Solar hot water system, help choosing right approach

[Sverige]

Hi all,

I see a few threads in this section which cover this subject, but nothing which seems fully relevant to my circumstances. I am trying to research what might be the best approach to make use of solar to heat water for showering during the 9 months or so each year when we have enough sunshine here in Sweden.

I think I can rule out options which use PV panels and electrical heating of the water. There are 2 reasons, first my available space for PV panels is already consumed and the output of those panels doesn’t give me the kind of excess which water heating would need. Second, my existing hot water installation is a ground source heatpump system which I can’t run on my solar electricity as it needs a 3 phase supply and my inverter is single phase and paying for a big expensive 3 phase inverter is off the table.

So.. I’m exploring solar thermal systems and I see it’s normal to have some kind of collector panel with an antifreeze fluid then to make use of a heat exchanger to warm water in a collector tank. Well my ground source heat pump already has a hot water tank conveniently plumbed into all the taps and the shower in the house, so really it’s the water in that tank which I would like to heat up.

I can think of two different ways to do this, and I don’t know what’s best. First, I could use the warm antifreeze fluid from a solar collector to warm the brine which comes out of the ground, so my heatpump is then using that much warmer input fluid to heat my hot water and in doing so, will use much less electricity (but still some). Is this feasible? I don’t know - I’d better try to find out what brine temperature range my heatpump can operate with, as it normally only receives fluid in the range 0-8C which comes from the borehole in my garden.

Option 2 is to apply a heat exchanger to the output side of the heatpump system and with the use of a circulating pump to slowly extract energy from the solar collector circuit and warm up the water in the water tank itself. I like this option as the big energy thief heat pump can remain switched off all summer and I only have to electrically power a small circulating pump, which my existing PV panel installation would cope with.

Whichever option I choose, I would need to rig some kind of heat exchanger transfer the heat from the solar collector circuit into either the brine (ethanol) circuit or the hot water pipes. This might just be running the pipes from the solar thermal circuit directly alongside the other pipes, as in both cases I have about 5m of copper pipe length exposed within my garage ceiling area. Does anyone have experience or info on such a DIY heat exchanger where pipes are just laid alongside each other?

Edited to add concept sketch :

 

[Acuario]

I guess your existing tank has an internal heat exchanger and the heat pump.
One way would be to use this single heat exchanger for both the heat pump and solar water heating.
Adding a mixer valve on one side and a common exit on the other you could then control the mixing valve depending on the source of the heat.
I use this in my greenhouse with a gravity feed system but it could be used on pressurized as well.
I guess in Sweden it gets pretty cold so you'll need an anti-freezing program for the panel unless you use a drain-back system.

Solar water works very well but it is an accumulation system rather than instant heat so circulating the heating medium slowly will work but not as well as you might expect. The delta can go as low as 3 degrees between the tank and panel. At lower temperatures many controllers switch to pwm mode to control the pump, as my pump doesn't accept that I switch it on/off at a slower frequency to attempt to extract maximum heat - it seems to work well.

 

[Sverige]

I guess your existing tank has an internal heat exchanger and the heat pump.
One way would be to use this single heat exchanger for both the heat pump and solar water heating.
Adding a mixer valve on one side and a common exit on the other you could then control the mixing valve depending on the source of the heat.
I use this in my greenhouse with a gravity feed system but it could be used on pressurized as well.
I guess in Sweden it gets pretty cold so you'll need an anti-freezing program for the panel unless you use a drain-back system.

Solar water works very well but it is an accumulation system rather than instant heat so circulating the heating medium slowly will work but not as well as you might expect. The delta can go as low as 3 degrees between the tank and panel. At lower temperatures many controllers switch to pwm mode to control the pump, as my pump doesn't accept that I switch it on/off at a slower frequency to attempt to extract maximum heat - it seems to work well.

View attachment 126534

Thanks for your reply and sketch. In fact my hot water tank is built into the heatpump unit, so all I have access to are the pipes which come out from that unit. The brine circuit pipes which carry ethanol and go off to the 176m deep borehole in the garden and the hot water pipes which go from the heatpump in the garage to the house.

Please excuse my terrible sketch and bad handwriting but I’ve had a go at drawing it. My idea is to use a cheap / easy diy heat exchanger of some kind to get the heat into the hot water pipes. Could I maybe just use garden hosepipe for the solar collector circuit and lay that hose pipe alongside the 5m of exposed pipework in the garage? I haven’t added pumps to my drawing, but both hot water and solar collector circuits would need some circulation pumps.

Any tips on whether this might work and how to make a suitable easy inexpensive heat exchanger would be very welcome. Also, since the attic space in the garage reaches 50C in the summer, I wonder whether I can avoid having to make a solar collector at all, but simply run my hosepipe through the garage attic and pick up enough heat that way, without an externally mounted evacuated pipe collector at all? I can live with my hot water only being heated up to about 40C, if that might be achievable?

 

[Hedges]

Second, my existing hot water installation is a ground source heatpump system which I can’t run on my solar electricity as it needs a 3 phase supply and my inverter is single phase and paying for a big expensive 3 phase inverter is off the table.

Is your HPWH inverter-drive? Or just a normal 3-phase motor?

If inverter drive it may run just fine on single phase. Capacitors would ripple more fed rectified single phase rather than 3 phase.

If 3-phase motor, consider an inexpensive 3-phase motor drive (but maybe only to feed compressor itself, not electronics.)

I use a 208/240V version of this Hitachi VFD for my pool pump:

Hitachi AC Drive SJ200-015HFU - Hitachi AC Drives / VFD Drives - Hitachi AC Drive - Hitachi VFD | Owned and Operated by AutomatedPT

Series: SJ200 2 Horsepower(HP) 1.5 Kilowatt (KW) 460 Volts 3.8 Amps 3 Phase 3.74 pounds This product is no longer manufactured or sold. The Hitachi recommended replacement is the WJ200-015HF drive. Please check the dimensions and specifications on the replacement drive page or contact us at...

hitachiacdrive.com

So.. I’m exploring solar thermal systems and I see it’s normal to have some kind of collector panel with an antifreeze fluid then to make use of a heat exchanger to warm water in a collector tank. Well my ground source heat pump already has a hot water tank conveniently plumbed into all the taps and the shower in the house, so really it’s the water in that tank which I would like to heat up.

Consider evacuated tube collectors. I haven't used them but they seem nice. Would extend usable season.
The tubes themselves are freeze protected. Manifold you could use a glycol loop, or drain back. Or mount such that it is in a conditioned space? (propylene glycol is non-toxic so not as critical to have redundant isolation. Could be tube in tube. If ethylene glycol something like two separate copper tubes twisted together would provide double isolation.)

Evacuated tube solar collector, Vacuum tube solar collector

Solar thermal collectors, Evacuated tube solar collector usually consist of a heat pipe inside a vacuum-sealed tube, Solar energy is collected by evacuated tube

www.onosisolar.com

California once did a study comparing cost of solar-thermal to electric (resistance) and gas, for hot water at roadside rest areas. They found solar thermal saved money compared to electric, but not compared to gas. Energy costs and alternative energy equipment costs are changing. We think the claimed efficiency of HPWH and PV can produce hot water with same collector area as solar-thermal. HPWH costs are high but PV is cheap, and solar-thermal isn't particularly inexpensive. A single-phase (or inverter drive) HPWH as addition to your system may be a way to go. Solar thermal pre-heat may be useful.

Here is a small, maybe moderately expensive 3-phase inverter for residential use. But I think compatible high voltage batteries are expensive:

Sunny Tripower Smart Energy | SMA Solar

www.sma.de

https://files.sma.de/downloads/STPSE-Batteries-TI-en-10.pdf

SMA Sunny Tripower 10.0 Smart Energy

SMA Sunny Tripower 10.0 Smart Energy STP10.0-3SE-40 Three phase hybrid inverter for homeowners The Sunny Tripower Smart Energy hybrid inverter is the two-in-one system for supplying solar power at home. With this, SMA has combined smart technology and integrated services to create a space-saving...

www.solartopstore.com

https://www.solartopstore.com/collections/byd-battery/products/byd-battery-box-premium-hvs-12-8 (Ouch!)

There are small 3-phase Chinese made inverters which work with 48V batteries.


Heat recovery? Where do you dump your hot water when you are through using it?
If significant energy is going down the drain after showering or using dishwasher, how about a reverse-flow heat exchanger to preheat incoming cold water?
I once saw a heat exchanger consisting of copper tube wrapped around tall vertical drain pipe, to be mounted under shower. Theory was that water going down the drain clings to surface of pipe, leaving air space in the middle. So fairly effective heat transfer.

 

[Acuario]

I don't think you will get enough heat transfer just by running the pipes in parallel. I think the delta would drop off too quickly so you would get an initial amount of hot water then it would go cold.
With solar water you really do need to accumulate in a tank for it to be of any real use.
With the system you have I would probably be inclined to put a buffer tank in the system with the heat pump as one source for heating and the solar water panel as another. There are tanks that have multiple coils so you could set up with a coil for the heat pump and another for the solar panel. Check out oeg.net they have loads of different tanks.
Also, ecorenovator.org has a forum where loads of people (including me) play with different designs for heating/cooling etc.
I'm sure I remember someone talking about using the heat from their attic.

 

[Shimmy]

@Hedges beat me to everything I thought of. Evacuated tubes are likely the only viable approach. You could run a pre-heat coil in the garage ceiling, but it might not be enough.

Having a solar hot water system (in a tropical climate though), I would tend to say the ground source heat pump is a better solution, and making it work with your PV is a better approach. You might even want to consider adding a mini-split air conditioner to super-heat the borehole in the summer, if you feel a need for cooling.

 

[PreppenWolf]

Our grandparents in Connecticut have evacuated tube solar hot water. 10 months of year they make all their hit water this way and heat their pool in summer. It offsets a decent portion of their electricity use even in the coldest two months. I'm planning on doing a system in the future.

 

[Acuario]

I have a flat panel system in North East Spain that I've had running for the last 17 years. It's a single 2x1.5 metre panel flat on the roof (at about 15 degrees angle) and pointing south. It heats a 160 litre tank. We have free hot water for at least 10 months a year.

I also have a 4 panel drain back (soon to be 8) connected into my heating system for the underfloor heating.

Hot water systems are definitely the most efficient but they are falling out of favour due to the falling price of solar electric.
If you have excess capacity, and a way to detect and control its use, then diverting the extra power to a resistive heating element or to a heat pump is a good alternative and (certainly with resistive heating) can be substantially cheaper to set up.

 

[Sverige]

Is your HPWH inverter-drive? Or just a normal 3-phase motor?

If inverter drive it may run just fine on single phase. Capacitors would ripple more fed rectified single phase rather than 3 phase.

If 3-phase motor, consider an inexpensive 3-phase motor drive (but maybe only to feed compressor itself, not electronics.)

I use a 208/240V version of this Hitachi VFD for my pool pump:

Hitachi AC Drive SJ200-015HFU - Hitachi AC Drives / VFD Drives - Hitachi AC Drive - Hitachi VFD | Owned and Operated by AutomatedPT

Series: SJ200 2 Horsepower(HP) 1.5 Kilowatt (KW) 460 Volts 3.8 Amps 3 Phase 3.74 pounds This product is no longer manufactured or sold. The Hitachi recommended replacement is the WJ200-015HF drive. Please check the dimensions and specifications on the replacement drive page or contact us at...

hitachiacdrive.com

Consider evacuated tube collectors. I haven't used them but they seem nice. Would extend usable season.
The tubes themselves are freeze protected. Manifold you could use a glycol loop, or drain back. Or mount such that it is in a conditioned space? (propylene glycol is non-toxic so not as critical to have redundant isolation. Could be tube in tube. If ethylene glycol something like two separate copper tubes twisted together would provide double isolation.)

Evacuated tube solar collector, Vacuum tube solar collector

Solar thermal collectors, Evacuated tube solar collector usually consist of a heat pipe inside a vacuum-sealed tube, Solar energy is collected by evacuated tube

www.onosisolar.com

California once did a study comparing cost of solar-thermal to electric (resistance) and gas, for hot water at roadside rest areas. They found solar thermal saved money compared to electric, but not compared to gas. Energy costs and alternative energy equipment costs are changing. We think the claimed efficiency of HPWH and PV can produce hot water with same collector area as solar-thermal. HPWH costs are high but PV is cheap, and solar-thermal isn't particularly inexpensive. A single-phase (or inverter drive) HPWH as addition to your system may be a way to go. Solar thermal pre-heat may be useful.

Here is a small, maybe moderately expensive 3-phase inverter for residential use. But I think compatible high voltage batteries are expensive:

Sunny Tripower Smart Energy | SMA Solar

www.sma.de

https://files.sma.de/downloads/STPSE-Batteries-TI-en-10.pdf

SMA Sunny Tripower 10.0 Smart Energy

SMA Sunny Tripower 10.0 Smart Energy STP10.0-3SE-40 Three phase hybrid inverter for homeowners The Sunny Tripower Smart Energy hybrid inverter is the two-in-one system for supplying solar power at home. With this, SMA has combined smart technology and integrated services to create a space-saving...

www.solartopstore.com

https://www.solartopstore.com/collections/byd-battery/products/byd-battery-box-premium-hvs-12-8 (Ouch!)

There are small 3-phase Chinese made inverters which work with 48V batteries.


Heat recovery? Where do you dump your hot water when you are through using it?
If significant energy is going down the drain after showering or using dishwasher, how about a reverse-flow heat exchanger to preheat incoming cold water?
I once saw a heat exchanger consisting of copper tube wrapped around tall vertical drain pipe, to be mounted under shower. Theory was that water going down the drain clings to surface of pipe, leaving air space in the middle. So fairly effective heat transfer.

Thanks for your ideas. This is my GSHP, it is an inverter driven model. I have tried to run it on a single phase, just by switching off all three phases then powering just one at a time. With phase 1 live, the heatpump will power up and be contactable via network, so my control app can display data from it, but it shows an error message “inverter not contactable“, which tells me the inverter needs a second phase simultaneously powered. With only phase 2 or phase 3 powered, nothing works. With the unit having cost around €15,000 to get installed (incl. drilling the borehole), I’m not brave enough to try powering it from a pair of non-synchronised single phase inverters, for fear of blowing the thing up!


Because this unit provides hot water in an energy efficient way, my conundrum is that if I am to put something in place to replace it, in search of lower energy bills, I cannot afford to spend much in doing so as my energy consumption on hot water is only around 40-50kWh /month. Options such as a new three phase inverter and replacing my current 4S LiFePO4 battery bank with higher voltage ones are simply not going to be on the table, and even evacuated tubes, nice though they might be, are probably too expensive for me. Hence I was considering a simple system with heat recovery in some form from garage roof and a DIY built heat exchanger to get that heat progressively into the hot water tank within my Thermia Diplomat heat pump via accumulation (by running a low power circulator pump, which my PV panels can readily accommodate).

Heat recovery from shower drain is not likely to be economical either. There’s just myself and my wife taking a 5 min shower every couple of days, and the hot water source is 25m away from the house in the garage.

 

[Sverige]

I don't think you will get enough heat transfer just by running the pipes in parallel. I think the delta would drop off too quickly so you would get an initial amount of hot water then it would go cold.
With solar water you really do need to accumulate in a tank for it to be of any real use.
With the system you have I would probably be inclined to put a buffer tank in the system with the heat pump as one source for heating and the solar water panel as another. There are tanks that have multiple coils so you could set up with a coil for the heat pump and another for the solar panel. Check out oeg.net they have loads of different tanks.
Also, ecorenovator.org has a forum where loads of people (including me) play with different designs for heating/cooling etc.
I'm sure I remember someone talking about using the heat from their attic.

Thank you - I don’t think I made it clear via my sketch, but my idea is not to rely on instantaneous heat transfer but rather to accumulate the solar heat into the tank which is integral in the heat pump unit within the garage. The water pipes to the house already have a U bend connection at the manifold within the house and a circulating pump which brings hot water to the house, so running this pump should allow the water which transits through the garage space to get circulated all the way back to the water tank.

If I can then find a cheap way to add some solar heat at an appropriate temperature during this circulating route, it will accumulat and raise the tank temperature. If this takes hours, so be it, the energy involved all comes for free if the circulating pump is run from my current PV panels.

So the two nuts I need to crack are how to pick up the heat from the garage roof or attic space and how to get it into the hot water pipes via a DIY heat exchanger which doesn’t cost me much to put in place. As I commented to @Hedges , my current spend on hot water is low, so I cannot spend much on trying to convert to solar, nice though it would be to lower my energy consumption.

ps.. although I’m quite literate in electrical matters, plumbing is a mysterious and scary world to me, so part of my desire to rig a heat exchanger which relies on pipes laid alongside one another is that it will avoid the need to cut into or drain my current system!!

 

[Sverige]

@Hedges beat me to everything I thought of. Evacuated tubes are likely the only viable approach. You could run a pre-heat coil in the garage ceiling, but it might not be enough.

Having a solar hot water system (in a tropical climate though), I would tend to say the ground source heat pump is a better solution, and making it work with your PV is a better approach. You might even want to consider adding a mini-split air conditioner to super-heat the borehole in the summer, if you feel a need for cooling.

Are there very cheap DIY designs for home made evacuated tubes, or are they always an item which must be bought?

I‘d love to be able to run my heatpump on my PV solar, but I can’t currently think of a way. I have 375Ah of 4S LiFePO4 and two cheap High frequency inverters (one “3000W”, one “6000W - both Chinese so in practice only half those ratings). Unless there’s a way I can synchronise their outputs so they run as two phases of a 3 phase supply, I don’t think I can power my heatpump. While running a hot water cycle, the heatpump will begin by using around 2kW and culminate at around 2.4kW 25 mins later. My battery bank could sustain that power draw, especially if I do it while it’s sunny, so my PV panels provide half the power, but my inverter solution is not suited to running a 3 phase high power device and I don’t know of an affordable solution to this.

My Chinese inverters both use a small control board which I assume generates the sine wave, and I day dream about fiddling with them to get the sine waves syncronised and 120 degrees apart from one another, but it’s beyond me and could blow up my very very expensive heatpump, so that shall remain just a day dream!

 

[Acuario]

Ok, so you have a recirculating system so there is movement of water - this helps.

So, here is an idea, buy a roll of uninsulated soft copper pipe and roll it around one of the hot water pipes - thinner pipe will be easier to manipulate. The return pipe would be better as it is cooler due to losses. Insulate it when it's done.
Buy a car radiator with an electric fan from a scrap yard and mount it in the roof space. Connect the ends of the coil to the top/bottom of the radiator with a circulating pump - if you can get a 12V pump even better.. Fill with antifreeze.
You'll need a 12V power source and a differential controller to switch the pump/fan on/off when there is more heat in the roof than in the exit from the pipe.

Cheap and it should work while there is heat in the roof space.

 

[Sverige]

Ok, so you have a recirculating system so there is movement of water - this helps.

So, here is an idea, buy a roll of uninsulated soft copper pipe and roll it around one of the hot water pipes - thinner pipe will be easier to manipulate. The return pipe would be better as it is cooler due to losses. Insulate it when it's done.
Buy a car radiator with an electric fan from a scrap yard and mount it in the roof space. Connect the ends of the coil to the top/bottom of the radiator with a circulating pump - if you can get a 12V pump even better.. Fill with antifreeze.
You'll need a 12V power source and a differential controller to switch the pump/fan on/off when there is more heat in the roof than in the exit from the pipe.

Cheap and it should work while there is heat in the roof space.

Thanks - this is very much within my budget level and possibly within my capabilities too ? It will be spring before I get around to trying any of this, but I’ll give it thought and will no doubt try a few things out before i get a working solution, such is the nature of this kind of homemade stuff.

Thanks also for the link you mentioned before to ecorenovator.org, I’ll have a look

 

[Acuario]

Good luck, for a differential controller you will find several Arduino and ESP projects that are easy to make.

 

[Sverige]

Good luck, for a differential controller you will find several Arduino and ESP projects that are easy to make.

Thanks, I saw one already on ecorenovator. I also have a few ideas of how I might be able to do that with some parts I have already, which might not be the most efficient solution but if it costs nothing because I’m using components I already have, then it could be the best approach for me.

I’ll keep an eye on local Facebook groups to see if I can score a car radiator and water pump for giveaway prices since I have a few months before I’ll need to try any of this.

 

[Hedges]

If you have two independent inverters not designed to be synchronized, don't even try. If they are designed to take a cable for stacking them in series/parallel/3-phase then you can.

I was looking to see if it is power factor corrected, or not. Don't see it mentioned.
The following two specs are probably for different models.

"Electrical data 3-N Mains power supply Volt 400"
"Electrical data 1-N Mains power supply Volt 230"

http://www.tcmadmin.thermia.se/docroot/dokumentbank/Thermia_Inverter_data_ENG_13_01_2017.pdf

Since you said you tried just one line connected, I assume it is wired with L1, L2, L3, N, G.
Just one line would be L1,N for 230V.
Try L1, L2. Those would be 400V apart and could power the inverter & motor, while L1, N would give 230V for control electronics.
Do that with your grid connection, because unsynchronized inverters would deliver varying voltage up to 460V and might hurt each other.

My VFD takes in 120/240V split phase (or could have been 3-phase if I had it) and rectifies to a DC bus. No power factor correction so just gulps current whenever line voltage is above DC bus. It drives variable speed 3-phase to the motor. Normal 3-phase is three sine waves, but this is PWM into each wire to get the current desired. I haven't looked with a scope; if into an inductor I think that would smooth out to sine wave, but motor load may filter it differently, or not at all.

Because it is VFD, should ramp up slowly so inverter doesn't have to supply a starting surge.
If it can run off two phases of the grid, maybe it can run from the "6000W" inverter, given a transformer to make 170V (of opposite phase) from 230V so it sees 400V between phases. And 230V from one phase to neutral. Or isolation transformer to make 200/400V split-phase.

Power factor correction comes in various kinds. Some synthesize sine wave current from input with a boost converter to charge HV caps. Some are a 6-transistor H-bridge from input to output, and can send power back to grid (e.g. elevator going down.) Some are rectifier/capacitor front ends but inject cancelling harmonics.

Older Dell computers worked with MSW UPS, while newer ones require sine wave.
We have some SMPS with PFC at work, and found they run just fine with a DC bus powering them.

No guarantees!

 

[Hedges]

Evacuated tubes have thin film coatings, and vacuum of course. Not a DIY fabrication project (for most of us.)
Some have a heat pipe making contact with inner wall of the thermos tube. Or, a working fluid can be used (must avoid freezing)
The heat pipes have much less fluid volume than empty volume, so no harm from freezing. They stagnate around 240F (above 100C), so stop transferring heat. Of course, at that point water in manifold has either been driven out as steam or is around 20 psi like a car radiator.

They are cheap enough to buy. I see ads that include a manifold and a few more tubes than it needs as spares.
Some ads in my area. Used to see other models. Think many are made in China.

solar water evacuated tube heater system with 80 gallon Rheem solar and electric | eBay

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10 Evacuated Solar Vacuum Tubes 1800mm 6 ft 72" DHW for Water Heater Collectors | eBay

And most importantly: Evacuated tubes don't FREEZE or need antifreeze. They're great quality with a heavy solid conductor inside. The efficiency is incredible, as compared to PV solar. This is the large 6 foot size, usually for the big 30 tube collectors.

www.ebay.com

Case Of 10 Evacuated Solar Vacuum Tube 1800mm 6 ft 72" DHW | eBay

Find many great new & used options and get the best deals for Case Of 10 Evacuated Solar Vacuum Tube 1800mm 6 ft 72" DHW at the best online prices at eBay! Free shipping for many products!

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[Don B. Cilly]

I use an on-demand butane heater. I keep it outside the house. It works really well.
For solar, at the local hardware store:

-

 

[sparrowhawk]

The way I see it is very simple. I know you say that you don't have any room to add anymore panels to generate enough power to heat water. So I have to ask. Do you use every watt of solar power you generate everyday?
If you have any excess solar why can't it be used to heat water? You want to add a solar water heater collector, so why can't that space be used for more photovoltaic panels? Does your existing water heater tank have an electric backup heating element in it?
There are things that can be done with any excess power not being used. Some people use some sort of system that diverts excess power into a heating element in the water heater tank. Myself I use a timer that turns the water heater on at around noon time when the sun is highest and let my inverter power the water heater. I'm able to use my inverter because I replaced the original heating elements with lower wattage ones that allow water heating along with battery charging and powering of other loads simultaneously. The system has worked great for over 2 years now.

 

[Hedges]

"Auxiliary heater, 3 steps kW 0/3/6/9"

Using the backup heating elements of the HPWH as dump load would be a simple approach. Just need to have suitable thermostat connected (and a signal when battery is full.)

If 3kW is too big a dump load, step-down transformer from 230V to 115V would make it 750W.