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124 posts
Posted Yesterday, 10:52 PM
Hi Neil

Those application notes seem to point to DC to DC conversion, using H-Bridge circuitry. It would be preferable to use switched DC or simulated AC to power the HWC element, so the thermostat could be placed in series with the element in order to make it legal in NZ.

What I think would be required is an H-Bridge motor driver circuit, with the forward\reverse switched at 50 HZ rate, the width of the driver pulses are then varied by a cpu, eg PIC to vary the load on the panels for optimum power point tracking. The load can be easily sensed with a pass through current probe, these are quite inexpensive and have a linear DC output with current. Unlike a motor the element is pure DC so no reactance involved, thus no problem with 50 HZ switching.

See the following links to give you some idea. here the element goes across M1,M2. this is all the bits combined into a custom chip.

I would only be too happy to contribute to this as something that others could use, however like yourself I am fully committed for some weeks until my house renovations are complete.


5 posts
Posted Yesterday, 8:34 PM
Texas Instruments' SolarMagic integrated circuit family seem to be good candidates for an MPPT controller for feeding a HWC element. I am working up a PV MPPT system based on the devices listed below. The maximum input and output voltages would be limited to 100V, which is an OK fit for keeping below the Prescribed Electrical Work limit here in NZ. The controller works in buck, direct, and boost modes, depending on whether the output voltage to the element (for MPPT) needs to be below, the same as, or above, the voltage input from the PV panels. The buck and boost options use 4 MOSFETs in an H-bridge. The direct mode can use 2 additional MOSFETs, but I suggest the slightly less efficient, but cheaper, option that uses the 4 MOSFET bridge.

SM72445 MPPT controller
SM72295 MOSFET bridge driver
SM72485 Buck converter (for (10V supply)
SM72238 5V regulator
SM72240 Reset circuit
SM72480 (or LM26LV) O/temp switch
SM74611 Smart bypass diode

Useful application notes for the SM72445 and a similar controller SM72442

Is there anyone there who has experience with these, or who can check them out and confirm whether I am on the right track?

I believe these TI devices could provide an almost 'universal' MPPT controller that would cope with a reasonable range of PV array sizes while keeping below the PEW voltage limits.

I am now fully committed to other projects until mid November, but after that I plan to push ahead on this project.

Neil G

PS: A word of warning: the low side output current sensing shown in the Typical Application Circuit in Fig 2 of the SM72445 data sheet will not work using the SM72295 current sense amplifier which has a common mode range for SI and SO from (VDD + 1V) to 100V.

21 posts
Posted Yesterday, 7:43 PM
Hi Mark,

re your panel orientation: I suggest if you have a play with the BRANZ PV calculator I mentioned earlier, it will give you a pretty good idea of how panel alignment affects average output. That way you can see if it may be worth your while to make adjustments?

Re sharp dropoff due to shading etc, that does line up with this whole MPPT business? I also think I read somewhere that for series panels, if one panel takes a hit it has a fairly big effect on the whole?

I'm going to continue trying to chase the numbers on MPPT vs non-MPPT as that way I can budget how best to chase it.

I also have some background in electronics and control. Having said that, it is a (very) long time since I did any circuit design, and right now I'm not too keen on starting from scratch: while a good final design will probably be quite simple, I certainly don't see myself as being able to go very directly to it.

So far, then, we have a short but growing line of folk hoping someone else will come up with the goods. But at least you're heating some water. And $10 is a fairly economical dog too, i'd say...


1 posts
Posted Yesterday, 10:33 AM
Hi all! We are about to build a very small home on Mt Iron in Wanaka.

We are planning grid fed PV and wetback. We have just bought a sunmar ne composting toilet (secondhand - not used for 2years). The literature on it says we need peat moss - is anyone using anything similar to this toilet? Do you use peat moss? Interested to hear any experiences of it. At worst it will be an experiment as I didn't pay much for it, and the other option would have been a long drop......

On another topic, we are building a 66m2 home with a detached office space. We are looking to put a good quality burner with wetback in and are considering a pyroclassic. We do have some concerns that this may be overkill (and also quite a large volume for our space) but I can't find anything anywhere near as efficient. Any suggestions? If anyone has one - do you have a lot of issues with overflow onto the roof? We'd get a 300L HWC, and are both very much bath people, but there are only two of us......again, interested to learn from you all!


14 posts
Posted Yesterday, 7:57 AM
A simple way to get mppt vs non mppt values would be to compare systems side by side and collect data. A few months ago I came across a Thesis paper about an mppt device. The student had done extensive data collection. The most telling graph was a line chart with your standard bell curve for mppt vs non mppt. Problem is I can't find the link now. I requested the same kind of information from the guy at techluck, but he kept skirting the question which makes me wonder if:

1. His devices doesn't work very well
2. It doesn't work at all

Cost is the other issue. For a 10 - 30% gain an mppt device needs to be affordable, or else you might as well just add another panel or two. Based on pictures of the device I believe it to be very overpriced at $265 US.

If someone can develop one there is a worldwide niche market. I have an electronics background from, but am very weak in the area of buck converters, pic programming, etc.


14 posts
Posted Yesterday, 7:43 AM

The orientation of the building dictated the setup as the main mounts are directly mounted to the roof supports. I've considered making some simple extensions for one side of each panel to 'kick' it forward. The back is easy as I built a telescoping system that can be raised or lowered in 2" increments. Just not sure if I will gain anything by lowering that 20 degree offset.

And on the dog thing, pretty sure it is a conspiracy, but the official story is that it gives incentives to license at the beginning of the year.

Back to the mppt problem and power drop off. If there is any shading or reduction in sun light, whether by the cherry tree, clouds, or purposely, the power drop off is sharp.

I have other data sets where there is a 2 hr block of sub 100 watt output due to thin clouds. Would be nice to be able to optimize for those conditions.

21 posts
Posted 15 Sep 14 7:41 PM
Hi Uhtrinity, and thanks again for all your info. You have a tidy looking setup, I wish I had a roof with that orientation. I also wish I didn't live in a high wind area: I'm still considering how best to prevent 6 square meters of panel from leaving the property.

1. I am sitting here astonished (yet again) that with a few keystrokes I can tell that your street is about 20degrees off due north, which points your panels +80mins from 12.23pm, which is your current solar midday. Imagine trying to put that together from NZ 30years ago?
So, your panels are facing the afternoon sun, and the readings you took would seem to reflect that.

2. Please ignore my earlier attempts to calculate non-MPPT 'losses'. I overlooked at least 2 factors that will have made them totally inaccurate. I will continue working on this (or maybe I can find some figures, they surely must be out there).

3. Earlier in this thread, I posted a link to the BRANZ solar PV calculator. BRANZ are the NZ building regulations body, so the calculator is probably a reliable source. While our weather etc (and so outputs) will obviously differ from yours, I found it interesting and easy to tweak the PV orientation angles etc and see how that affected average output. It occurred to me you could also use this (or something similar) to see if it is worth adjusting your panels?

4. We promise not to stalk you. But only if you will tell us why your dog licenses are $5 for January, and $10 for all the rest of the year?


14 posts
Posted 15 Sep 14 2:47 PM
This site really needs an edit feature. The last link got truncated. If you just zoom in to the top of Stephen St you can see how much off rotation it is. Mine is last house on the left. Panels are on the shop (not shown in satellite view).

14 posts
Posted 15 Sep 14 2:44 PM
One more post, this is the google map view, shows how far south / southeast the panels are rotated. Just promise non of you will stalk me from New Zealand :),+ID/@42.7952914,-112.2494403,68m/data=!3m1!1e3!4m2!3m1!1s0x53554aa3443d0e43:0xe1b098a133f94569

14 posts
Posted 15 Sep 14 2:40 PM
Oh, and btw, that first pic is the full 180 degree panoramic. That pic was approx 6 or 6:30 pm with the sun behind the edge of the cherry tree. I have considered giving the panels a slight rotation south, it would allow them to operate at peak sooner and the tree would be less of an issue, though it is borderline now.

14 posts
Posted 15 Sep 14 2:22 PM
No worries, I heavily participate in the InsightCentral Forums. It is for the Honda Insight Hybrids and we are the same over there.

This is my first time in a solar forum.

14 posts
Posted 15 Sep 14 2:19 PM
Here is a view of the panelsthemselves.

14 posts
Posted 15 Sep 14 2:16 PM
The panels are tilted roughly 40 to 45 degrees and are actually slightly overtilted for this time of the year. The view of the sun is pretty clear except for shading from a cherry tree that starts about 6 pm, but by then the power is already tapering off.

I might have some morning shading in the winter, but the houseline should be fine

21 posts
Posted 15 Sep 14 1:08 PM
Oops....that's not suggesting there is anything 'wrong' with your's just that we tend to swap an awful lot of theory in forums like this, and it's great to be chewing on some real numbers.



21 posts
Posted 15 Sep 14 11:58 AM
May I ask what vertical angle your panels are mounted at, also whether they are part-shaded at any part of the day?
The reason I ask is that I am a self-employed automation eng, working on industrial hotwater, refrigeration etc. I sometimes spend a great deal of time looking at temperature trends (graphs) etc as a way of understanding the 'behaviour' of such systems, as a means of tuning and troubleshooting.
So, in the case of your figures, I find myself trying to build up a mental picture.



21 posts
Posted 15 Sep 14 11:31 AM
Thanks uhtrinity.
I love numbers, though they only stand behind the hard realities, not in front. Still, they can be a very economical way of testing some ideas, and making some decisions.

And now I wish i could nip round with a compass: it looks as though your 'solar midday' is around 2pm, a bearing of 210' or higher?
In which case your figures for 2, 3 and 4 hrs either side of that are not dissimilar to my earlier guesses, except you seem to have had a long period of sustained generation after your 'midday'.

More on this later: I'm supposed to be putting in raised beds for the potato crop!


14 posts
Posted 15 Sep 14 10:17 AM
Here is data from a fairly clear day from about a week ago .My panels face slightly South / Southwest, so I get peak around 1:00 - 2:00 pm

Voltage (DC) Load (ohms) Amps Watts
9:00 AM 9 12.8 0.7 6
9:30 AM 22 12.8 1.7 38
10:00 AM 35 12.8 2.7 96
10:30 AM 49 12.8 3.8 188
11:00 AM 62 12.8 4.8 300
11:30 AM 73 12.8 5.7 416
12:00 PM 88 12.8 6.9 605
12:30 PM 94 12.8 7.4 696
1:00 PM 99 12.8 7.7 764
1:30 PM 100 12.8 7.8 781
2:00 PM 108 12.8 8.4 908
2:30 PM 103 12.8 8.1 834
3:00 PM 104 12.8 8.1 850
3:30 PM 10 12.8 0.8 855
4:00 PM 10 12.8 0.8 850
4:30 PM 105 12.8 8.2 861
5:00 PM 79 12.8 6.1 481
5:30 PM 60 12.8 4.7 281
6:00 PM 41 12.8 3.2 131

5 posts
Posted 15 Sep 14 9:30 AM
Hi uhtrinity, I am glad to hear you are having some success without the use of an MPPT controller. I am thinking I might start out the same way, keeping it simple until a fixed resistance MPPT controller becomes practically available.
I have sourced element that roughly matches 3x24VDC 250W panels in parallel . I have three options as to where I am going to mount the panels ranging from ~3m-10m away. Apart from the cable cost and cable losses are there any other down sides to keeping it low voltage?

I will also attempt to capture data along the the way for comparison. DealExtreme has a range of V/A meters with similar specs to the one you are getting:
I was thinking of pointing a wireless webcam at it and using an OCR library to datalog the readings remotely, but this is pretty crude. Does any know of a remote V/A datalogging device for DC voltages and currents we are talking about?

I also found this document:
which, among other things, answers my question on the cutoff voltages to avoid inspection.
The cutoff voltages are 50VAC and 120VDC.
Unfortunately there are few consumer inverters that output 50VAC.

Thanks, Ken.

21 posts
Posted 15 Sep 14 9:15 AM
Hi SolarMike, what you describe will work fine, so long as your panels are receiving 1000w/M2. That is, at midday on a clear day. However, as the irradiance drops below that, either because of sun angle, or cloud, then an increasing mismatch between what the panels are producing and the fixed load results in 'lost' wattage:
As the irradiance drops, obviously the panels will produce less wattage, but they will produce still less as panel voltage shifts away from the max power point. Which is what will happen with a fixed load. The idea of MPPT in this situation is to make the HW element appear like a varying load (or resistance) in the circuit, essentially 'taking' less power as output drops, and so maintaining the PV at max power voltage.

I said before I had tried some rough figures. I am not very confident of my results, but this may give you some idea for 4 X 250W PV into fixed load:

At 12am, clear, PV possible 1000W, actual 1000W.
At 2pm, clear, possible 800W, but actual 680W.
At 3pm, clear, possible 600W, but actual 440W.
at 4pm, clear, possible 400W, but actual 200W.

(These are rough approximations, I would very much like better figures if anyone has them, or at least some indication that they are approximately correct.)

So, as you can see, as the PV receives less light, the possible output drops off, but with a fixed load, the actual output drops still further. This will happen every day due to sun angle alone, and will happen still more when you factor in cloud cover etc.

Hey uhtrinity, since you have the measuring gear, how about some actuals at hourly intervals on a clear day? That would see us into the ballpark here.
Effective plate area with sun angle is something like:
12am 100%
1pm 95%
2pm 85%
3pm 70%
4pm 48%
5pm 24%
But we will see the output into fixed load dropping away faster than that. And we can then calculate the possible savings with MPPT.

Two other things:
1. There is a bit of confusion around the commonly available MPPT charging regulators. While these are doing what we would want at the PV end (holding the PV at max power point), they are then producing an output optimised for the charge cycle of batteries. This output is not a good fit for direct wiring to a HW element.
2. Most regulators give a DC output. Since this raises various problems with thermostats, safeties etc, I am convinced that we would be better with AC output, or at least 'chopped' DC.

Finally, the very necessary disclaimer: I'm only at the beginning of all this myself, with a a great deal to learn: all thoughts, ideas and corrections most gratefully received.


14 posts
Posted 15 Sep 14 1:11 AM
Ooops, made a typo above and didn't see an edit option.

My panels are Solarworld, 240 watts each.

30 volts ideal @ 8 amps .... not 30, though that would be nice :)

Picked them up a few months ago on Ebay for $200 US each shipped.

14 posts
Posted 15 Sep 14 1:03 AM
As far as matching up panels to an element, that is pretty easy. Using Ohms law I made a chart matching up the ideal voltage and current for both series and parallel configurations. There are numerous AC and DC elements out there ranging from a low of .48 ohms to over 20 ohms.

Running low DC voltages in parallel wasn't practical since I have a 60 foot conduit run between the panels and the house. The voltage drop was too high.

14 posts
Posted 15 Sep 14 12:55 AM
I'm at about 42.9 degrees North, so not far off. Wattage is being calculated by measuring voltage pretty close to the element. Since the resistance is known amperage and wattage can be easily calculated. I also verified current early on using a DC clampmeter.

I then did some testing over multiple days taking readings every 30 minutes to build up a solar power profile. Averaged the values over the day to estimate kW/hrs. We are in fall atm, so weather has been pretty good, other than occasional thundershowers. I've been able to check performance under cloudy conditions where usability drops to 10% for an hour or two. The last week has been clear all week, so performance has been consistent at about 4.5 - 5 kW/hrs /day.

The top element is derated from 4500 W @ 240 volts to 1125 watts @ 120 volts and set at 125F. I'm running it on a kill-a-watt meter. It seems to average about 2kW/hrs per day, lows of 1.5 and highs of 2.5.

I ordered one of these off of Ebay, but it hasn't arrived yet:

But there is a chance I might be outside of its DC envelope since I hit highs of 110 volts at the element.

124 posts
Posted 14 Sep 14 11:28 PM
I'm seriously thinking of doing some experimentation with PV driving a HWC, currently in the process of replacing all the old black DUX plastic plumbing pipes and fittings and installing a solar ready SS 300 liter cylinder in a house I'm renovating. Due to the location of the cylinder the copper pipe run to the collectors will be 40 meters in total, with a difficult route, seems a lot of work and may be cost effective to use PV.

If the PV panel output is a close match to the element resistive load at the mppt point, then is there any point in using a tracking inverter. Eg EccoInnovation sell 28.6 ohm heating elements, if 2 of these are placed in parallel = 14.3 ohms. 250 watt pv panels from AASolar have a mppt voltage of 30v and a current of 8.33A, 4 of these in series will deliver 120v and deliver 8.39A using a 14.3 ohm load, this is so close to the mppt point, will it matter.

I have a large bank of 400 AH Lifepo4 batteries charged by PV with a mppt charge controller, the mppt tracking on it is not that good, the algorithm doesn't seem to be the best, what I'm alluding to here is mppt is only as good at the control system that's running it, PV is very low priced, to make up for an inexact match just an an extra panel.

The only obstacle I can see is the high voltage DC and the thermostat, a simple H bridge switch running at 50 hz would solve that problem and can be made for less than $50.


3 posts
Posted 14 Sep 14 10:11 PM
Hi. The star ratings can be a useful general guide (more stars is better) but need to be treated with care - the rating system gets revised every few years, so a 3 star today would be much better than, say, a 3 star from 5 years ago.

Size is also important - if the ratings here are the same as the European one, they will be be adjusted for size, so that a large fridge that uses less kWh/litre would have a higher star rating than a smaller fridge with a higher kWh/litre, even if overall the large fridge uses more kWh in total.

So, kWh per year is the most reliable comparison (although not always easy to track down).

There is also the 'energy star' label which is used to identify the most efficient appliances (but again, be careful about size). The EECA website has a list of energy star appliances available:

Just as a side issue, coming from the UK, I've found that the efficiency of most appliances over here is way behind what is available in Europe. If they can make them more efficient over there, why don't we have them here?!

Hope that's some help :)

3 posts
Posted 14 Sep 14 9:59 PM
I have some beautiful old rimu and kauri doors that I want to put into my new house. My plan was to strip them back (some painted, some varnished) and then finish with tung oil...however, I have discovered that both the paint and varnish contain lead (the varnish surprised me). I also tested one of the kauri doors that had already been stripped and it was also positive for lead, so it seems that the lead has penetrated into the wood.

My dilemma is whether it is safe to go ahead with just oiling these doors or whether it would be better to polyurethane (reluctantly) in order to seal the doors. Can anyone advise?

Am also now thinking that not wise to try and strip these doors myself, so am considering chemical dipping - does anyone know if that would get all the lead out of the wood as well (then I could happily oil away...)?

It's a bit ironic that I have been working had to avoid VOCs, wood preservatives etc in building the house and thought it would be a good thing to reuse some old doors, only to find I am at risk of bringing lead into the house!

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