I have a 28gal, single element, 120v, under counter water heater out in the "auxiliary" kitchen on the back of the garage where we do our meat cutting. Normally, I don't keep the water heater on except when we're gonna be working up a pig or cow or whatever for a time. I'll flip it on then to have hot water for washing up and all. Heater is about 10 years old now. I have twenty 175w panels I put up on my solar power system back in 2007, and took down last year to replace with lot higher watt/efficient panels, so they are sitting unused. I bought a 48v 800w element with an integral thermostat from Missouri Wind/Solar, and wanted to see how it would do. My thinking is I would replace the water heater in the Aux kitchen with a dual element, stick the DC element in one hole, and kept the AC element in the other hole to run when we need a lot of hot water. The DC element would heat the tank all the time, keep it it at whatever I set the thermostat at, so if we want to use just a little water (wash dog dishes, for example, or just wash hands out there), it would be there at no electrical cost, then flip on the AC element just like now when we're gonna use the room heavy for a few days.....and not be dependent on the sun on panels to heat enough water for that use. I assume the DC element (800w) wouldn't keep up with our water use at those times (AC element is 1500 watts, plus connected to enough power to run it) SO, I tried to get the info out of Ms Wind/Solar about HOW many panels can I connect. Final answer: they don't know.....ahahahaaa. The panels are 35.5v, 4.95 amp max output. Ms W/S said "max 56v, 14amps, so that means two panels in parallel". That sounded kinda low to me. But I did a test run yesterday with just that....two panels in parallel (two in series would get up up to 70ish votes, exceeding their 56v max....so I get that). Measuring at the element, I get 35v, and only 1.5 amp draw. Huh ?. The element barely warms, I could stick my hand down in the test bucket I set up and hold it on the element. Not much going on there. So I connected 4 of them in parallel. Still 35v, but now the amps doubled. Element was heating enough I could not keep my hand on it (real scientific test huh ), and the amp draw doubled to 3amps....still not a whole lot, but definitely cooking compared to two panels. I left it an hour, panels in full sun, and the approximately 3 gallons of water in the test bucket heated from 62 degrees starting temp to 125 degrees ending temp (what I set the dial on the thermostat) when I came back to check. Still got some more testing to do and combinations to try. The test bucket has a 120v 1500w element and 240v 2000w element that came out of AC heaters (got a neighbor that works at local American Water heater plant and can get me all the elements/etc like that I need), and I'm curious to see what happens when I connect the panels straight to them (no thermostat). Panels are temporarily clamped to a hunk of Unistrut and leaned against the shop wall for this test. Your thoughts, suggestions, comments welcomed. andy
Sounds like a standard high voltage 10 ohm heating element. If it really was 48v 800w it would be like 3 ohms.
Measuring the ohms: 48v 800w element measures 3.8 ohms and what I had from various tanks/etc: 120v 1500w element measures 9.2 ohms 120v 2000w element measures 7.1 ohms 240v 3000w element measures 17.5 ohms 240v 4500w element measures 12.6 ohms
you need 5 or 6 of those panels in straight parallel , fuse each with a 6 gang glass fuse in 15 amp and then use the heavyset wire to the heating DC element , Use a 40 amp relay to control power(so now you need a small 12Vdc panel as well to control the relay. I have a DC amp gauge in series to see the load / draw [ Mechanical ] ) I use the standard DHW thermostat to control the DC relay . Wind sun stuff can't handle the amps in DC , fire is about to happen . Sloth I did mine 2 years ago and dropped the collectors and the hassle
Well, I ended up mounting 4 panels in parallel for this particular experiment. Got the tank plumbed and wired in yesterday morning, connected power around noon. Water inlet temp as I filled the tank (28gal) was 67 degrees F. Element was drawing 11.4 amps of the potentially 20 amps those four panels are able to produce. We had fair sun, but the atmosphere here is sorta hazy due to smoke from western forest fires....so we're not getting good bright sun. Roof where the panels are mounted faces southeast, so best sun on them is 9am to around 1pm, and then it goes away fairly fast......so I didn't get much production yesterday. Water got to around 83 peak yesterday afternoon. I checked the tank this morning, it was reading 76 degrees, so it cooled off 7 degrees overnight. Tank has foam insulation. Checking it just now at 11:00am, it reads 106, drawing 10.1 amps. Still hazy sun conditions. Panels on the roof in parallel come into a Midnite combiner box with a 10amp breaker on each input. I'll be adding a 40amp breaker and cleaning up the wiring install a bunch once the experiment is over. Element is a 48v 700w one from Missouri Wind/Solar with built in thermostat, adjustable from off to 190. Next element I'm going to try with the tank going in at the house is a 36v 1200 w element from Amazon. I plan to use at least 6 panels with it, and maybe 8.
The LED thermometer is also from Amazon. $14.50 https://www.amazon.com/gp/product/B07X37WH5Z/?tag=survivalmonke-20 Comes with a K type probe for external use, but I bought another probe that screws directly in a 1/2 pipe thread I put in the line out from the top of the tank. $13.99 Twidec/2M NPT 1/2"inch (6X50MM) Pipe Thread Temperature Sensor Probe Two Wire Temperature Controller (0~600℃) 304 Stainless Steel K Type Thermocouple MT-205-1/2: Amazon.com: Industrial & Scientific I used a old 12vDC 'wall wart' transformer I had laying around to power the thermostat. Just cut the original plug off and wired to the two power leads that come on the thermostat. I have a 120v receptacle handy for it. IF you wanted total off grid, you could simply use a small 12v solar panel to give you a tiny amount of power needed to run it....a 10-20w panel would be plenty.
So yesterday, it climbed to 128 degrees max. Held to 118 degrees over night in the tank, and today has climbed to 145 degrees max as of around 2:30pm, which is a little past peak solar time on the panels. And today the sun has been in/out with clouds. Plenty good enough for a little hand washing water in the shop bathroom, so I'm pleased. Heck, 145 and you can scald chickens ! Experiment # 2 is a new, same gal size (28gal) heater I'll be installing in the auxiliary kitchen off the back of our garage where we do meat cutting/etc. This is where I removed the tank for the shop bathroom experiment above. The new tank was a dual element 240v model in which I installed a single 120v element with thermostat in the lower hole so it can be used on demand (flipping the breaker on to it) the same way we used the previous one....the day we plan to use the room, I flip the breaker on so we have plenty of hot water to clean up/etc, then cut it back off so it don't sit there drawing power when we really don't need it much. The 36v 1220w DC element, I'm sticking in the upper hole. Purpose is so that anytime we venture out there to wash a dog dish, or just need a small amount of hot water, it will be available. Since this element does not have a built in thermostat like the one in the shop, I'll be using the set up recommended by Cruisin Sloth, which is to use the original thermostat that came with the heater. You can't connect (as I understand it) DC directly to the thermostat, as the contacts in it won't take it for very long. To solve this, I'll be running low voltage/milliamp DC from another wall wart thru the thermostat contacts to a 120amp (overkill, but it was cheap) relay. The solar infeed will go to the relay, then on to the DC element with a 40amp breaker between the two. Relay from Amazon: Amazon.com Best place I have to mount the panels is on my garage roof, which is west facing, 42 degree slope (10/12). I'm thinking I'll put 6 panels on this array as I've got 14 of them in storage. 6 x 175w = 1050w. Sun available this time of year from around 11am-noon to sunset.
IMO you would b much frther ahead doing thermal solar, Either a glass covered tank or a pipe array that is covered in glass both are more efficient in making hot water from the sun than electricity. I have done both.
I've done both too with this PV set up. Here's my greenhouse setup for hot water in the floor of the house. I'll take maybe a bit less efficiency for the lack of plumbing, messing with glycol in the solar loop, boiling it out if the pumps or controller quits, the pumps, the controller, expansion tanks, pressure relief valves, and all that other crap that AIN'T in the PV setup. PV, run wires to breaker, connect to element with built in thermostat. Done. Dead simple, whole lot less to fail. No plumbing.....GOD I hate plumbing..... And the setup on my garage roof would involve some extensive pipe runs to get from the collectors to the tank which is close to 40' away....run thru garage attic, attic of kitchen added on the back, then down to the tank under the counter. Running wire is gonna be bad enough. This is the roof of the auxiliary kitchen on the back of the garage while being constructed. Not impossible to run plumbing from the front of the garage roof, but certainly not easy. Did I mention how I hate to plumb ? Then after going thru 2 attics, I'd have to get the pipe down the far right corner to where the heater is located under the counter. Doing it while under construction wouldn't have been to bad I guess, but that ship sailed 15 years ago. My plan now is to run some flex conduit with a couple #8 wires up the inside corner of that left wall cabinet, then on up into the attic of the kitchen, then on into the attic of the garage where the combiner box will be located for the roof panels. Still a bit of work, but not running 2 pipes, and insulating them well enough in unheated attic spaces or back to the glycol thing. Nope....it's PV electric for me !
Just for reference, I have a solar collector in Central Florida with an old galvanized 80-gallon water tank in it. The kind used on water wells before bladder tanks got popular. It faces south and is about 4' X 6' in area. It preheats the water that goes into my electric water heater. For the last ten years or so it's been saving me about $100.00 a month in electricity. Call it a $12,000.00 return on less than $100.00 invested. It was mostly built from scrounged materials. Worth doing.
Control box setup in the auxiliary kitchen. Solar will run (when I get the roof panels mounted) the lower element, the upper element I can flip on with the toggle switch to run 120vAC if the solar isn't keeping up enough (which it probably won't) when we're using the kitchen heavily. Got the wire from the tank thermostat disconnected right now since no solar infeed, and the relay would stay engaged all the time. Feed for the 120v AC element runs thru the receptacle box, thru the toggle switch and down to the element. Pilot light tapped in to remind me the 120 is on. LED temperature read out is from sensor mounted in the hot water line just outside the tank.
Wiring cleaned up and finished in shop bath. Added 40amp breaker under the combiner box, mounted the LED thermostat on the wall. Foamed in around tank where old thermostat used to be located.
Control box in auxiliary kitchen modified to add main breaker. 6 panels now on the roof (6 x 175w 35v 5amp). Today was overcast all afternoon and still managed a 22 degree rise from 73 to 95 degree. Should cook on a good afternoon. This project done.
Update. The 6 panel deal is working like a charm. Have barely had to cut in the 120v element while using the kitchen this past week.....plenty of 140 degree water.
