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Old 19-08-2015, 14:22   #1
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12 volt hot water

So . i have my 825 watt solar power system up and running. It easily makesmore power than I can use. so I was thinking, maybe I can put a 12 volt heating element in my hot water heater ,my Outback controller has an axillary Dump who has done this and does it work?
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Old 19-08-2015, 14:51   #2
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Re: 12 volt hot water

Just because I am an engineer....
When the 825 Watt is the listed power of your solar panel you should consider yourself lucky if you can get 30-40% of that. Similar to 35 Miles per gallon listed on your car sticker.
So lets say you are in Latitude 0 during the equinox, 12:00 AM on a crystal open sky day. Your solar panel is perfectly flat, aligned with the horizon and are no waves. You will be able to get 200-300 watt from that panel. Please deduct the efficiency of your resistance heater and the wiring transmission losses and you will get 50-100 Watt in the water. Now you have to deduct the heat losses on the tank insulation and the result will be similar to the temperature increase in New York when you lit a candle in Boston.
Please buy a "solar shower bag" and don't drop the soap.
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Old 19-08-2015, 15:18   #3
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Re: 12 volt hot water

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Originally Posted by GJori View Post
Just because I am an engineer....
When the 825 Watt is the listed power of your solar panel you should consider yourself lucky if you can get 30-40% of that.
I get full power rating from my 290W panel lots of times.

But that's nothing to do with the question: Yes you can use the Aux relay output on the Outback controller to supply power to a relay to connect a DC heater element in you water heater (if there is one that fits it) to the DC system. You would program the Aux relay to only do this when the DC volt has reached absorption voltage meaning that you aren't using the full solar power anymore to charge.
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Old 19-08-2015, 15:38   #4
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Re: 12 volt hot water

My 8.45A rated solar panels have on occasions put out 9A each. (With MPPT) Seems that quality PVs are rated conservatively. Yes, there are losses in wiring, but it may be argued that electric water heater is 100% efficient, since electric power goes in, heat goes out but no other energy of different form does. Of course, the heat that goes out through the insulation is lost to you, so well insulated water heater tank will require less energy to keep the water at set temperature.
Some people (particularly of the LiFePO4 crowd) did or tried to divert excess charging capacity (solar or wind) into their water heater when batteries are full. It gets a little complicated though, the system would need to deal with number of possible situations, like batteries full/need charging or thermostat on /off when the water at/below the set temperature. And with LiFePO4 batteries, reliability is a must.
Some major brands of marine water heaters offer replacement 12V elements. Maybe you could put it on timer? The one I have doesn't, so I have considered setting up 400W inverter, 2:1 step down autotransformer that would feed 60V~ to the standard heater element. That would reduce the output of my 700W water heater element to roughly quarter, say175W, or approximately 13A draw from a 14V solar outup. Just about right, no?
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Old 19-08-2015, 16:16   #5
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Re: 12 volt hot water

It's not uncommon at all for RVs. You won't get lots of hot water, but hey, it's free except all the parts.

My solar panels often put out ~100% of their rating. The 200w on the boat will put out nearly 200w for 2-4 hours per day on a sunny day. The 480w I have on my RV comes pretty close too. Nowhere near 30% of capacity till the sun is pretty low. (now a 30% _per day_ is being discussed, that's likely true)
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Old 19-08-2015, 16:18   #6
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Re: 12 volt hot water

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Originally Posted by GJori View Post
Just because I am an engineer....
So lets say you are in Latitude 0 during the equinox, 12:00 AM on a crystal open sky day. Your solar panel is perfectly flat, aligned with the horizon and are no waves. You will be able to get 200-300 watt from that panel.
Wow! That's amazing! I wouldn't have thought moonlight could give that much power. Just think how much power that panel will put out at noon!
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Old 19-08-2015, 17:15   #7
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Re: 12 volt hot water

It all depends too on how much time you are willing to wait for your hot water and how well your hot water tank is isolated, etc.

Anyways go to you nearest caravan chandlery and investigate.

You can build a homebrew 12V water heater from a Waeco water kettle too.

Or skip one electric solar panel and substitute it with a water heating panel.

b.
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Old 19-08-2015, 19:44   #8
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Re: 12 volt hot water

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Originally Posted by barnakiel View Post
It all depends too on how much time you are willing to wait for your hot water and how well your hot water tank is isolated, etc.

Anyways go to you nearest caravan chandlery and investigate.

You can build a homebrew 12V water heater from a Waeco water kettle too.

Or skip one electric solar panel and substitute it with a water heating panel.

b.
You may be on to something with that! I wonder if there are units small enough to work on a boat... most I have seen are too large.

But man, does that ever make sense.
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Old 19-08-2015, 19:50   #9
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Re: 12 volt hot water

An extra 200W for an hour (so 200Wh) will heat 30 liters of water about 5 degrees C (10F). So, if you have that kind of extra power you can see what you'll get out of it. It works, but without a lot of extra power the best you'll get is tepid water.

Solar thermal water heating panels can be from 40-60% efficient (compared with 16-22% for most PV panels), so you'll get much more heating bang for your space. Of course, have to balance that with the electrical power required to pump water through the solar thermal system.
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Old 19-08-2015, 20:54   #10
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Re: 12 volt hot water

I occasionally plug the water heater into the AC socket to heat it from the batteries/solar ( so does Nigel Calder )
1500W waterheater, 1600W inverter, chews about 50Ah
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Old 19-08-2015, 21:57   #11
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Re: 12 volt hot water

Here ya go and yes it will work
http://www.amazon.com/Watt-Submersib...heater+element
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Old 19-08-2015, 23:06   #12
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Re: 12 volt hot water

We run our water heater off our solar. We have 800W of panels. On a typical day our LiFePO4 5KWh battery gets above 90% by 10am. I run the water heater off the inverter in 15 minute bursts, running it down to 85% or so... then let it charge back up, then bump it again... keeps the water warm.

Probably won't do that long term, want to come up with a more energy efficient way to do it. Our water heater isn't connected to the engine, so I'm thinking about running a small pump to a black hose connected to the engine heating circuit and then around the deck and see if that works... but that's low on my priority list.
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Old 20-08-2015, 04:46   #13
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Re: 12 volt hot water

Yes you can use the Aux on the FM to drive a SSR (solid state relay) for water heating etc,
There are 12v ,1" elements available on ebay, This is an old very long post about it OutBack Power Technologies User Forum • View topic - How do I use every last electron? may be worth an new question, probably some simple answers save you digging through,

Tim
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Old 20-08-2015, 08:16   #14
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Re: 12 volt hot water

Autumn breeze, another option I've considered is to change the 1200W heater element to a 600W element. This would allow the heater to use half the Amps for twice as long. Benefit being not such a fast battery draw and likely that the whole draw could be offset by solar without any assistance from the batteries. It's not a priority on our list as we don't need hot water in the tropics. A timer switch would be a good way to activate it to make sure it doesn't remain on overnight or when not needed.
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Old 20-08-2015, 09:19   #15
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Re: 12 volt hot water

I have solar panels, wind generator and a loop from my engine to the water heater. My blue sky system has the dump option for overcharging protection. Now granted I and the Boat live in South Florida so we have SUN and sea breeze most of the time. After doing a lot of research and getting way to much info from "engineers" I asked a old timer electrician what is the difference between 12v and 120 heating elements. After he quit laughing, "price". He said just disconnect the 110v wires from your element and hook up your 12v dump wires to element. Guess what, I have water that is warm enough to take a shower wash dishes and what ever..
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