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Replacing water heating solar panels with pv?

  • 01-07-2022 1:55pm
    Posts: 0

    Ten years ago I installed solar water heating panels on my south facing roof. They've worked fine for a decade, but I'm now considering installing pv solar, and the optimal location for them is on the roof where the water heating panels are. (there isn't enough roof space to install both).

    Would it be crazy of me to remove the former and replace them with pv which would, I assume, both heat my water more effectively as well as providing additional electricity?

    (the alternative is a ground-mounted system which would be dearer, I'm told.)


  • Registered Users Posts: 6,002 ✭✭✭Ubbquittious

    I would not get rid of them if they are working fine.

    You need about 3x the surface area worth of PV for the same water heating capacity as tubes. Ground mounted is not a bad idea

  • Registered Users Posts: 1,231 ✭✭✭DC999

    Or do both. Get solar pv on the better roof (sun wise) and use to generate electricity but not heat water as they are less efficient for heating water.

    And move the existing solar water heating panels to another part of the roof. Others with them would know what (less sunny) roof orientation would still give you benefit. I don't have them. So you're only paying for the cost to move them on the roof. You can fit them on the ground or a wall but ones I've seen like that can look like an eye sore and take up garden space. I've no idea how long those panels work (need to replace the fluid every so often). But if there's still years left in them, then keep them and free hot water (bar cost to move them).

    What other direction do you have roof wise?

  • Posts: 0 [Deleted User]

    Solar water panels are on a south-facing garage roof. Main house roof faces East/West; it's a dormer bungalow with veluxes so locations for pv panels on the roof are limited and there are issues with shadows. Only viable alternative seems to be ground mounted, S facing panels.

  • Registered Users Posts: 63,640 ✭✭✭✭unkel

    You should leave thermal on the south facing roof. It makes at least 3 times as much energy as a PV panel per m2 and it produces most energy (that you all use in the form of stored hot water) when south facing. For electricity (self use, without storage), PV panels on east + west is ideal, you can use optimizers on all the panels that get shading. Ground mound is fine as well, but far more expensive to install unless you can do it all DIY and fabricate the structure yourself from materials you can get cheaply.

  • Posts: 0 [Deleted User]

    Thanks, but would installing panels facing due east and due west not mean that I'd have to install a greater number of panels to achieve the same output than if I opted for the south facing ones? (Another potential issue is that I don't have any attic space in my bungalow, due to vaulted ceilings.)

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  • Registered Users Posts: 63,640 ✭✭✭✭unkel

    Yes, of course a south facing panel produces most. That said, if you can not store the electricity you produce, it is best to produce when you need it. With an EW setup you produce (a good bit) throughout the day when you and your house use it. With a S setup you produce a lot in the few hours around noon (usually far more than you use) and you don't produce enough in morning and afternoon

    Personally I have a large power wall (20kWh) and over 100kWh in batteries in my EVs, so my 100% south facing arrays work well for me. But without storage EW is ideal

  • Posts: 0 [Deleted User]

    Thanks for that. I'm torn between selling my surplus (if any) to the grid or investing in storage. It's hard to know where to get objective advice.

  • Registered Users Posts: 1,231 ✭✭✭DC999

    Its not an easy decision for sure and each house is different. If you have an ev or plan to get one, you can use that to soak up the excess (if it's in the driveway when sun is shining) using something like a zappi charger. So the ev is my 'battery' for now.

    I'm gonna see what surplus (if any) I have for the grid and then in year2 look at adding a battery if the maths makes sense or depending on how it's all working. I only get my solar next week. Really depends on the FIT rate (more on that in another thread in the renewable energy forum).

  • Registered Users Posts: 5,702 ✭✭✭Chris_5339762

    We have the same.

    Old solar hot water panels and new solar PV panels on the same roof. We decided to leave the old solar hot water panels as directly(ish) heating hot water is always going to be more efficient than getting PV to heat hot water.

    It took a bit of tinkering, but we ended up setting the PV (eddi) thermostat to switch off at 55C, allowing enough headway for the old solar hot water panels to top it up for the rest of the day.

    We now get a ridiculous amount of hot water very quickly. But it is certainly possible to marry the two systems up together. I definitely wouldn't be pulling the old solar hot water panels down. Our only worry is if they fail, antifreeze will go down the drainpipes into the pond.

  • Registered Users Posts: 63,640 ✭✭✭✭unkel

    @Chris_5339762 - solar tubes are about 60-70% efficient, PV panels only about 20% in capturing the energy from the sun.

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  • Posts: 0 [Deleted User]

    So my quote is ~€10,000 (incl VAT and before grant) for the following 4Kw (ground mounted) system

    10 x 400w Trina Solar PV Panels

    Ground Mount Frames

    Huawei Hybrid Invertor (SUN2000-4KTL)

    Mounting Brackets, Cable and wiring

    Includes all Application and Admin fees for NC6 Connection to the Grid and for the SEAI Grant.

    Any informed views on this price would be very welcome, thanks. 

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  • Moderators, Education Moderators, Home & Garden Moderators Posts: 7,919 Mod ✭✭✭✭Jonathan

    Seems expensive for what it is. Why a hybrid inverter? Are you planning on getting a battery down the line?

  • Posts: 0 [Deleted User]

    I have no idea why! At this point in time, I have no plans to install batteries but am open to it when I have seen how much surplus the system produces. (I had mentioned to the installer that I hoped to sell surplus power to the grid so I assume that's why it was included.) Is it an expensive piece of kit?

  • Registered Users Posts: 63,640 ✭✭✭✭unkel

    A non-hybrid inverter is the guts of a grand cheaper. I would get a few more quotes, that one seems very expensive.

    That said, I don't know what a decent cost is for a ground mount setup, but there is quite a bit of material costs and a good bit of labour in fabricating and installing one. You sure you don't have a bit of roof somewhere where the panels can go? It's far, far cheaper on a roof.

  • Posts: 0 [Deleted User]

    Once again, thanks. I'm pretty set on the ground mounted panels, for a number of infrastructural reasons, and am willing to pay extra for it.

    However the above quotation excludes the cost of me arranging for someone to install the metal frames and dig a 25m trench (presumably with ducting) to take the cable to my meter box in the garage. So yes, I think that I definitely need some more quotes.

  • Registered Users Posts: 1,224 ✭✭✭Galego

    It isn't a straight cut 60-70% Vs 20%. The PV panels work better than thermal across certain weather conditions. There are videos in YouTube proving this point. It is a straight winner for the thermal if you only want to heat water with the panels but PV panels are way more versatile and that's why the industry has moved to them and ditched thermal panels (in a way).

  • Registered Users Posts: 63,640 ✭✭✭✭unkel

    It's not really that the industry has moved to them. It's that PV panels cost only a fraction now of what they cost 10 years ago (tubes haven't really gone down much) and where there was a decent subsidy on tubes back then and zero subsidy on PV, now there is a ridiculously generous subsidy on PV. Also for the sunk cost of thermal, you will need to have a very large hot water demand to have a reasonable pay back time (think big households), PV is far more scalable

    Secondly, for many installs it's not either PV or thermal, but a combination of both, obviously nobody just wants to heat water. Personally I have 40 tubes and nearly 11kwp of PV

    So yes, I wouldn't really recommend anyone with a large roof area to go thermal these days unless they have a huge water requirement. I'm a family of 5 with 4 adult / teenage females. My brother in law has a large indoor swimming pool and 120 tubes. It helps a lot there, where PV would be completely inadequate.

  • Registered Users Posts: 4,381 ✭✭✭Gerry

    I've just taken thermal panels off and replaced with PV. Yep - this might seem crazy.

    My house came from new with thermal on SW and PV on NE ( yes - I know. The builder didn't care, it was easier for installers, and like the rest of my neighbours, we didn't pay enough attention )

    The south west facing roof with dormer + velux - 3/4 PV panels max might have fitted.

    On the north east facing roof, there's already 1KW PV, honestly didn't see the point in moving the panels there. I've offers to buy the thermal panels already.

    So now I've 8 panels on the front and 4 on the back. I'll use electric immersion though not going Eddie just yet. Will look at night rate or getting home assistant to switch on during periods of large export.

    3 factors did for the water panels:

    We don't use massive amounts of hot water. In the summer, the thermal panels produce loads, in fact too much, and there's nowhere for this to go.

    Also they really don't work that well in winter, despite all of the hype about them being more efficient than PV.

    PV excess can always be stored or fed back to the grid

    The maintenance cost for thermal panels ( which I've seen every few years ) eats into the financial savings. If you don't refill the coolant and flush every few years they stop working. the pump also failed after a few years. it will fail again.. had enough.

    I'm saying this from 6 years of experience with them.

  • Posts: 0 [Deleted User]

    So I'm back again with my latest brainwave! (I've given up on ground-mounted due to cost and payback.)

    The photo below shows the only suitable roof for PV panels in my home, it faces due south and gets uninterrupted sunshine all day long.

     As can be seen the water heating panels in the middle take up a lot of the roof, however, I have space available for solar panels on both sides of those thermal panels. The space available on either side of the existing panels is approx 2.1 metres long by 4.5 meters high. However, given the SEAI grant conditions, I can't install panels closer than 500mm to the edges of the roof or 200mm from the top. This leaves me with two areas of 1.6m X 4.3m on which I can install panels - that's a total of about 13.5 sq m.

    My questions are as follows:

    Can I mix and match panel sizes to maximise the use of this roof area or would that be impractical?

    Roughly how much energy (kWp) could I expect to generate from this area of PV panels?

    Would I be crazy to proceed? (Yes I know that depends greatly on the cost of the installation!)

    Thanks again.

  • Moderators, Sports Moderators Posts: 18,131 Mod ✭✭✭✭slave1

    Can I mix and match panel sizes to maximise the use of this roof area or would that be impractical?

    You can put whatever you want but when mixed together the panels max output will be determined by the lowest wattage panel. Best was to manage this is to have low planels on one string and high panels on another as the limitation is per string.

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  • Registered Users Posts: 63,640 ✭✭✭✭unkel

    Not the lowest wattage, but the lowest current. But yes, if you have two identical size panels from the same manufacturer, one say 355W and the other slightly newer 375W, it means the newer has slightly better efficiency, which expresses itself as a slighly higher current (at the same voltage as they have the same number of cells). If you put them in series, both output the same current, which means they both produce 355W max.

    You could put them in parallel though, then both will produce their own max output and you don't lose anything...