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Old 20-08-2015, 09:45   #16
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Re: 12 volt hot water

I was going to bring up a 120V AC element will heat water if it's wired to 12V DC, but I think it's heat output and current draw will be greatly diminished.
In other words a 1000W 120V element won't draw 1000W at 12V, or at least I don't think it will.
For example a European 220V coffee pot will work in the US on 110, but it's slow, doesn't provide nearly as much heat
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Old 20-08-2015, 11:24   #17
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Re: 12 volt hot water

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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?
I went through a similar thought process earlier this year. After giving consideration to all the possible options I decided to leave the AC heating element (800w, 230v) in place and simply use an inverter. Just need to keep an eye on the battery charge state and remember to switch it off!
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Old 20-08-2015, 11:42   #18
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Re: 12 volt hot water

A 1500 watt 120 volt heating element will have aproximately a 24 watt output at 12 volts IIRC. So yes a 120 volt element will work. However it would be worth the cost to purchase a dedicated 12 volt element of say 300 to 400 watt output.
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Old 20-08-2015, 11:49   #19
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Re: 12 volt hot water

Your not going to heat much at 24 watts, if that's all it is, then it ain't gonna work
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Old 20-08-2015, 14:49   #20
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Re: 12 volt hot water

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Originally Posted by newhaul View Post
A 1500 watt 120 volt heating element will have aproximately a 24 watt output at 12 volts IIRC. So yes a 120 volt element will work. However it would be worth the cost to purchase a dedicated 12 volt element of say 300 to 400 watt output.
It would seem logical to me that 12V is a tenth of 120V, so a tenth of 1500 watts would be 150 watts output. Could you explain where 24 watts was derived from?
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Old 20-08-2015, 14:52   #21
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Re: 12 volt hot water

of course if you have a lot of solar, once the batteries are charged you could just turn on your inverter and heat the water
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Old 20-08-2015, 15:31   #22
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Re: 12 volt hot water

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It would seem logical to me that 12V is a tenth of 120V, so a tenth of 1500 watts would be 150 watts output. Could you explain where 24 watts was derived from?
One would think that but actually doesn't work that way.you have to factor the resistance omms into the equasion
12 VOLT BATTERY SYSTEM DIVIDE BY 64 * (2000W@120V= 31W@15V)
24 VOLT BATTERY SYSTEM DIVIDE BY 16** (2000W@120V= 125W@30V)
48 VOLT BATTERY SYSTEM DIVIDE BY 4 * * (2000W@120V=500W@60V)
Here is the site with all the actual math and the explanations

http://solarhomestead.com/ac-elements-as-dump-loads/
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Old 20-08-2015, 15:42   #23
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Re: 12 volt hot water

Why not just fit a small solar water heater like the Heliatos made for RV and marine market . Has a 12 v circulating pump that's runs of a small solar panel . Have seen a review of this in the Ozzie Cruising Helmsman magazine , looks good .
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Old 20-08-2015, 15:45   #24
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Re: 12 volt hot water

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Why not just fit a small solar water heater like the Heliatos made for RV and marine market . Has a 12 v circulating pump that's runs of a small solar panel . Have seen a review of this in the Ozzie Cruising Helmsman magazine , looks good .
That would be a nice addition to a larger vessel however it would not do what the op wanted. He wants to use an electric water heating element as a dump load for his solar after batteries are fully charged.( making power so may as well use it)
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Old 20-08-2015, 16:02   #25
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Re: 12 volt hot water

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A 1500 watt 120 volt heating element will have aproximately a 24 watt output at 12 volts IIRC. So yes a 120 volt element will work. However it would be worth the cost to purchase a dedicated 12 volt element of say 300 to 400 watt output.
We need to find the resistance of the element @ 120 volts because that's the actual rating. Take the actual wattage of 1500 and divide by the amperage squared will give us the resistance. The amperage is wattage (1500) divided by voltage (120) equals 12.5 amps. Now take the wattage (1500) and divide by amperage squared (12.5 X 12.5)=156.25. 1500 divided by 156.25 equals the resistance 9.6 ohms. Now take the 12 volts and square it to equal 144 and divide by resistance of 9.6 ohms to get the wattage 15 watts.
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Old 20-08-2015, 16:15   #26
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Re: 12 volt hot water

OK then but any way you slice it spend the funds for a dedicated 12 volt element with a 300 or 400 watt 12v rating
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Old 20-08-2015, 16:21   #27
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Re: 12 volt hot water

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Originally Posted by EllisElectric View Post
We need to find the resistance of the element @ 120 volts because that's the actual rating. Take the actual wattage of 1500 and divide by the amperage squared will give us the resistance. The amperage is wattage (1500) divided by voltage (120) equals 12.5 amps. Now take the wattage (1500) and divide by amperage squared (12.5 X 12.5)=156.25. 1500 divided by 156.25 equals the resistance 9.6 ohms. Now take the 12 volts and square it to equal 144 and divide by resistance of 9.6 ohms to get the wattage 15 watts.
Figure at the actual output of a solar panel at battery float voltage of 14.5volts d you get about 22 watts output and at the top end of 15 volts from the panels you are at the 24 watts output point
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Old 20-08-2015, 20:09   #28
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Re: 12 volt hot water

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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)
Yes, it is done on RVs, but it is very uncommon. The way I know it is done on RVs is that I believe you when you say you did it. But you have the ONLY RV that I know of that has 12v water heating. I have never met an RVer in any of the RV campsites or National Park campsites or National Forest campsites or state park campsites that didn't have LP hot water. I have a sailboat that has 420 rated watts of solar and I get hot water on the boat from either a byproduct of running the engine or plugging into 110V AC shore power. You are the only person that I know in the RV community or the sailing community that uses 12V to heat water. Even among the power boaters that I know I've not heard one that uses 12V to heat water. 110V AC yes. 220V AC yes. Engine cooling yes. 12V DC no.
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Old 20-08-2015, 20:38   #29
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Re: 12 volt hot water

Yes the outback will do it and yes you have enough solar. You need a SSR / switch that needs to be turned on and off manually and yes they sell dual voltage water heater elements in 120/12volt.around $100.00
You can call Alex at seatech in marathon and he can walk you through it. Hes been heating water via an outback 80 and pv for a few months now.

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Old 20-08-2015, 20:42   #30
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Re: 12 volt hot water

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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.
:
There is plenty of incorrect info in this post. Resistive heaters are 100% effective in turning electricity into heat, which is very easy to make according to the rules of themodynamics. Even my cheapo solar panels from Ebay make 115% of rated watts in the Caribbean at noon. I use 40% or 45% factor to turn rated watts into Ah generated per day, but that is another story.
Still, if you want to heat water in the sun nothing beats a black bag full of water exposed to the sun. The question is whether you want to also use the bag to shower naked in front of a Greek taverna as some Scandinavian cruisers do

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