Home Forums General Discussion Using PV systems to heat hot water


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    Richard Popenhagen

    I recently asked one of our local PV experts the following questions:
    Is it possible to configure a PV setup so that it heats the hot water cylinder during the day?

    The reason for this is that during the day when there is little or no power demand from the house, instead of exporting all that excess electricity back to the grid, for often a variable or rather small buyback rate from the power company, that excess power gets used to heat the hot water cylinder. Sort of the reverse of having a HWC only heating at night. Obviously the circuit supplying the HWC would need to be changed from ripple control.

    Is this technically possible?

    What are the problems or challenges that would need to be overcome? (capacity of system, load control, supplementary back up for winter/ weeks of cloudy weather, overriding the element from coming on at night…..?)

    Is it even worth considering doing this?

    Here is the response:

    Hi the attached link has got some chatter about getting the best value from solar once Meridian go away from the 1 to 1.


    Re hot water would be a simple case of putting a time clock on the tank to come on during the optimum solar power production 11 to 1. I would say the tank needs to be big enough and well insulated to hold the heat up until evening and morning. Of course needs to be a sunny day if not then it just uses power from the grid as it usually did. Yes it is all about using the power when it is getting produced.

    It does not answer all my questions but it is a start.

    A couple of other points that I would want considered:

    You would need to make sure that only two hours of power to the hot water cylinder a day would be enough to meet the hot water demand of the house, for a larger household you may need to extend the timer so that the element is on for a longer period.

    This would also put you on the higher “uncontrolled” electricity price tariff. So for days when there is not enough power being generated by the PV system to cope with the demand from the HWC element, the element would be drawing power from the grid, at the higher uncontrolled tariff compared to the normal lower controlled water heating tariff, at present this is about 18% lower.

    Another way around this would be to use a dual element solar cylinder with the PV connected to the bottom element and the normal ripple controlled water heating supply connected to the mid element.

    Those are my thoughts so far, I am keen to explore this option further if anyone else has some more info on this subject.


    Norman Smith

    Hi Richard, a quick first response.

    A few months ago there was an extended conversation on-line on the Sustainable Energy Forum (do others know about this group?) lead by the doyen of solar water heating in NZ Arthur Williamson of Chch which discussed a cut down PV system. Rather than an inverter, direct current from PV would heat a (second?) coil in the hot water cyclinder. I’m now a bit hazy on the details and have no idea if it is possible to retrieve the various posts on this which discussed technical issues. Could this be relevant?


    Richard Popenhagen

    Hi Norman

    Thanks for the response; I posted that article a few hours before I disappeared away on holiday, hence my delay in replying.

    This certainly could be relevant; I guess I am trying to get my head around the practical details of how to make it work. Questions I have are:

    ·         Are DC elements available that would screw into a standard HWC element port?

    ·         I assume that this DC element would need to be controlled by a thermostat to prevent cylinder overheating during periods of low hot water demand (when people are away on holiday!!!)

    ·         You would still need a backup element connected to the 230 volt AC supply to provide heat during times of inclement weather. This would also need to have thermostat control, so you would need a hot water cylinder with dual element housings and dual thermostat pockets.

    ·         Obviously prioritising the PV supplied element to work first would be essential to maximise the gain, controlling the secondary boost element with timers as has been suggested in other posts as one way of achieving this (see below), but from my experience with solar hot water systems this is fraught with problems, especially if hot water demand profiles change, as they often do, even between week days and weekends.

    There has been some chatter on this subject on ecobob, link attached for anyone that is interested:






    Norman Smith

    Hi Richard, I trust your holiday sees you refreshed. From memory, and I didn’t follow the SEF debate too closely, it did drill down into the kind of detail of interest to you. Will see what I can retrieve. Norman.

    Scott Willis

    Hi Richard,

    Direct PV to heat hot water is known as “Hot PV” I understand. Worth contacting Barbara Elliston at Elliston Power Consultants Limited to find out more as this is something in progress. Best, Scott

    Norman Smith

    Prompted/reminded by Scott’s posting I took up my promise in an earlier one to see what I could retrieve from the SEF forum and which kicked  off thus:

    ” PV based resistive water heating is better value than solar water heating. Capital cost is less, and the council overhead for potable water building consent / and leaky building risk is eliminated. You could probably also deploy it without notifying the electricity company, as it will not be exporting into the network. Also gives an upgrade path for later on when inverters reduce further in cost, allowing for higher value grid tie energy to be delivered to the customers load. Regards, Malcolm Souness”

    I could retrieve the various postings and links and compile a ‘quick and dirty’ document to be forwarded to people who are interested. I would not want to lodge it in the Best Practise Library; getting something to that level would take some hours, but it would be a starter. Interest?

    Cheers, Norman

    A debate followed with

    Fred Braxton

    Hi Richard, Norman and Scott, thanks for your contributions.

    We’re using PV to heat most of our hot water — via 240V AC rather than DC.

    When our old (15 years), inefficient solar HW died we took a gamble.

    We don’t use a lot of hot water anyway so instead of replacing the SWH we spent the equivalent money (and then some!) on beefing up our small 15-year-old  0.75kW PV system to 3.6kW along with a new off-the-shelf mains electric HWC. (Interestingly the additional 2.9kW cost less than the original 0.75kW!) We are grid-tied.

    Heart of the WH system is a Chinese SS4 SWH controller. This clever device has a range of user-adjustable timers and temperature differential settings. I’ve configured it to do an electric boost between 11am and 3pm (when PVoutput is normally at its maximum) but only if the HWC temperature is below 45C. On low-PV days of course it sucks mains.

    The NZ-designed and -made Solastat and Ecostat can probably do a similar job.

    In addition we program the dishwasher and washing machine to operate in that time window also.

    The controller also manages the wetback pump (it thinks the wetback is a solar WH). The wetback has its own heat-exchanger in the cylinder. In addition it has a legionella booster function and there’s a manual boost with adjustable cut-off temp.

    Critical to the system working well was super-insulating the HWC, valving and pipework, to minimise standing loss. There’s no need for the whole system to sit at 65C all day long when tap temperatures need only be 50C or less.

    In case of extra HW demand it’s easy to change settings or just push the manual boost. The keypad is in the kitchen, so very easy to do.

    I see this has turned into a rant about efficient waterheating systems! A small investment of about $500 in a decent controller and a bit of user education should have a quick payback. It has certainly given me a feeling of being in the driving seat of this major energy user.

    Best to all,   Fred


    Phil Squire

    Brilliant posts people. Just what has been running around in my mind too. With no security of supplying our energy retailers with green elec. using the energy at source is the bomb!

    I would love someone to model this – anyone got a nifty programme that would calculate rates of return/payback etc for a PV system tied to the grid.

    FYI http://www.pvcalc.org does a reasonable job of calculating the returns on varying PV systems, lifetime, cost of money, elec tarrifs etc. The main issue I have when comparing a PV systems financial returns when comparing to the” risk-free” (unless you’re in Cyprus) rate of return in a bank acct, is that depreciation rates are not straight line. You lose at least $3000 in the first year, since the install cost and removal cost are lost in asset if you need to move or sell the system. But for  a homeowner with a long tenure (over 15 years, it’s a money maker on a big scale (percentage wise).

    Interesting to see the Vector offer in Auckland. $2000 up front and $80/month for a Vector owned system 3kWp on your roof – also has a lithium ion battery that they can use for load control.

    Scott Willis

    Wow, so many good angles here! Easy one first: I hear the Vector offer has catch at the end of life ($1000) – anyone know the full details of Vector’s programme? As I understand it there are only a dozen or so households so far.

    Fred, I love your explanation of how you’ve become a very active ‘prosumer’ and I’m going to spend some time looking more closely at my own hot water system issues in light of your information with that fantastic detail.

    Yesterday in a meeting with Paul Encell of Solarcity I heard more about ambition in the solar engineering world to create smarter solar PV but lack details.

    The big issue is not the technology  though – all the solutions exist. Rather, its the simple package that is missing. Here in Blueskin we’re seeing a number of solar PV installations going in at present, all individual install arrangements so far, but a new standardised model being proposed and rolled out next week. Both are probably necessary to get broad acceptance and grow interest in the more complete, hands-on options.

    Norman Smith

    Presumably people have heard about this initiative  announced last September:
    Kiwibank, the Warehouse and SolarCity launch solar power push with Warehouse vouchers and Kiwibank ‘sustainable energy’ loans
    Kiwibank is launching “sustainable energy loans”- tied to home loans – to help people buy solar power systems provided by SolarCity from the Warehouse. In a press release today the three companies said they were aiming to help New Zealand families instal solar power for as little as NZ$2.50 a day.


    The initiative, dubbed “the Warehouse Solar Roof Shout” is aiming to instal 27,000 systems on rooftops to generate up to 65 million kilowatt hours of renewable energy annually, which the companies say is the equivalent of powering more than eight thousand homes. They say this would provide up to 30% of each homeowner’s energy use at a price below what they are currently paying for power.



    SolarCity solar power systems will be showcased in six Warehouse stores every weekend, starting today, reaching 42 stores across New Zealand over the next year, and offering a NZ$250 voucher for every system sold.


    Meanwhile, Kiwibank’s “sustainable energy loan” will see the bank contribute up to NZ$2,000 towards the cost of financing each system installed. The companies said the Kiwibank loan would allow eligible customers to pay for their system as part of a Kiwibank home loan, with Kiwibank “kicking in” NZ$2,000 over four years.


    “The sustainable energy loan can be included when you move your current home loan to Kiwibank, as a top-up on an existing Kiwibank home loan or on a new Kiwibank home loan. Kiwibank will contribute NZ$2,000 (over four years) towards the cost of the system and if you’re already a Kiwibank home loan customer, Kiwibank will waive the top-up fee.”

    etc etc : For more see https://practitionershub.org.nz/forums/topic/using-pv-systems-to-heat-hot-water/#post-1226



    Scott Willis

    Hi Norman,

    Yes. These are financing options, to facilitate the uptake of sustainable energy options. The Kiwibank offer is available for “solar hot water, solar photovoltaic, wind, hydroelectric and geothermal. Domestic only”. “Kiwibank will contribute a maximum of $2000 over four years ($800 at the end of year one and $400 at the end of years two, three and four)”. “For more information, please contact Peter Lowe on 04 460 5338.

    Prometheus offers “Eco-purchase loans for home owners” for “insulation, wood burning stoves, wetbacks, solar installations and double glazing”. The interest rate is fixed and is higher than the current Kiwibank loan, but it is for the product only and is not tacked onto an existing mortgage, so it may end up costing less, depending on the size of the mortgage.

    Good that you have given these financing offers some profile, as uptake has been relatively low.

    Eion Scott

    Great to read your post Fred, have been talking with Vector about it and they appear to be telling me they won’t allow people to go off ripple control to allow setting it up for daytime heating. This would give more options for people who don’t want to sell excess home generation to the grid. In my case, Mercury only offers 3.5c per kWh, which makes an investment in PV marginal.

    So I’m looking at the VectorSolar programme which has the big benefit of being able to store up to 4 kWh overnight in a lithium battery/inverter cabinet which can also be islanded (i.e. used in the event of a grid outage). I’ve used the PVCalc tool that Phil pointed to (great tip Phil) and it appears to be an excellent investment, even with my east/west roof (Vector claim it should operate at about 88% of a northern elevation, and the 3kWp panels will produce 10.7 kWh a day on average). (I can’t seem to attach the details to this, but will see if I can put it under resources). Yes you don’t get to keep the system at the end of the 12.5 year contract but there is no decommission fee (that applies to the 6 year contract).
    Yes you don’t get to keep the system at the end of the 12.5 year contract but there is no decommission fee (that applies to the 6 year contract). I’ve done a similar calc on the return if I bought the equipment myself (assuming I could buy it for $10,500, which is the total cost of the lease ($70 a month) over 12.5 years – probably unlikely) and I think the lease arrangement is better, despite the lower return in the long run, as it’s going to save a lot of design uncertainty, will generate profits from day one, and we don’t need to worry if the house is sold or the panels or system need maintenance/upgrading/replacement.

    If you want to know more about the Vector Solar pilot http://vector.co.nz/solar/service-offering-and-pricing





    Hi Richard P

    I’m interested to know if you ever found the answers to your original question (back in March).



    Richard Popenhagen

    In a nutshell yes, all the subsequent posts have been brilliant.

    This how I would do it if starting from scratch:

    ·         Use a large (depending on household size) 270 or 300 litre dual bottom element/ mid element hot water cylinder (with thermostat control on both elements) & two bottom entry wetback ports (explained below).

    ·         Connect the PV supply to the bottom element, with timer control so that this element can only operate during the day when the PV system is producing, initially say from 10.30am to 3.30pm (the duration of this period can be fined tuned to fit higher or lower demands). You could also install a relay so that if the PV system stopped producing (due to cloud cover), it also cuts out this element to prevent it “sucking” power from the grid supply. So essentially this element would only work when the sun is shining and the water temperature in the bottom of the HWC is below the thermostat set point.

    The bottom thermostat will cut off the power supply once the water temperature reaches the set point, so if it only needs two hours of input, that is all it will get, to avoid the HWC overheating. This will then free up further day time power generation from the PV system for other uses in the home.

    ·         Connect the mid element & mid thermostat to the standard ripple control (or night rate) electricity supply. This will boost the top half of the cylinder only, up to 60oC, at night time using the “cheaper” ripple controlled electricity, if there have been some inclement days and the water in the top half of the cylinder drops to below 60oC. This also satisfies the Legionella protection requirements.

    Normally this element would never work, only in the event of bad weather or high water draw off. This is similar to how you should configure a solar water heating system, but using PV instead.

    ·         Ideally I would also have a wood-burner with a wetback also connected to the HWC (for South Island and cooler N.I. locations), which would help boost the hot water temperature in winter when the PV system might struggle to provide enough power.

    On that note it is very important to use wetback connections fitted to the bottom of the HWC, not side connections (as are common on solar ready cylinders). This is to prevent reverse circulation occurring when the fire is not going. With side entry connections, the temperature differential that occurs between the side entry ports causes reverse circulation to occur. Side entry connected solar systems get around this problem by including non-return valves. That is fine for a pumped solar system, but with natural thermo-siphon wetback systems the pressure differentials are too small to overcome the resistance in most non-return valves. Yes you could fit a pump to the wetback circuit, but I would be trying to keep the system as simple and robust as possible by avoiding pumps and valves, which all have a finite life and will eventually fail.

    The other point with wet-backs is that the base of the hot water cylinder must be a minimum of 300mm above the level of the top of the wetback, also to prevent reverse circulation occurring. So where you position the hot water cylinder is important in the planning stages.

    If anyone wants further information on the intricacies of thermo-siphon systems and problems that can occur, I can bore you to tears on that subject.

     So for a small extra investment in the right sort of hot water cylinder with the correct connections and ports/ pockets would allow a simple robust system to be installed. The only other bits (apart from the PV system and the wood burner) are a simple timer, a relay, and setting it up correctly.

    Obviously there are other ways of doing it and I welcome others thoughts and opinions on alternatives, after all it is an ever changing field as we know.

    I am a great believer in keeping things as simple as possible, and to avoid unnecessary mechanical devices which add complication and generally have a relatively short life before they need servicing or replacement.



    Wow, that’s a fantastic summary of where you’ve got to Richard! Thanks so much for writing it up and sharing it. I hope others can put all that experience to good use too.





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