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Solar panels for water heating

Solar panels for water heating

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Peter Bell05/04/2021 10:08:01
376 forum posts
164 photos

I have 4, 270w solar panels which I have used with a dual element 24v immersion heater for a while. The panels are wired as 2 parallel pairs, I have never managed to achieve a decent loading measuring the voltage and current.

Now one element is failing looks like its time to try something else and I wonder if anyone else has better experience?

I also wonder if I need to get a 240v 1000w heater but use a 240v solar inverter rather than trying to match the panels to a lower voltage immersion heater?


panel info.jpg

John Haine05/04/2021 10:18:30
4638 forum posts
273 photos

We've had solar panels for nearly 4 years now with inverter, FIT to grid, battery, and a solar adapter ("iBoost" on the immersion heater. Over the past week when it has been sunny the water has been toasty every day with little extra input from mains overnight, and from now on then our water heatig (and evening cooking) will be effectively free until autumn. We are on our 3rd iBoost unit, the previous 2 blew their main fuse for no apparent reason after a few months, but I think they must have fixed that.

Bob Stevenson05/04/2021 11:28:21
579 forum posts
7 photos

If you just want hot water then you might be better off with a different system.......there are lots of thieves out there peddling solar power so take a look at this site of a 'one man band' developer who has excellent figures to back up his solar panel designs.......

Speedy Builder505/04/2021 11:43:44
2593 forum posts
208 photos

The problem that I see with solar energy is that if the sun doesn't shine, you have to top up with mains elec. However if you knew that the sun was going to shine, you would not top up with the mains and just wait for the sun. If you have a tank full of hot water, then the solar power has to be sold to the grid (and you loose out). Somehow, your solar system needs to be linked to an accurate weather forecast to make those decisions for you.


not done it yet05/04/2021 12:05:29
6744 forum posts
20 photos

Unless you are restricted to electric water heating a better bet might be to use the leccy generation to reduce your grid use.

If you exceed your base-use, an Earthwise Solic 200 could divert excess leccy to your usual mains powered immersion - but it would have a longish pay-back time. At this time of the year, your 4 panels are likely heating your water excessively, unless thermostated, when sunny and cold. Running two strings could explain why your element had failed - firstly you might be subjecting it to excess voltage and secondly it may be scaled-up if the element is a short one (high density).

Gas is costing less than 2 1/2p per kWh (at present), so replacing grid leccy (which is likely worth 5 or 6 times more, at daytime rates) is by far the more economic way to go, if on natural gas heating. Installation costs may seem prohibitive with the current regulations which should be followed.

A lot depends on your life-style, of course - whether the home is occupied during the day, for instance...

Martin Kyte05/04/2021 12:17:15
2728 forum posts
48 photos

or you could opt for actual solar water heating or Solar Thermal as it's know and described on this site:-


The system intrinsically responds to solar heat to raise the tank temperature. So effectively it harvests all the heat it can for your tank and reduces the load on whatever else you heat with. Some lifestyle cahnges may benifit you as clearly first thing in the morning you will not have has the sun all night so for example showering in the evening instead would benifit.

The website was just the first one I could find with a diagram. A freind of mine did his own version and has actually removed his gas boiler completely and run on a log burner and the odd use of an emmersion heater during the depths of winter. He is something of a 'green' thouh and lives on his own so cannot be regarded a typical.

The think I like about the system is that it just takes load when it can.

regards Martin

Bazyle05/04/2021 12:42:13
6300 forum posts
222 photos

If you want alternative energy advice search for the 'Navitron' forum. It is the equivalent for that topic of this site for ME and is likewise populated by more sensible contributors than the 'eco' forums. Last time I looked at it solar water heating by vacuum tube collectores was still not ecenomical payback/life of equipment unless you were already in need of a new plumbing makeover or new build house. Solar elctric > water is less efficient but the electricity is multiuse so works out a more ecenomically sensible choice.

Peter Bell05/04/2021 13:10:59
376 forum posts
164 photos

Thanks for all the constructive replies. To give a bit more of the picture I initially bought a "piped" panel system but it proved difficult to plumb in to my system as it was then. We now use an air source heat pump to a thermal store and have underfloor heating from that, all home grown and installed. The hot water cylinder is heated by the ASHP with the supply controlled by motorised valves on demand. It's stored at 35c and the final water to taps is boosted to 50c by on demand by elec heaters.

The on demand elec heaters can take feed water at up to 80c but obviously would be switched off so I decided if I could boost the cyl temp feeding them anything would be a bonus. I have never achieved more than 60c even in the summer hence my question about better matching of my panels to an immersion heater. I'm trying to avoid batteries and grid tie and had wondered if one of the solar type inverters would accept a dc supply direct from the panels and output 240v for the immersion?


martin haysom05/04/2021 14:22:39
90 forum posts

i have solar water heating, wile i cannot say it don't work it is so inefficient it will need to last about 70 years to recuperate it cost, assuming it never requires any maintenance. so for anybody considering solar water heating unless you live somewhere much sunnier than the UK i surgest don't waste your money

Nigel Graham 205/04/2021 14:57:34
2053 forum posts
28 photos

I read this thread having had my interest raised by a friend in the Fire Brigade recently telling me something I'd not previously known about solar panels.

I'd already know, as Martin says, that solar panels are not all they are cracked up to be, though many people do claim positive (!) results from them.

It seems the utility-bill saving can be at least partially offset by the rise in house insurance consequent on fitting them. That is because even if the fire is not caused by a fault in the system, a roof covered in the panels creates considerable additional problems and hazards in fighting it.

("And as for battery cars..." he added, part-way through a course on dealing with those safely.)

My brother fitted a small solar-panel to the roof of a shed in their back-garden in Southern Scotland. He mounted it at the best angle for latitude, on a frame rotated by motor to keep it facing the Sun.

Paul White 305/04/2021 15:41:30
109 forum posts
23 photos

Nigel, can you give any more info/comment on your brothers small panel installation.


Nigel Graham 205/04/2021 20:59:02
2053 forum posts
28 photos

Paul -

Not very easily but I know the panel is fairly small, maybe 1.5m square. It feeds large storage batteries but though it augments the mains supply I don't think it has a feed-back function. I doubt it would run the whole house alone for a day, but that wasn't the idea.

The panel is rotated at clock speed by a small motor and reduction-gear, taking little power.

The system he built for the electrics and oil-fired heating looks wondrously complicated, with a control panel that would grace part of an oil refinery, behind a glass door; and a catering-size heat-exchanger / storage tank he'd obtained second-hand at a fraction of its new price.

He worked for a time for a "green energy" company, so learnt a lot from that. I asked him if they installed many small-scale hydro-electric schemes of the types now quite common in England, since Scotland is hardly short of burns and the precipitation to keep them flowing. He said the planning system didn't actually ban them but made them not worth the effort. Ironically there are whacking great wind-turbines all over the place!

John Haine05/04/2021 21:44:01
4638 forum posts
273 photos

FWIW I informed our insurers when the panels were installed and there was no discernible effect on premiums. As I said we essentially heat our water and do much of the evening cooking by solar power from spring through to early autumn, and export a bit to the grid. Much better to use it yourself if you can though.

Solar electric is a better way to heat water than direct solar I think as you get high grade heat though the efficiency is possibly less- but since you can only heat 206 litres (according to the label on the tank) it's better to do that to 60 degrees at say 50% efficient than 50 degrees at 60 % in terms of the usefulness of the stored heat.

John Haine05/04/2021 21:52:01
4638 forum posts
273 photos

Peter, it would help if you could give us some information on what voltage and current you actually get from the panels and what the resistance of the immersion elements is. Maybe a different connection would be better to give a better "impedance match" - such as all the panels in series with the two elements in series. Perhaps it needs to change depending on the light level with a couple of relays?

Georgineer06/04/2021 00:20:20
567 forum posts
32 photos
Posted by martin haysom on 05/04/2021 14:22:39:

i have solar water heating, wile i cannot say it don't work it is so inefficient it will need to last about 70 years to recuperate it cost, assuming it never requires any maintenance. so for anybody considering solar water heating unless you live somewhere much sunnier than the UK i surgest don't waste your money

Absolutely. I also have solar water heating and I would have done better to leave the money in the bank.

  • It only produces measurable amounts of hot water in the summer, when the need for it is least.
  • Although it has a bigger storage tank than the original, it cannot store all the heat available on a sunny summer day and shuts down.
  • It delivers water at 80°C which would be a serious safety concern if there were children or frail adults in the house.
  • It still relies on mains electricity to operate, so when the sub-station opposite our house burned out we had no source of hot water until it was restored (the gas boiler and immersion heater also need electricity).
  • Modern washing machines have only a cold water inlet so no saving there.
  • Maintenance costs more than the saving in gas bills.

I'm sure there are other drawbacks I could think of if pressed.

George B.

Peter Bell06/04/2021 08:32:17
376 forum posts
164 photos

Thanks for all the interesting replies. So far I havent spent much as its an ongoing experiment really rather than an investment so I'm not terribly worried on the cost.

John, my panels produce 8.69a at 31v each according to the spec. I did some testing when I first got them and I'm sure they produced something like that but I need to do more. This video goes into it and the chap also has an interesting spread sheet.


Paul White 306/04/2021 08:59:50
109 forum posts
23 photos

Nigel, thanks for your response , the “following the sun” aspect is very interesting.

Again thanks.

Neil Lickfold06/04/2021 09:50:42
836 forum posts
166 photos

A guy in Hamilton made a rotating solar shed. He used a sensor of some sort, that moves the unit when the sun has moved more than some amount of angle. When the sun shines it rotates to that angle then stops. In the evening it rotates to a place to catch the sun in the morning around 8 or 9 am sun position. Before that , there seems to be very little energy being captured. Like 2 hours before sunset, also very low energy. He found keeping the panels clean was very important.

not done it yet06/04/2021 09:57:51
6744 forum posts
20 photos

Let me analyse a few comments - mostly the adverse ones.

Firstly Bazyle is spot on re Navitron - although it is sponsored by the company there is no interference as long as competitors are not advertised on their forum!

Pay-back time is, as reported, more if replacing natural gas heating - although gas installations only running intermittently and for short periods are not as efficient as some claim. Replacing electricity at day-time grid rates can provide a much faster ROI (return on investment).

MH does not say what form of system he has. There are some suppliers that install ‘chocolate teapots’ at exorbitant cost. Many have been ‘taken in’ by hyped up marketing, in the past (and still are).

The panel mentioned by NG 2 is huge, as panels go. 1.5m square is 2.25m^2. Panels are rated at around 160W per m^2 at standard insolation of 1000W/m^2, so that single panel would be rated at around 350W. I call a small panel as one rated at 20-30W. Most panels are typically 180-200W, with some now around 300-330W.

Scotland ‘s lattitude means very short winter daylight hours and looong summer hours! Hence more use as replacing leccy than heating water, the further north one lives. 15 hours at full power (unlikely) would only provide 5kWh of electricity, so say 4kWh maximum usable. Base load during the day would likely more than halve that available for storage in a battery system (depends on lifestyle, of course). Effects would likely be less in southern Scotland.

There needs to be no problem, if prepared to install mini inverters (or possibly other devices before the main inverter to reduce the DC voltages within the system), of very high DC voltages found in most string rooftop inverter PV systems. Mini inverters invert the individual panels to mains voltage and frequency, so avoid the (usual) up to 500V DC, or more, feeds to the more usual string inverter.

There is no reason to expose any occupant to scalding hot water at 80 degrees Celsius!. TRVs (Temperature Regulating Valves) are cheap and a safe way of managing a hot water system with higher temperature storage. There is then only the risk if the hot water tank ruptures (has been known).

All solar thermal systems should be sized to maximise the annual hot water needs (not necessarily the total for all the year!), thus needing some form of bypass for surplus heating which occurs in the summer (and while hot water is not being used - holiday periods, for instance). Most systems offered are under-sized (marketing, quick sale and profit again!).

Only battery systems, with an off-grid inverter, can avoid everything shutting down during power cuts.

Edited By not done it yet on 06/04/2021 10:05:23

Alistair Robertson 106/04/2021 10:00:59
144 forum posts
6 photos

A house in the next street has for about 30 years had water heating solar panels.

My wife used to go in and care for the woman and on sunny days they had to run off hot water from the system as the tank was absolutely roasting, in fact boiling and the excess water was spraying from an overspill vent on to the side of the house next door.

I recently spoke to a new owner who has been there a couple of years and he says that the system is still working well and he has fitted a hot water generator that powers his garage tools and his robot mower etc. I will have a look when we are allowed to visit safely.

The system still works very well in the winter on cold sunny days and he doesn't need any backup electric heating.

I will update when I have looked in to how it all works.

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