Water heater modification to use solar and electricity

Had an idea.  Is it possible to modify a standard 40-50 gallon electric water heater to also run on solar?  Currently it has two heat elements.  Can you disconnect one and leave the other one to be controlled by the water heater brain.  Then replace the other one with a DC heat element (if there is such a thing???) that runs off a dedicated solar panel.  

Then I could turn off the breaker for the water heater in sunny seasons.  The sun would heat the tank up (I hope) and if company comes and we need more hot water, turn the breaker back on.

Has anyone done something like this?  Do they make DC replacement heat elements?  Can a water heater work with one element hooked up?  Should the upper one be on solar?  Would one "standard" panel do enough or would I need a bunch of them?  

Thanks!

Hi Mike.  I'v done this with a 300 watt dc element hooked straight to a solar panel.  I got it from eBay. It heats up when the sun is shining and you have the old backup 240v ac for when the sun don't shine. We also took an extra tank and put it in front of the normal propane water heater.  Preheated water saves lots of gas.

You will definitely need some control circuitry - not just a direct hook up. You don't want to inadvertently turn your tank into a bomb by overheating it.

Our system is currently heated using 110AC. We only turn it on when the sun is out. And i only have one element hooked up. After about 4 hours using about 4KWH the water is nice and hot.
How does the 300Watt dc work for you Christopher?



Our system is also set up to get heat from our wood stove. so in the summer its electric and winter its wood stove. I much prefer our waste solar to heat the water.


So i am wondering which system you could set up to be equivalent to 4 hours of VAC.

Maybe there are higher voltage systems?

I wonder what would happen if you used say 800 watts of panels and put it into the element. Lets say they are 80VDC and put out 10amp. Could the wiring hold? Would the element work?
I imagine using 12Vdc would not work great. Plus you need larger cable.
With it being 80VDC i think you can get away with smaller cable. I also assume a 120VDC unit would work with DC however iam not 100% on this.

If you could mount the panels close to the heater would be best practise.

Yes you can.  It is a common thing as a dump load for after the batteries are charged.

It is more efficient to use a heat pump water heater, but they are over a grand where dc heat element is under a hundred

Cool, so it is a "thing"!  

Ok, I could put a panel (or 3?) about 30' from the water heater.  Is that close enough?  

Next question...  What kind of panel should I be looking for?

I see DC water heater elements that are 12v, 24v and 48v.  I'm assuming that would match the panel(s)?  If the panels put our more wattage than the heaters can deliver, is that a problem?  I'm assuming so...

I'm hoping that by having just one of the two heater elements on solar, and 40 gallons of water to store the heat, that I should be able to avoid explosions by monitoring it on sunny days to see how hot the tank gets.  They do make thermostats that you can put in line but I'm not sure if they'd need the hole that the other element is installed in.

Mike Haasl wrote:Cool, so it is a "thing"!  

Ok, I could put a panel (or 3?) about 30' from the water heater.  Is that close enough?  

Next question...  What kind of panel should I be looking for?

I see DC water heater elements that are 12v, 24v and 48v.  I'm assuming that would match the panel(s)?  If the panels put our more wattage than the heaters can deliver, is that a problem?  I'm assuming so...

I'm hoping that by having just one of the two heater elements on solar, and 40 gallons of water to store the heat, that I should be able to avoid explosions by monitoring it on sunny days to see how hot the tank gets.  They do make thermostats that you can put in line but I'm not sure if they'd need the hole that the other element is installed in.

To me it would depend on what is available.

The panels i just got can produce 40VOC. so maybe the 48v.
I know there are panels which produce 49VOC. so one of those would work as well.


In my opinion you would want the panel as close as you can get it to the water heater.

I would try and match the panel to the heater element.
IE 450W panel i would get maybe a 600w element. I am not sure what is available though.

It seems like you have an answer to the question you asked. Could I pose another question? What about, instead of solar electric, building a solar hot water heater that preheated the water for the electric heater tank? You wouldn't need to modify the tank itself, just add the solar hot water heater on the in side of the tank. If its sunny, the water is already hot, and the tank doesn't turn on. If its night time or cloudy, then the water coming in is normal temp and the electric kicks in.

EDIT - Now that I think of it, you would probably need a bigger tank, but I'd still like to throw it out as an idea, since we can go solar - hot water, instead of solar - electric - hot water.

PV panels are SO much cheaper than they used to be it's usually cheaper to do solar electric hot water these days.

Make sure you get the system HOT every couple days to avoid Listeria.  It can be a nasty problem in poorly designed systems.

The problem with heating a tank prior to the water heater is that, at least in my case, we don't use hardly any hot water.  A shower or the dishwasher might consume 5 gallons.  So most of the time the tank is using electricity to maintain it's temp  and fight off the cold basement.  Yes, cold water coming in is also a load for the tank but that is pretty rare in our household (once a day?).   For heavier users of hot water, yes that would definitely work.

I think the area I'm stuck on now is the actual hardware.  If I can get a couple panels, how do I wire them up (series/parallel), how do I figure the wire size, fuses go where, etc.  And is one or two panels enough to do anything?

Even if the solar just helps and I leave the other element running to heat the tank up to 140, I'd be happy.  So I'd rather undersize the solar than risk overheating the tank.

Alternately, if I somehow had a DC thermostat somehow rigged up to keep the tank from overheating, can it just cut off juice from the panels or would the panels blow up if they can't let out the electricity?

I am also using a DC hot water system. I have a 20 gallon tank with a single DC element. I think I have only seen them go up to 600 Watt versions for my 24v system voltage. I have batteries that buffer my voltage, but you will burn them out if you feed them too much current, so under-sizing the array a bit is a good idea.

As for the thermostat, here is what I do. I feed 24v dc from my batteries through the AC thermostat and use it to control the coil on a relay. Since the coil only draws milliamps, you can safely use the AC breaker. The relay is rated for 40Amps DC so once the thermostat opens, it breaks the DC current at the relay. If you have an outlet available, you could just run the relay off a wall-wart that put out 12v or whatever. Panels will not explode if they are disconnected under load. They will however try and maintain current flow, so whatever interrupts them better be designed for DC. A DC arc is basically what powers an arc-welder - it gets very hot, and will sustain an arc a lot farther than a similar AC current would.

If you are going to endeavor to fiddle with a hot water system I have 2 words of warning. 1) make very certain that a hot water relief valve is installed and functioning properly, and 2) seriously consider installing an anti-scald valve on the outlet of the water heater. When I was first tinkering with my system I did come back to find 185 degree water sputtering out of my overflow pipe once. I have a mixing valve, so there would not have been a risk of scalds, but the relief valve should be the very last line of defense. If it had not worked right, I might have had a much scarier story to tell.

I was also a little paranoid about stuff growing in my tank, but my system is so small that it gets flushed out pretty fast. It either gets hot, or it stays pretty cool - rarely does it ever simply get warm and stay that way. I think having an AC element going in there would likely make that a non-issue, but there are a lot of factors. Incoming water temp is one, and I recall reading that those bacteria are simply more common in places with warmer water. If it is a problem in your area, its worth thinking about.

Anyway, seems like a doable retrofit. Let us know how it goes.

Mike Haasl wrote:Had an idea.  Is it possible to modify a standard 40-50 gallon electric water heater to also run on solar?  Currently it has two heat elements.  Can you disconnect one and leave the other one to be controlled by the water heater brain.  Then replace the other one with a DC heat element (if there is such a thing???) that runs off a dedicated solar panel.  

Then I could turn off the breaker for the water heater in sunny seasons.  The sun would heat the tank up (I hope) and if company comes and we need more hot water, turn the breaker back on.

Has anyone done something like this?  Do they make DC replacement heat elements?  Can a water heater work with one element hooked up?  Should the upper one be on solar?  Would one "standard" panel do enough or would I need a bunch of them?  

Thanks!

Mike, this is both something old and something new. Old school off grid DC water heating elements were common as wind turbine dump loads. The wind controller would flip a relay once the batteries were charged and then make hot water. Solar panels were expensive so we didn't think that way. Do you already have solar? I ask because there is a way to not use batteries or controllers if you just want to heat water with panels. You would need to install a 36 volt heating element like this one
https://www.hurricanewindpower.com/dc-water-heater-element-36-volt-1200-watt-submersible-diversion-heater/#aoh=16434922781315&referrer=https%3A%2F%2Fwww.google.com&_tf=From%20%251%24s&share=https%3A%2F%2Fwww.hurricanewindpower.com%2Fdc-water-heater-element-36-volt-1200-watt-submersible-diversion-heater%2F
Use the existing thermostat on your water heater to control a relay like this one
https://www.amazon.com/SSR-100DD-Solid-State-Relay-Module/dp/B07PFDJQLV/ref=mp_s_a_1_9?crid=2JQJK71C3I6RM&keywords=dc+solid+state+relay&qid=1643493270&sprefix=dc+aolid+%2Caps%2C118&sr=8-9
Using your existing thermostat keeps the water from getting too hot but it cannot control DC amperage it would arc too much so you connect a small trigger DC voltage to it like a 12 volt wall plug in and run that to the relay and hook up the solar to the relays output. The listed element and relay would be good for up to roughly 1kw of 60 cell panels hooked up in parallel. The panels when hooked to a load run between 28 and 33 volts.
How you run it is your choice some people replace the lower element in their hot water heater with the DC element some hook up a separate small tank.
I prefer replacing an element so you always have the ac upper element to automatically take over...
Cheers,. David

Thanks David!  I don't have solar currently so no batteries to worry about.  I'm a little confused about the thermostat...  I do want to keep one of the two AC heater elements and the thermostat it runs off of.  Is there a way to use that same thermostat to control the DC for the solar?  Or are you talking about a second thermostat?

To really simplify things, I'm fine with not having a thermostat and seeing how hot it gets on a full sun day.  I'm assuming with one or two panels I won't get over 150F.  If I'm off base with that and it would get hotter, then I'd be fine going with one panel.  I'd rather underheat the water and use electricity to finish the job, than make the system too complex for my simple mechanical brain.

What about going at it another way.  Add a large naturally vented(no explosion risk) hot tank(insulated box lined with EDPM) next to the water heater.(the bigger the box the better, the deeper the box the better)  Use solar thermal water panels to heat it it. 60% to 80% efficient instead of 20% so smaller panel area needed for the same heat gain.  The tank and the panels you can build.  Single low wattage DC pump to run it.   If you build the panels for drain back nothing but water so no expensive fluid.  You heat the insulated box of water with the collector and run your cold water  in to hot water tank inlet line thru it so the pressurized drinking water is always completely separate.  Add a conditioning value to the heater outlet so you can get the water heater tank hotter than scalding some of the time.  Apparently flat plate collectors top out at about 140 degrees so it is self regulating.

Mike Haasl wrote:Had an idea.  Is it possible to modify a standard 40-50 gallon electric water heater to also run on solar?  Currently it has two heat elements.  Can you disconnect one and leave the other one to be controlled by the water heater brain.  Then replace the other one with a DC heat element (if there is such a thing???) that runs off a dedicated solar panel.  

Then I could turn off the breaker for the water heater in sunny seasons.  The sun would heat the tank up (I hope) and if company comes and we need more hot water, turn the breaker back on.

Has anyone done something like this?  Do they make DC replacement heat elements?  Can a water heater work with one element hooked up?  Should the upper one be on solar?  Would one "standard" panel do enough or would I need a bunch of them?  

Thanks!

The panels have the heating properties, like from 0 degrees to 20 (centigrade) in the details, but I'd use a 12V heating element, and an electric thermostat (very cheap) that cuts the pwer when you get to the temperature you want. If you have enough sun in the day, and bury your tank in some sort of isolation, or the ground and cover it with an isolated panel it should keep hot or warm through the night. You'd only need one solar PV panel, a small one, and, on a farm you must have a battery or two lying around. You need a charge controller (ebay) and a 12V regulator, idem. I don't know if you have water pressure or not... without it you need a 12v pump too. If you want I'll do you a plan if you cotaact me through FB.

Mike Haasl wrote:Thanks David!  I don't have solar currently so no batteries to worry about.  I'm a little confused about the thermostat...  I do want to keep one of the two AC heater elements and the thermostat it runs off of.  Is there a way to use that same thermostat to control the DC for the solar?  Or are you talking about a second thermostat?

To really simplify things, I'm fine with not having a thermostat and seeing how hot it gets on a full sun day.  I'm assuming with one or two panels I won't get over 150F.  If I'm off base with that and it would get hotter, then I'd be fine going with one panel.  I'd rather underheat the water and use electricity to finish the job, than make the system too complex for my simple mechanical brain.

a dual element hotwater tank will have an upper and a lower thermostat. Disconnect ac power to the lower thermostat and use it as your safety overheating protection. Always better to have a mechanical safety backing up human memory whenever possible.
Cheers, let me know if you go for it!
David

It is possible you may run into a small issue over time using only one element (at a time.) They are designed to do half the tank, if that makes sense, so they will be working harder and may wear out faster. You could lay in a supply of replacements... and if you figured out some preheating system, that could take some load off. I am reading this thread with interest for my own use, I want this idea to be a thing! Thank you all for sharing.

Yes of course you can use existing 2 element water heater. Super easy.  Solar direct is super way to go.  
Check out this link and video.

https://practicalpreppers.com/product/cyboenergy-off-grid-inverter/

No need to worry about DC elements.

Use the existing thermostat on your water heater to control a relay like this one
https://www.amazon.com/SSR-100DD-Solid-State-Relay-Module/dp/B07PFDJQLV/ref=mp_s_a_1_9?crid=2JQJK71C3I6RM&keywords=dc+solid+state+relay&qid=1643493270&sprefix=dc+aolid+%2Caps%2C118&sr=8-9

Have you used any of those, David? I ordered several different SSRs on amazon a couple years back, and they were all junk. Either they would not activate, or they had failed open, I do not recall. I wondered why the were like 1/10th the cost of ordering one from Grainger. The ones I paid 100 bucks for have been running flawlessly for years. And dont forget to put them on a heatsink. I imagine a high amperage contactor from a starter motor or something would also work.

Carl Nystrom wrote:

Use the existing thermostat on your water heater to control a relay like this one
https://www.amazon.com/SSR-100DD-Solid-State-Relay-Module/dp/B07PFDJQLV/ref=mp_s_a_1_9?crid=2JQJK71C3I6RM&keywords=dc+solid+state+relay&qid=1643493270&sprefix=dc+aolid+%2Caps%2C118&sr=8-9

Have you used any of those, David? I ordered several different SSRs on amazon a couple years back, and they were all junk. Either they would not activate, or they had failed open, I do not recall. I wondered why the were like 1/10th the cost of ordering one from Grainger. The ones I paid 100 bucks for have been running flawlessly for years. And dont forget to put them on a heatsink. I imagine a high amperage contactor from a starter motor or something would also work.

That particular one no. And yes one from Grainger would be better for sure. I have used relays from Amazon in the past with good results. In my haste I might have used a poor link. It can be a crap shoot.

I think all standard water heater elements are purely resistive (not inductive) so they will provide the same amount of heat with either AC or DC as long as the input voltage is the same. There are inductive heat elements for other purposes. THAT changes everything.

Ohm's law always applies. What this means is that the load (the heating element) will not draw any more current than it needs. Since the resistance is manufactured into the heating element & the solar panel voltage will be constant (when it's sunny) you would need to have enough panels wired correctly to provide the desired voltage & current the element needs to reach full heat. In the example below the element won't try to draw any more than 16.7 amps unless it is provided more than 120 volts.


https://www.homedepot.com/p/Rheem-PROTECH-120-Volt-2000-Watt-Copper-Heating-Element-for-Electric-Water-Heaters-SP10874GH/205650205

Ohm's law     voltage = current x resistance           E=IR     120 = 16.7 x R               7.18 ohms

A power formula    watts = amps x voltage             P=IE     2000=I x 120                 16.7 amps

Do not power an AC hot water heater with DC, as it will fuse the thermostat contacts. It is a fire hazard.

The contacts in an AC thermostat don't part wide enough to break DC, as it will arc, create heat and fuse the contacts shut.

Solutions:

1. Replace AC thermostat with DC thermostat (Limit: 50VDC): $12.83 @ https://windandsolar.com/dc-water-heating-elements/
2. MPPT controller: $299.95 + $18 S/H @ http://techluck.com/
3. DC -> AC inverter: $1697.00 @ https://greenetechenergy.com/product/sun-bandit-sb-1200-b-ppk-off-grid-solar-water-heater-inverter-system-1-4-pv-panels/

David Baillie wrote: a dual element hotwater tank will have an upper and a lower thermostat. Disconnect ac power to the lower thermostat and use it as your safety overheating protection. Always better to have a mechanical safety backing up human memory whenever possible.
Cheers, let me know if you go for it!
David

Ahh, thanks, I didn't realize there were two thermostats on a water heater.  Mind blown :)

Good point Thomas.

I was trying to answer Mike's specific question

I see DC water heater elements that are 12v, 24v and 48v.  I'm assuming that would match the panel(s)?  If the panels put our more wattage than the heaters can deliver, is that a problem?  I'm assuming so...

This is so interesting though I probably won't be doing it. For good info about solar and control panels, I really enjoy watching Will Prowse on YouTube. The kid is amazing and looks like he's 20 years old. Good luck!

You beat me to it Cindy! I was going to recommend Will, his videos are excellent and he explains things in layman’s terms.
To the subject at hand: electric water heaters with an upper and lower element have the 2 elements connected in such a way that they work in tandem to keep the entire tank hot. ( https://www.youtube.com/watch?v=_YZkgZVCZn0)
If you eliminate one or the other it would seem you may disrupt the proper operation. If you are going to use solar seasonally, but electric the rest of the year, it might make sense (if you have the space) to simply have a second tank, an indirect style like the ones used with boilers where the heating is done with a second water coil running through the water in the tank. Easy enough to separate the systems with a couple ball valves.

I’m late to the party here… but I have about 300’ of 3/4” poly pipe just sitting on my black shingled roof.  I have the water circulating when the sun is out with a pump in the basement by the hot water tank.  It does enough to get the water to 125f.  I would suggest not making electricity with the sun… just use the sun to heat water.  There are many ways to make a solar hot water heater.  Think about how hot a garden hose gets when it sits in the sun in the summer!   I am installing a commercially made hot water solar panel that was made by a now defunct company in Texas.   See image.  

I've got 1050 watts of solar going directly to a 120vac heat element at 128vdc..and it works good.. I didn't even have to replace the controls on the water heater.. So yes you can do that.. Just wire your panels for the same voltage as the element calls for and you have it whooped

I didn't even have to replace the controls on the water heater

How long have you been running this way? The heating elements don't care if it's AC or DC, but I'm thinking the contacts in the thermostat(s) will! DC tends to arc and burn things, thus heavier switches for DC devices.

Jerry Wood wrote:I've got 1050 watts of solar going directly to a 120vac heat element at 128vdc..and it works good.. I didn't even have to replace the controls on the water heater.. So yes you can do that.. Just wire your panels for the same voltage as the element calls for and you have it whooped

Jerry usually you install a relay to switch on the heating element. Maybe not yet but your thermostat will get all Carboned up from the DC arcing.

Hi. You will want to optimize it by matching the resistance with your solar panels. Here is a handy link.
https://welovetheearth.com/matching-water-heater-elements-to-solar-pv-panels/

Jaz

This is a high-tech solution to the integration problem:

http://paladin.co.nz/

We've got one and it keeps us from exporting power to the grid at least part of the time.

Hello brain trust, I'm back!!!

Ok, I got my hands on two used solar panels.  From the label on the back it says:
Rated power   Pmax     327 W
Voltage            Vmp       54.7 V
Current            Imp       5.98 A
 Voc   64.9 V
 Isc    6.46 A
Maximum series fuse  20 A

I think that means that if I put them in series they'd generate something under 109.4 volts and 5.98 amps and a total wattage of 654 W.  That's if they were new and these ain't.  
Or in parallel they'd make 54.7 V and 11.98 amps.

They have the normal looking solar connectors on them that I've seen in videos.  They're a black cylinder.  If there are multiple kinds, please let me know, otherwise I'm assuming they're mc4.

To run these in to my water heater I think I'll have a run of 54 feet (give or take).  I'm sure 49.9' would be much cheaper than 50.1'...

How fat a wire do I need for this?  Seems like ones on Amazon are all 10awg which seems pretty beefy.  Could I use UF-B (underground gray romex) for the majority of the run or do I need to do a straight shot with the solar wire?

Do I need a fuse or circuit breaker in this solar circuit?  If so, what and where?

The grid power goes into the top of the current water heater.  I suspect each leg of the 220 feeds one of the elements.  So I can just disconnect the lower one and put wire nuts on to disable it.  Then I run my fancy solar wires down to that spot on the water heater and.....

The current element says 240V and 4500W on it.  I'm pretty sure it's only getting 120V since there's only a red and black wire hooked up to it.  My suspicion is that I can use this element for my solar feed without any issue.  Or is the element too big for this little bit of juice and I should replace it with a DC element?

An unanswered question is the thermostat.  If I'm going to use it, I'd need to get a DC thermostat.  The current AC one slides into a clip system on my Richmond (Menards) water heater.  They seem to make 12, 24 and 48V thermostats.  But even in parallel these panels can do 54.7 V.  Is that a problem or is it one of those things where nominal and actual dimensions are different?

The DC thermostats seem to need to be bolted on to the tank.  I guess I'd have to fiddle around to make sure it can fit on my heater.  I'm trying not to modify it in case this whole pipe dream is a failure.

Thinking out loud about the thermostat here:
If I skip the thermostat, I'd be putting about 600 W of power into a 40 gallon tank of water.  These would be pointed south so I'm guessing they would be making peak power for 4 hours, decent power for 4 more and a trickle for 4 more.  Let's say 600W x 4 hours = 2.4kwh.  Double it to account for the rest of the day to make 4.8 kwh.  That's 16,400 BTU.  Divide by 40 gallons and 8 pounds per gallon gives 51 degrees of temperature rise.  

We use very little hot water each day.  Maybe 5-10 gallons.  Worst case (explosion wise) we're on vacation and it's sunny every day.  I'm not sure what the heat loss of that water heater tank is...  Seems like if it was at 140 and I turned the power off, that it wouldn't drop below 90 in 24 hours.  So 4.8 kwh of heat could, maybe, possibly get dangerously hot after a few days.  

Did I do that math right?  Am I greatly overestimating the max power I'll get from these panels?

Thanks for all the help with this!

I just collected a very interesting bit of data.  22 hours ago when I looked in the water heater, I turned off the breaker.  I forgot to turn it back on... I took my normal shower last night and we used no hot water since.  I just checked the hot water temp and it's 115F.  The heater is set to 125F.  So, somehow, I took a shower putting 5? gallons of cold water into the tank, didn't supply any heat, and it held the heat in well enough in my 55 degree basement that it's still at 115.  I'm quite amazed.  

I'll turn the breaker on now and see how hot the water is when it's in full operation.

I am now certain I'll be putting a thermostat in the system :)

I think I figured it out.  Last night the water was at 127F.  

I think the stratification in the tank was confusing me.  While the power was off, the water at the top of the tank was staying warm but I bet the water down at the bottom was getting much colder.  So on average there was more than a 12 degree cool-off in the tank.  I think I still need a thermostat though...

Zeek McGalla wrote:I’m late to the party here… but I have about 300’ of 3/4” poly pipe just sitting on my black shingled roof.  I have the water circulating when the sun is out with a pump in the basement by the hot water tank.  It does enough to get the water to 125f.  I would suggest not making electricity with the sun… just use the sun to heat water.  There are many ways to make a solar hot water heater.  Think about how hot a garden hose gets when it sits in the sun in the summer!   I am installing a commercially made hot water solar panel that was made by a now defunct company in Texas.   See image.  

That is certainly a more economical way to make hot water - unless you just happen to have a bunch of solar panels lying around. Correct me if I'm wrong (and i often am), but I had it in my head that using electricity to heat something is not efficient/economical. Or maybe it is just here in Canada that electric baseboard heat is way more expensive than natural gas. But at any rate, the talk of heating hot water made me think about RMH (I'm a little obsessed with them at the moment) and what if we could use hot water to create electricity?!  A RMH could be modified to create electricity. There is this camping stove that does just that! It also uses small twigs. The people at Wheaton Labs are tinkering with that concept this July!! At the Permaculture Technology Jamboree. See https://permies.com/s/permaculture-technology-jamboree  and there is a BOGO sale for that event happening NOW! See https://permies.com/s/ptj-bogo-sale

Well, the two solar panels cost me $100.  I have the electric water heater already.  Any sort of direct solar to water system (tubes, drain back, evaporated tubes) carries the risk of freezing in winter.  I have 7 months of freezing here.

Electricity heating water is nearly 100% efficient.  The solar panels aren't as efficient as a direct water heat system but that's a trade off I'm willing to make.  For  maybe $200 in parts I'll save $100-300 a year in electricity bills alone.  Yay!

Plus by doing it this way I'm making very minor changes to my house and the water heater.  I can go back with minimal issues.

A RMH making electricity would be very cool.  A RMH heating water would be a pain in my butt that I wouldn't even consider for indoor/house plumbing.  How to keep it from going boom-squish?  How to keep it from freezing and going split-leak?  Why build a separate fire when the sun is there much of the time?  Many issues that wouldn't work for my particular set of circumstances...

Mike Haasl wrote:Hello brain trust, I'm back!!! Ok, I got my hands on two used solar panels.  From the label on the back it says:
Rated power   Pmax     327 W
Voltage            Vmp       54.7 V Current            Imp       5.98 A
 Voc   64.9 V  Isc    6.46 A
Maximum series fuse  20 A
I think that means that if I put them in series they'd generate something under 109.4 volts and 5.98 amps and a total wattage of 654 W.  That's if they were new and these ain't.  Or in parallel they'd make 54.7 V and 11.98 amps. They have the normal looking solar connectors on them that I've seen in videos.  They're a black cylinder.  If there are multiple kinds, please let me know, otherwise I'm assuming they're mc4.

So when I look at those stats I see grid tie panels all over. I would dislike using those panels because of the high voltage for the VOC alone. Since panels are rated at 77*F(25*C) The VOC will be much higher when cold. You can always plug the panel numbers into this string tool to find out what the VOC will be in the winter time. https://www.midnitesolar.com/sizingTool/ Chances are it will be quite significant.

Mike Haasl wrote:
To run these in to my water heater I think I'll have a run of 54 feet (give or take).  I'm sure 49.9' would be much cheaper than 50.1'...
How fat a wire do I need for this?  Seems like ones on Amazon are all 10awg which seems pretty beefy.  Could I use UF-B (underground gray romex) for the majority of the run or do I need to do a straight shot with the solar wire?

I always find https://www.calculator.net/voltage-drop-calculator.html to be helpful. 50 feet is not very far. Try typing in 12awg wire.
I believe solar panel wire is rated to be in the sun and to withstand 90*C. So its designed for it. I believe you can use what you have. The solar panel wire I used was easy to work with, it bends easily and didn't seem to have a "memory" Unlike some coils of wire.

Mike Haasl wrote:
Do I need a fuse or circuit breaker in this solar circuit?  If so, what and where?

I would put the circuit breaker near your water heater

Mike Haasl wrote:
The grid power goes into the top of the current water heater.  I suspect each leg of the 220 feeds one of the elements.  So I can just disconnect the lower one and put wire nuts on to disable it.  Then I run my fancy solar wires down to that spot on the water heater and.....The current element says 240V and 4500W on it.  I'm pretty sure it's only getting 120V since there's only a red and black wire hooked up to it.  My suspicion is that I can use this element for my solar feed without any issue.  Or is the element too big for this little bit of juice and I should replace it with a DC element?
An unanswered question is the thermostat.  If I'm going to use it, I'd need to get a DC thermostat.  The current AC one slides into a clip system on my Richmond (Menards) water heater.  They seem to make 12, 24 and 48V thermostats.  But even in parallel these panels can do 54.7 V.  Is that a problem or is it one of those things where nominal and actual dimensions are different? The DC thermostats seem to need to be bolted on to the tank.  I guess I'd have to fiddle around to make sure it can fit on my heater.  I'm trying not to modify it in case this whole pipe dream is a failure.  

Sorry mike I don't know the answer to these questions. I do know that those panels will put out more than 54.7 VDC  when its -30 out.

Mike Haasl wrote:
Thinking out loud about the thermostat here:
If I skip the thermostat, I'd be putting about 600 W of power into a 40 gallon tank of water.  These would be pointed south so I'm guessing they would be making peak power for 4 hours, decent power for 4 more and a trickle for 4 more.  Let's say 600W x 4 hours = 2.4kwh.  Double it to account for the rest of the day to make 4.8 kwh.  That's 16,400 BTU.  Divide by 40 gallons and 8 pounds per gallon gives 51 degrees of temperature rise.  

We use very little hot water each day.  Maybe 5-10 gallons.  Worst case (explosion wise) we're on vacation and it's sunny every day.  I'm not sure what the heat loss of that water heater tank is...  Seems like if it was at 140 and I turned the power off, that it wouldn't drop below 90 in 24 hours.  So 4.8 kwh of heat could, maybe, possibly get dangerously hot after a few days.  

Did I do that math right?  Am I greatly overestimating the max power I'll get from these panels?

Thanks for all the help with this!

I am not quite sure on all the math and whatnot.
One thing you could do when you go away(vacation) is to turn off the solar panels with a circuit breaker.


Personally mike, unless you want to make a small solar system. I would suggest getting lower voltage(VOC) panels to comply with the DC thermostat. I would run the lower voltage panels in parallel.

Thanks Jordan, I plugged it into the MidNite Solar site and it gives a VOC at -30C of 76.7V.

That sucks, too bad I bought those, I hope my buddy knew what he was getting (he bought a bunch).

I'll have to dig into the thermostat issue some more to see if there's a higher voltage option out there.....