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Using a Power Diverter Off-Grid

Does anyone have experience using a Power Diverter on an off-grid setup?
I am thinking of adding a thermal storage heater to my off-grid supplied home to store available excess energy for use overnight. There appears to be a range of these on the market with storage capacities between 7.5-35kWhr.
My power supply is an ac-coupled system with 8 kW of PV and 55kWhr of SLA batteries controlled by Sunny Island Battery Inverters. The Sunny Islands use rising supply frequency to limit the power of the solar inverters when the battery voltage limits are reached. The whole power supply is remote from the house and monitored using an EmonTx.
I already have a number of simply switched loads that only operate when the battery limits are reached, using using such things as WiFi smart plugs signaled from node red when the supply frequency indicates excess PV is available. With the loads I have there is still another 15-20kWhr of additional generation capacity on a sunny day that with the right supply switching, could be ‘diverted’ into an electric storage heater.
Does anyone have thoughts on appropriate design arrangements or problems to avoid?

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I seem to remember that Robin Emley has mentioned that he’s supplied a diverter kit to someone with a similar set-up. But I can’t remember the details.

It should be quite easy to modify MartinR’s diverter to make a stand-alone unit because that already measures the frequency. My instinct says that if the inverters are controlled by the frequency, then it’s going to be easier to work with another system that’s also controlled by frequency.

The obvious problem is stability - you need to make sure that the control ranges are such that there is no ‘hunting’ for the right operating point.

easiest way is just using your charge controller. it should have a dump setting. what the voltage of your battery? or what the voltage of your solar panel configuration

you can hook it directly to an ac type element . the only thing to remember is the output volts if 1/2 voltage rating of the element it outputs 1/4 the wattage . if the element is 2.5 kw 120 volt element and your dump voltage is 60v then the element size is 600w just use a series of element to get the right kw of dump you want or use a step up converter to take it 120 volt other ways is use the dump function to drive an SSR ( as some controller support that) to pulse the AC or DC to your tank element … If you use the charge control then you should still maintain proper charging cycle

to control the temperature of the tank just run low voltage through the temperature controllers that drives an DC relay or SSR suitable for your element load


If you intend to use a “PV Optimiser” sort of device which varys the voltage to the elements of a heater (the sort that use heavy Magnetite bricks with elements sandwiched inbetween), I would go careful what storage heaters you try out for this task. If it is an old fashioned storage heater (the type using a simple bi-metal stat for input control and no onboard electronics), then using them as a thermal store is potentially possible. If it is a storage heater with onboard electronics such as a Dimplex Quantum or similar, then they can only accept a straightforward 230-240v on the “off-peak” terminals and not a varying voltage / chopped AC voltage. The reason is that the Charge Controller on said storage heater uses Triacs to trigger and switch the storage elements and feeding that controller with a voltage that’s not a 50Hz 230-240v supply can cause issues.

I get asked this question quite a lot in my day job. Given most domestic UK PV setups have a relatively small grid-tied array over here and get relatively low output from that array in Winter when most heat is needed & conversely Max output from the PV panel when there is no space heating needed, it has generally been accepted as not worth persuing connecting it to a space heating setup. In Australia, how much space heating do you need (in what I assume are 6 weeks of “winter” down under) vs how much Hot Water do you need all year round?

What has always been encouraged for those users is to make use of the excess PV energy heating a Domestic Hot Water cylinder as that will be used throughout the year, and diverting what is left over, to say an Electric Vehicle. Over to others on here when it comes to designs of PV diverting equipment.

Thank you for thoughts.
I suffer from what I imagine distresses many owners of off grid systems, that being watching the spare generating capacity of a well designed system not being utilised on a sunny day and imagining ways to use that capacity. :wink:
We have a heat pump hot water system, induction cooking and reverse cycle air conditioning. The system also operates two pumps for the swimming pool during the day when power is available Unfortunately an electric vehicle for daily use is not yet practical due to the cost of the additional batteries required to charge it overnight.
During our '6 weeks of winter’we do get some sub zero nights and the storage heater looked like a good option to store and use some more of the available energy. I will engage the local heater suppliers to see how their heating elements are controlled.

“Unfortunately an electric vehicle for daily use is not yet practical due to the cost”
best option for this buy two used electric cars and just swap back and forth. you can get some used electric cars now pretty cheap… best for an off grid would be PHEV like a chevy volt or opel/vauxhaul Ampera…

I used this type of electric car ( i paid 10k for mine but I see them going for 5k locally ). I can do 90% of trips on electric unless I do long hauls . plus there no charging infrastructure for hundreds of miles

I have a similar(ish) AC coupled off-grid system and use the Sunny Island frequency shifting for all my diversion loads. Generation is from a mix of PV/Wind (peak 2.5 and 6 kW respectively) and we are in the UK so diversion issues/opportunities/priorities differ somewhat. We have a high heating demand which is delivered by wet system (primary heating source being a gasification log boiler) so this is our priority for diversion.

For primary diversions I use a frequency transducer 0-10v signal to control proportional SSRs supplying immersions in DHW and Pre-heat tanks and a Buffer/Thermal store (1500l in total). These are local to both the SI grid CU and the heating storage/distribution area so nicely hardwired, including necessary automatic isolation for genset operation. This takes the majority of excess generation - averaging 5,000-6,000 kWh per year, 30% of our hotwater and space heating requirement and varying 15-95% with the seasons.

Secondary diversion is to various mobile space heaters (oil filled rads, convectors etc) and controlled by frequency control/protection relays once the primaries are at saturation. We move these about to wherever we think most useful. Only have a small data set as yet but I think these are contributing 5-15% of our heating. I looked at storage heaters a few years ago but baulked at the prices of even used items and decided they would be no better fit for our situation anyway.

The secondary relays have been operating in various configurations for some 10 years and the current set-up with the SSRs for about five. No particular issues, other than selecting the right load values and some laborious fine tuning the trigger points, and no replacements have been required in that time.

I have a reasonably extensive emon system for monitoring which has proved invaluable for “guiding” our off-grid activities. Domestic strife has been much reduced as a result. Monitoring and isolation of secondary diversions for genset operation is now through a combination of Emoncms, Node-RED and HA and there are other possible automations I may get round to muddling my way through.

I have a relatively small battery bank - total 15kWh FLA @ 24v. Although it will be upgraded at renewal time to around 30kWh @ 48v to better match our system, I am not a great believer in battery storage for anything other than filling the day to day renewable generation gaps. We try to keep our consumption consistent with generation and time shift our heavy loads accordingly. I have played with some pre-emptive automation but this often ended in disaster - localised weather forecasting is simply not reliable enough.

At current prices I remain unconvinced about an EV for mopping up our surplus generation - just another expensive battery that would need replacing. When EVs become a bit more mainstream perhaps.

Despite best efforts we are unable to use all our generation or, more sadly, avoid operating the genset which contributes 3-5% annually. I doubt there is anything reasonable that we could do to change that. I estimate we lose between 5 and 10% of our available renewable generation annually to atmosphere and/or inverter back-off when everything is full/hot and all domestic duties are fulfilled. Such is life living with renewables - feast or famine.

Let me know if anyone finds a way of time shifting the weather itself.

you might be surprised those batteries are quite durable - Tesla loop a Tesla taxi/leasing service most of their Tesla have over 300000 miles on them currently . they say they started failing once they reached 450000 miles mark . but they think it the super charging that does them in. As they super charge them twice a day from empty on some routes . 40 BYD e6 electric taxis have have over 500000 miles on them 12 years in service in Shenzhen and no batteries have failed yet. my volt is rate for +15 years battery life - as i live in cool climate- if a hot climate it rated for 12 years. most people get a new car at around 7-10 years . if the used volt I have last to 2027 ( 250000km estimate) I would be happy. as even with a gas jobber I would be replacing and fix stuff on it by then and eyeing up a new “car” as currently i still get full travel distance when it above 15c ( which is normal) … if I could my car of choice would be BYD Tang 600 or BYD Han which you can currently buy in Norway or China which gets 600km to a charge and battery life is estimated at 1.2 million km ( pretty tough for a modern gas jobber to do that many miles today as most of the engines components today are built out of plastic and start failing around 200000km mark) – BYD Blade Battery Now Entering The European Market

Hi Adrian, It appears we have a lot of experience and technology in common.

In our case it is more like 20-30%. Today we are at the edge of the design envelope in that it has rained for the past two days and there has been very little generation. Our SLA batteries are at 65% SoC and I suspect that the generator will auto start tomorrow as more overcast weather is forecast. This only happens about 6 times a year.

The technical problem of optimising the storage heaters appeals to me however I have just seen the price of the storage heaters in Australia and like you baulked. I could probably add some more panels to our system for a better overall outcome for same outlay.

I suppose the above ‘distress’ is really a first world problem and not a bad one to have :smiley:

Hi, I use the frequency of the ‘off grid’ to control diverters controlled by an Arduino, Pi, wifi sockets and a bell transformer. This is on top of a version of Robins PV diverter I converted to frequency control, though it didn’t leave much of the original sketch in.

Sounds like a similar set up to mine.