen/dining space, and a big lounge. So 6 different zones.

The Solar system, we're just getting documentation on from Micromall for the Deye system, installation instructions etc. After having looked through all the solutions, considering the price and the fact it's got 20kWh of battery for 18k, which was 4k cheaper than a SigEnergy solution with 10kWh, it is worth it to me.

The below doesn't include installation, which is being done by two of my friends who are certified to do so, so account for that. I've found install quotes to be around 5-6k where you've provided the kit.

You should not be paying $10K unless you are going for a high end CO2 system. If it is a high end CO2 system it will easily out perform an Infinity in energy savings. For a monobloc style system coming off an infinity I would be charging around $7 to $8 max for a 330litre Hot water heater pump in Napier or Hastings.

Saor Alba

fastbike

541 posts

Ultimate Geek
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#3489225 9-May-2026 19:16

tweake:

i looked at it a few decades ago as mates got it installed. the simple problem is it can boil the water. the systems where designed for peak summer conditions to avoid this. that worked ok in mates case because they had wetback on the fireplace. so electric water heating only kicked in when you had cold weather but not cold enough to run the fireplace.

but if you size the system to work outside of summer, or you have long times with no hot water usage, you can boil the water or usually the release valves opens and you dump the hot water. talking to the install crowd, on the larger ones they design in a thermal dump eg swimming pool or water tank. somewhere to dump the unwanted heat during peak summer conditions.

the PV equivalent is probably designing the panel size for winter use then feeding back to grid in summer.

Interesting, we've never had that. There is a working fluid in the panel so obviously keeps the thermal transfer limited.

Otautahi Christchurch

tweake

2732 posts

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#3489232 9-May-2026 20:11

fastbike:

Interesting, we've never had that. There is a working fluid in the panel so obviously keeps the thermal transfer limited.

a quick google, it looks like they can use a food safe glycol which has a higher boiling temp. so they stop the pump and let it increase in temp to stop the cylinder from overheating.

as you still need PV for the rest of the electrical loads, so is adding a solar hot water system worth the price ?

fastbike

541 posts

Ultimate Geek
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#3489910 11-May-2026 16:16

tweake:

fastbike:

Interesting, we've never had that. There is a working fluid in the panel so obviously keeps the thermal transfer limited.

a quick google, it looks like they can use a food safe glycol which has a higher boiling temp. so they stop the pump and let it increase in temp to stop the cylinder from overheating.

as you still need PV for the rest of the electrical loads, so is adding a solar hot water system worth the price ?

It uses a heatpipe with a heat sheet, details are here. I met Dr Williamson, a very interesting chap although has since shuffled off the planet.

https://atonengineering.co.nz/thermocell-technical/

But back to the PV vs thermal. When we installed the solar thermal the cost was IIRC around $2k plus a little more for the cylinder to have two elements and the ports for the thermal loop. Solar PV in 2002 was scifi, and only deployed in special cases where the economics could be ignored.

Fast forward to the PV prices of last few years - I think I would add another string of PV to where the solar thermal panels are (even though there is some shading in the winter the thermal copes with this) and just dump the heat into the cylinder. This is plan B if we ever have an issue with the HW system. I have a 4 MPPT inverter and only 3 are currently used - although I'd need to do some clever stuff with the battery to avoid clipping to 10kW AC capacity of the inverter.

Otautahi Christchurch

tweake

2732 posts

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#3489994 11-May-2026 20:32

fastbike:

It uses a heatpipe with a heat sheet, details are here. I met Dr Williamson, a very interesting chap although has since shuffled off the planet.

yes heat pipes, which where around decades ago, but they still heated a circulating fluid. back then it was just water. i see today they are using glycol. the gain must be worth it compensate for the efficiency loss of the heat exchanger.

fastbike

541 posts

Ultimate Geek
+1 received by user: 411

#3490005 11-May-2026 23:20

tweake:

fastbike:

It uses a heatpipe with a heat sheet, details are here. I met Dr Williamson, a very interesting chap although has since shuffled off the planet.

yes heat pipes, which where around decades ago, but they still heated a circulating fluid. back then it was just water. i see today they are using glycol. the gain must be worth it compensate for the efficiency loss of the heat exchanger.

Did you ever talk to Arthur ?

Otautahi Christchurch

Quinny

936 posts

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Trusted

#3490038 12-May-2026 08:54

Not sure if anyone has said but the Contact plan (Good Nights) cannot have solar/battery added anymore. You will loose that plan if change/move. Factor this into any numbers. I am one of the lucky ones with solar/battery and on it but if move (which was looking at so spoke with them) I cannot go back on it with any feedin. I currently off shift over 95% of my usage into that period so would miss it

MikeAqua

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#3490045 12-May-2026 09:07

timmmay:

At that point you'll be paying $70 a month just for the hob so it becomes easier to justify replacing, or maybe using bottles. I find cooking only partly uses solar, by the time we want to cook the sun is down or weaker and the oven uses a ton of power, a battery would be helpful here but just use the grid.

Switching to bottles would be cheaper - especially if you have the jets to convert your hob for LPG. We get through one standard BBQ type bottle every four months and I cook a lot.

There are a couple of options to take advantage of daytime power for cooking. A slow cooker is one. If your oven has a decent timer function, you can set it to turn on at x time and cook a roast or casserole. It depends on what/how you cook. I'll often sear a shoulder roast before work and set it up to come on at low temperature at 4pm for a 6:30pm dinner.

Mike

MikeAqua

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#3490055 12-May-2026 09:35

raytaylor:

Pretty much not economic
An evacuated tube solar collector is about 40% efficient so you can get double the heat for the same amount of roof space.
But i'd only look at it if you are really limited for space. Its just so much more costly than buying two or three extra PV panels.
You also dont need to get a plumber involved - the heat is transported using existing electrical wiring.
And the benefit with the PV panels is once the water has finished heating, you can do other stuff with the electricity like export it to the grid or another useful load.

If you were running a pool heater, motel or another system where you need to heat lots of water, then it would totally be worth it but for a residential installation where you only need four kilowatt hours of heat for water each day, easier just to add more panels.

Even with a pool, I'd use PV panels. Air to water heat pumps are effective for pool heating, as you're typically heating to <30*C. You can pump a lot of power into them, as long as you're below set point. It depends how controllable things are but (in summer) I'd set up to heat the HWC first - using a high set point and a tempering valve, and then heat the pool - again using a high set point.

Mike

tweake

2732 posts

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#3490290 12-May-2026 19:20

fastbike:

Did you ever talk to Arthur ?

no idea. i talked to a few companies way back then but i don't recall names or even the companies. sorry it was a long time ago.

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