How to / DIY > Everything else

Easy PV water heating

(1/7) > >>

Hi All
As posted else where I'm well into the off grid barn project, anything mechanical isn't a problem, but if it has anything electrically challenging I get to my limits in very short order!

Here's the first situation I could do with some input on.

At present I have a wood fired oven with a back boiler (a converted 1980's oil burning Rayburn) which supplies a 20mm (3/4'') coil in a very well insulated 110 litre (20 gallon) domestic hot water tank, before going on to feed 2 towel radiators a large cast iron radiator and underfloor heating. It works surprisingly well with thermo syphon, tho' I have built in a pump, just in case.

It gets the water up to 45 / 50 degrees c (110 / 125f)  from around 25c (left over from the night before) with no trouble and warms the rest of the barn too.

The cylinder has 3 standard (2 1/4 BSP) 1000w 220v immersion heaters fitted - as I had the tank custom made I had three immersion heater fittings and a few extra 3/4 BSP fittings for temperature gauges ect put in, so that I have the choice of heating water by PV panels, the generator or a hydro genny without having to have any change over switch gear. It also gives a large amount of redundancy.

 I did mention that I try to avoid anything complicated in the electronic department, didn't I? (It was only by being a member here that I discovered Mosfet wasn't a resort on the Red Sea)

A couple of weeks ago I did a bit of a trial and an hours run with a 3kva genny supplying one 1000w heater, it raised the temperature in the tank (at the very top) from 25 to 35 degrees c in an hour.

As the days are getting longer and warmer I'll be using the stove less and don't have time to start on a little hydro plant yet, I wondered if there is an easy way of connecting solar panels to one of the immersion heaters and dumping everything into heating the water. A system where a temperature gauge in the hot water tank switched the output into the batteries would be good. As of today we're getting in excess of eight hours sun a day, The barn is in a north / south valley and we loose a bit at each end of the day behind the ridges either side, we get quite a lot of sunny days and the air up here is clear too.

For 220v power I have an HGI remote work station genset I picked up (unused) off eBay which has a 3 kw Hatz single cylinder diesel charging four deep cycle batteries (in parallel) through a Victron Multiplus 3000 inverter charger, which all works automatically, as soon as the voltage in the batteries drop below the preset.

No, I didn't design, programme or set it up in any way, I just plugged all the connections in, added batteries and filled the tank, honest!

It would be nice to get a little  solar power into the batteries but I'd rather heat the water first (I'm only burning around 1.2 litres / 3 pints of diesel (with 20% wvo) a day, depending on power tool usage)

The one small issue I have encountered so far is that the Victron is so sensitive that it doesn't like to power the immersion elements, all three have a very tiny earth leak and and this causes the Victron to shut down automatically. I did the trial with a 1960's Villers powered genset that didn't object at all. I checked with the heater manufactures and the tech guy was most helpful, and said he's not surprised and that under mains fed circumstances the 'leak' would not be enough to trip an RCD. This leaves me to believe a totally separate circuit would be better.

I don't object to changing the immersion heater to a 12v model, I made a few enquiries and they seem to be available, with adaptors to 2 1/4'' BSP

So, as you can see I'm trying to keep everything simple, Any suggestions on size of panels,  controlers, size of cables and anything else I don't know about is more than welcome, I can probably stand a little abuse too!

Thanks in expectation....


Hi Stef,
It seems unlikely to me that the Victron is more sensitive than an RCD breaker (50 or 100 ma of current to earth as trip point) , and more likely that you have some bad or cracked heating elements.  A brief overheat (air exposed element or sediment encasement) can cause this, or it could have been a mfg screwup.

So something is off with your setup, or the Victron has a problem, which shouldn't be ignored either.  You didn't mention a model number so I couldn't search the manual for the specified RCD/GFCI trip current.

You can get a clamp on amp meters that will read milliamps, so could directly capture the peak leakage current. In this case it would go around both hot wires 2together, so the element current is self canceled and you only read the leakage current.  It's a useful tool to have.  I have a $50 chinesium clamp on amp meter  that does both AC and DC (true RMS) current sensing and I find that doubly useful, as you can shift to measuring DC current your PV, charge controller, or battery draw.

If you do decide to go 12V, it will be a challenge to get large enough cable and wire to the elements,  The usual  snap disk thermostats are NOT rated for DC current, but some are.  If you use an AC snap disk for DC, yous must keep DC current to microamps, which is readily doable via transistor.  I can help with this but I'm not a fan of high current 12V due to the fire hose wire/terminations required.

Best Wishes,
Bruce M

The most efficient solar water heating is vacuum tubes. These can run on thermosiphon, or pumped systems. The biggest thing would be either running an open system, or having a suitable over-pressure relief (they get seriously hot - boiling off hot)

Generally found more in charge controllers for wind turbines but "dump" or "diversion" loads/modes are an option. (Usually same thing: a heating element, which provides a resistive load and also brakes the wind turbine so it doesn't overspeed)

You could probably find a controller that would take your panels' input, wire the heater via a thermostat as the "main/ battery" and your batteries as the "diversion load"
(Though personally I'd do it the other way around)
I'd go for higher DC voltage, 24 or 48 or even more. This will mean you can use smaller gauge wiring for the same cable lengths, compared to 12V.


--- Quote from: mihit on March 27, 2022, 06:54:45 AM ---The most efficient solar water heating is vacuum tubes. ........

--- End quote ---

Yes, but the 5+ year failure rate on the tubes is abysmal.  I see way too many folks looking for replacement tubes, developing schemes to recharge the tubes with acetone or something ( I love sweat soldering a tube with acetone in it ) to even want to bother with any of the vacuum tube systems.   Often, many tubes have failed, but the rest of them keep the system going for a while.

Evening Gents

Thanks for the input, it's been helpful certainly.

I'm not sure how to reply, so I'll just womble through it all....

Bruce, I hadn't considered the ramifications of 12v dc, so that's out.

The Tech guy from Tesla UK (nothing at all to do with Elon) who supplied the elements, admitted that the earth leakage was common but liveable with, I didn't put power on them until they were in the cylinder and the it was full of water. prior to that they were in the original packaging and dry stored. Any damage, as you say, must me a manufacturing issue.

The Victron unit is a 12 volt 3000 va 120 amp, well that's what it says on the box.

I've got a clamp meter somewhere, I'll try and look it out, it's either in my other barn, about a mile down the hill or in my other Land Rover, in the UK ( I abused it really badly last year, setting this place up, so I ran it back to the UK where a mate with a dedicated Land Rover attitude is going through it so I can overwork it again)

The clip you recommended was quite apposite, i I'm a bit wiser now.

I've sent off for the Victron interface which a mate assures me he can make sense of, I've looked at a couple of YouTube clips and it looks like a worthwhile exercise.

Mike and Mihit,

The layout or the barn doesn't suit the use of a diy solar water heater, the wet stuff is on the north side and the sun isn't so i'd have to pump the hot water around the long way, maybe if it had been built with a bit more forethought i could have fitted it in, but the way the project has grown its a bit late.

I don't need the amount of hot water a vacuum system would generate, I'm trying to cut down my power consumption and support it with 50% efficient 'redneck' engineering that I can manage myself rather than build an expensive off grid system that will run an all electric twenty first century palace, a bit of a flowery turn of phrase but I'm sure you get the idea...


What would be the simplest system that would take the power from PV panels / a wind turbine / a pico hydro plant and convert it to 220v ac so I can feed a 1000w heating element directly?

A friend of mine, a Basque lady who lectures in physics at the university in Bordeaux, reckons it would be almost impossible for a 1000w element to boil 110 litres of water even if you ran it constantly, The heat loss as it got closer to boiling point  would get greater, I'm not sure if this is correct but as there's not a full days sun in the valley her opinion is that the need to divert excess power shouldn't arise.

All further thoughts and suggestions welcome!!




[0] Message Index

[#] Next page

Go to full version