Anyone setup a device for using excess solar for HWS


looking to use excess solar for my HWS.

i have an evacuated tube hot water system but winter it struggles. Even shitty on bad days in summer.
Getting a 12kw solar system and looking to pump some excess power into heating the tank mainly over winter. But don’t want to fully heat it and then the evacuated tubes sit there doing nothing.
Its a mid element so winter it can basically do as much as it wants as the tubes will heat the lower half but summer i can max out the whole tank to 85 or 90°
The HWS runs on off peak with afternoon boost which appears to do morning afternoon and night to heat the water
and i have 3 phase so not sure how that would work setting something up. would need some sort of 2 way switch to switch from controlled load to solar

would be nice if it could heat the tank to say 70° but the off peak cut in is @ 60°


I have one of these

Would that work for you?



I have an Apricus system consisting of 120 tubes. It supplies all of our DHW needs as well as some heat for our radiant floor system. If your evacuated tubes are not functioning ( and you should be able to tell by looking at the water temp entering the system compared to the water temp exiting the system) The Delta T of this has been between 10 to 60 degrees differential for our system. If your Delta T is very low, then either you do not have enough tubes, or the vacuum in the tubes have been lost or the heat transfer fluid is degraded. A thermal camera works well to see if the tubes are warmer than ambient temperature (They should not be with adequate vacuum)


Hi gregvet if you have a Fronius inverter you could use on of these a Finder Modular Contactor 25A 2 N/O 12V AC/DC Coil
Catalogue No: 12 V AC/DC run off the Fronius dater manger for about 15 -20 % of the cost,it should work well with 11 kw mine works well with only 6.6 kw


I use a similar piece of kit that Bobboulby mentions, except mine was home made. It uses an Arduino to monitor excess power and then diverts that power to a resistive load. In my case a 3kw heating coil in my hot water tank.
I don’t understand why you’re concerned that your tubes will potentially be sat there doing nothing. I would have thought the goal is free hot water, period. Also if you happen to be using lots of electricity on a sunny day, then the tubes will contribute, otherwise doubt worry about it.
I have my solar hot water set to a higher temperature than I have the gas heated hot water, that way I max out on the free stuff and the gas has less chance of kicking in. Also from May to September I switch the gas hot water completely off and run purely on solar. It’s saved me a fortune over the years.
As for you having 3 phase, does your solar export on all 3 phases or just one? And your property may have 3 phase coming in, but does it split, ie 1 phase for the house, 1 for a workshop, 1 for a barn?


the evacuated tubes work just need something extra for winter

that looks ok but cant find any Australian distributors and the only information i can find is its not up to Australian standard. and cant really even find a price on it. looks like it was $530AU about 5 years ago

it will be a fronius inverter so maybe that’s something i can look at. if i got a 1 NO 1NC and connected to the off peak but if the fronius inverter switches it on it switches it to the excess solar. if no excess solar it switches it back to off peak. then connect it to a PWM to drop the power to 1500w so i can set it to come on in lower generation

i have enough metering devices that monitor my power consumption so if i could and knew how to put it together with the generation data from the inverter that could work

i get 18c FIT so if my solar is heating the tank and the tubes are doing nothing im losing out on 18c/kw and the tubes that cost an arm and a leg aren’t giving an ROI

3 phase export on all 3 phases.
3 phase at house different circuits at each phase, and 3 phase to workshop with different circuits on each phase


I can send you the circuit diagram, component list and arduino program along with web links to the forums of the clever folk that actually designed it if you want? Probably best if I email you it direct.
As for the 3 phase issue I have no experience so all I can do is make suggestions (which I’m sure you’re probably just as good at). Your inverter does it directly churn out 3 phase or is there some sort of device between the inverter and your 3 phase supply? That said the piece of kit I have may work with it’s energy clamp around all 3 phases. Definitely worth pursuing.
I know the program code can be modified to allow a bigger gap between import and export, so if you widened this margin, the solar tubes would have room to work. Or you could put it on a timer and if the tubes haven’t contributed by a set time then the excess solar device kicks in. I’m sure there must be other ways of striking a happy balance.


just sent you a message,



just sent you a message.



Have sent you both 5 emails. I still need to forward you the program for the arduino


Check out
This guy is the brains and inventer of the mk2 robins pv router. He sells kits and I also noted he does a 3 phase version.


There is an Aussie built power diverter see


I presume your HWS includes a pump (usually a standard central heating pump) that only comes on if the thermal collectors are hot enough? Usually you will have a temperature sensor in the tank and another in the tubes - if the tube sensor is >10 degrees warmer, the pump comes on. So you could have a relay whose coil is wired in parallel with the pump, whose contacts open when the pump is on - wire this to stop the PV heating water if the thermal is working. Your electric heater will (should) also include a temperature sensor, which you set to cut off if the desired temperature is reached. In any case, you should NOT prevent your HWS pump running if there is sun on the tubes - otherwise it will overheat and could be damaged.


Ok so solar is going to be installed in a weeks time pending weather and how busy installer is.

I have a few main ideas in my head. (Trying to workout a low cost simple setup)
Use the inverter (fronius) to switch a relay at a set amount of generation. And tap into the offpeak cable to the hot water. This would switch the power input from offpeak to peak where the solar would be coming from.
This would be quite cheap.
I could try and add a pwm controller so i could lower the draw to the element which inturn would
Would let me have the switch on generation alot lower.
I could also get my data from generation and consumption and tally it then if its over my element rating or pwm controller setting turn on relay. More stuffing around but more accurate.
Would be nice to be able give it any excess generation no matter the amount.
This is loosely based off warren333 device. But ising inverter rather than arduino

Totally unrelated to solar install a wetback system on the fireplace and hope thermosyphoning works to get the hot water to the collector and turns on the pump.
This would cost alot more. But would be alot simpler. I dont know of there could be any issues like waterleaks etc.

The standalone controllers listed would cost alot more considering its 3 phase so have ruled them out for now.

Also is it possible to have 2 thermostats on 1 element? Switched by a relay?


Hi B man, i have fronius inverter with a relay controlled by the power management function in the data manger cost $250 all up single phase system, 6.625 kw size solar system in west Australia driving the element in the hot water tank 3.6 kw at the moment,changing for 1.2 kw element this will give me the ability to have the hot water 100% powered by solar 90% of year. atm i can only drive 50% of the hot water from the solar, so element size is very important to how well the solar powers the hot water system. my Frounius power management settings for winter are on at 2500 watts off at 2350 watts, min run time 5 min, don’t think its legal to have relay on off peak supply, so elc would probably not wire this up for you. i also looked at the 333 device and went with the above as it is much cheaper and very much easer to do especially if wired up when your inverter is fitted,see my system Jims maxim jinkos 6.625kW for further info on a working system with power management used, Jim


Hey i opened your system but the consumption data is hidden so unable to see how its working. I just sent u a msg through there.

Have you thought about a pwm controller for your hws element that way you can keep the same element but able to control the power. Turn it up and down pending summer and winter

If i got a Finder Modular Contactor 25A 1 N/O 1 NC 12V AC/DC Coil i cant see the difference between that and the old manual day/night time switch it’s either offpeak or peak just done automatically.
Its basically a timer that limits offpeak when solar is running.

Is there a booklet with information about the data manager? I can’t seem to find much about the terminal blocks on it and the recording frequency etc.


hi settings are in the inverter as below look up the vids on how to on utube.

Jim’s maxim jinko’s 0 en Fronius
Energy management

Output: IO-1State: on
by power production
by power surplus (in case of feed-in limits)
Minimum duration per on-signal:
Maximum duration per day:
Desired duration
per day:
is to finish by:



I have put one of these together and I am very impressed with it. It continuously monitors export and diverts excess to the HWS - on a cycle by cycle basis. We have a 4.8 kW HWS and 7kW PV with Fronius primo inverter (South East Australia). It comes as a kit, but I think you can also buy it pre-made.


I’m doing much the same with excess PV from a Fronius Symo 10 system heating a 315lt HWS. I’ve presently got the Load Management tripping the element on when there’s 3500W excess and off once grid consumption goes over 500W. I settled on those figures as they seem to work best right through the year, although I guess a summer/winter adjustment might get a bit more free hot water. It’s hard to figure the sweet spot when weather and household needs change daily.

Reheating the HWS after morning showers and maybe a load of (hot) washing takes 45 - 90 minutes. Across midday it’s often possible to have the A/C on and also reheat the HWS, all from the excess solar, especially in winter when the panels are cooler.


I’m glad you like it. Back in the “sunny uk” where the cells can be balls out one second and it’s chucking it down with rain the next the only solution is a bit of kit like this that constantly monitors the excess power and diverts it.
It’s save me a fortune in gas (my way of heating hot water)