Half power water heater setup


Solar Enthusiast
I'm in Aus, we're 240v & we're talking a household water heater setup. Its a full size electric hot water heater with 2 elements - the top one is a "boost" which is always on, the lower element is wired as off-peak or "controlled load" - which means the power companies turn it on-off when power is cheap, mostly at night. Both elements are 4.8kw elements (12Ohm).

So i've got 7.5kw rooftop solar, which during the day will have up to 5kw spare capacity available during the middle of the day, but mostly 2kw for a good 8hrs a day (depending on the season).
I get 6c/kwh for feeding into the grid, and during the night i'm using off-peak power at 9-15c to heat up the water in the heater. So wasting 3-6c heating water every evening. So wanting to see if I can use the excess solar to heat the water, the issue is the heater has 4.8kw elements - if i turn it on during the day i'll be paying 16-20c/kwh for any power from the grid not covered by solar. So i must be able to throttle it down.

Looked into all sorts of power control to bring down that 4.8kw - SCR's, PWM... but trying to throttle/control 5kw of power was a hassle.
So i decided to keep it simple - just use a transformer to drop the power, and run the heater element on the transformer secondary! Super simple & efficient, no heat sinks or other stuff to worry about.

Its 240v here in Aus, and luckily we can get cheap step down transformers for USA (110v), or Japan (100v) that are pretty high capacity. A 1000VA transformer was $100AU. Its a 12ohm heater coil, at 100v its 8.3A, so 830W - a 1000VA transformer gives the right headroom & putting 8.3A through the 10A relay (which isnt actually switched - as i switch the primary side before the secondary)

I've got the main element on a 30A relay (snubbers on all relays) which will enable/disable Off-peak. Off peak starts at midnight every night & it'll heat for 2hrs, then again at 5am for 30min. The output of that relay then goes to the secondary winding (100v) of the transformer, I did this so i'm not putting 240v through the 100v winding & generating 400v on the primary.
I then also switch the primary side of the coil.
I switch the relays in specific order so i'm switching the minimal current. I never switch the off-peak relay during its use (20A switch), I then switch the secondary (no current as primary is off), then i switch the primary coil - as its lower current than the secondary. (As above - all relays have snubbers)

Everything is controlled by a ESP8266 board with 2 relay boards, & a 5v Vigortronix supply.

Havent had a sunny day yet, but the plan is to run it as much as possible during the sun (can voice activate/web page as its WiFi connectred), I should get 8kwh or so - so that at night it only runs for 30min or so. I could adjust the switching so i can turn it off so it wont operate at night - but there's a risk if its not sunny enough. So i'll just get it as hot as i can during the day & then allow it to top up on the cheapest grid power.

It basically switches/replaces the off peak line with "throttled" peak power into the heater unit - so all the safety stuff remains & the thermostats are still the primary controls etc.

Circuit diagram & photo attached below.
Hopefully this helps some others.


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Photon Sorcerer
Have you considered just disconnecting the upper element? In our normal use (no children at home) the upper element almost never comes on.


Solar Enthusiast
Have you considered just disconnecting the upper element? In our normal use (no children at home) the upper element almost never comes on.
yeah it never comes on.
im trying to replace the offpeak with solar - the peak element has never done anything & wont be doing anything.


Solar Enthusiast
Some updates...
I now have 3 power settings - 1000W, 2400W & the usual 4.8kw.
For the 2400W i wire the elements in series, HOWEVER... it trips the overtemp on the top element. So i'll be installing a temp sensor at the top, when it hits 70-75deg (OTC is set at 80) - it'll switch from high power to low power (just the bottom element) to prevent the OTC tripping.

So far its working really well, i can dump 5-7kw/h into it during the day (its the middle of winter) & its taking a good chunk out of the night cycle.


Solar Enthusiast
why not just put smaller wattage elements in the water and run them off your solar powered panel ,same volts less watts ,may take longer to heat and and put them on a timer so they don't go on at peak ?


Solar Enthusiast
why not just put smaller wattage elements in the water and run them off your solar powered panel ,same volts less watts ,may take longer to heat and and put them on a timer so they don't go on at peak ?

yeah thats another solution for sure, but then you lose the ability to heat it quickly on cheap electricity rates overnight if needed. Or if you ran out of water the ability for a fast top up. (could leave the top element to fix that last one)

I also didnt want to stuff around, drain the 200L tank, try & find the "right" elements that would fit etc. Then maybe have a slight leak in a seal...
I was looking for an "external solution" if that makes sense - less problems, but you are 100% correct.

The need for lower power settings on everything is becoming more evident with solar/batteries (kettle, water heater, oven....)


Solar Enthusiast
OK here's the latest setup... I have 800W through the bottom element using the 100v transformer & 2,400w using both heating elements in series.

BUT... there's an issue...
when using both elements its tripping the OTC at the top, which must be killing the neutral & active (& stopping the lower element).
I'm amazed at the stratification inside he unit, using an IR thermometer i can measure 76deg at the top & 45deg at the bottom (degrees science).

So once I hit that 80deg point at the top, i only have 800W through the bottom element - which i can have running all day & takes me to 53deg or so at the bottom.
I really need to pump 2.5kw through that bottom element. Options are a larger transformer ($$$) or replace the element with a lower power one (hassle, drain, hope there's no leaks, non-standard element...)

For now i'll install a temp sensor at the top, so at least i dont have to manually reset each time, then think about next steps!
a 2.4kw lower element is $33 & you're supposed to drain these every year or so for maintenance... looks like i'll be going down that path!


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Solar Enthusiast
I replaced my lower 220 water element with dual element 36 volt/600 watt element each. I have 3 solar panels on the east side of the house and 3 solar panels on the west side of the house. I believe the panels are 305 watt each. They are connected directly to the water heater element. With sun shining every day, my upper element does not even turn on so basically the hot water setup is all on solar. I also have a set of feed wires coming from one of my solar controllers to feed one side of the hot water heater elements in the 1 to 4 o'clock time of the day. I send this energy to the side of the element that is fed off of the east facing panels since during this time of the day no amps are coming from the east facing panels. My two sets of batteries normally go on float at around 3 pm each day. So by heating water when the batteries go on float, I am using energy that would normally go to waste. 80 to 90 percent of my 110 power is on solar. 2100 sq foot home.
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