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Discussion Starter · #1 ·
I was able to snag a brand new Stiebel Eltron SHC 2.5 for $75, thinking I would just swap the heating element & thermostat with a 12v one from Missouri Wind & Solar and be set. Occasional quick shower and hot water hand washing/kitchen washing.

Originally I wanted to get an Isotherm SPA 15 but now I'm trying to avoid the additional carpentry to get it to fit inside my cabinet. Plus, the cost difference now is huge.

Bad idea? Worth it? The idea of "free" hot water after a drive is really enticing.
 

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I was able to snag a brand new Stiebel Eltron SHC 2.5 for $75, thinking I would just swap the heating element & thermostat with a 12v one from Missouri Wind & Solar and be set. Occasional quick shower and hot water hand washing/kitchen washing.

Originally I wanted to get an Isotherm SPA 15 but now I'm trying to avoid the additional carpentry to get it to fit inside my cabinet. Plus, the cost difference now is huge.

Bad idea? Worth it? The idea of "free" hot water after a drive is really enticing.
If you're going to run it while driving, why not just run it on 110v via inverter? It will likely run fine on a cheap msw inverter.
 

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Discussion Starter · #3 ·
If you're going to run it while driving, why not just run it on 110v via inverter? It will likely run fine on a cheap msw inverter.
I have 300aH of lifepo4 and a 2000w Kisae inverter.

It would take around 141ah to heat water with the 110v element it comes with for 2 hours a day vs around 20ah using a 12v element, hence why I'm debating trying this "cheap" water heater out while swapping to 12v.
 

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My experience is that you only need about 15-20 minutes to heat 2.5 gallons of water. If you need to run the element for 2 hours a day, that's a lot of hot water.
 

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Discussion Starter · #5 ·
My experience is that you only need about 15-20 minutes to heat 2.5 gallons of water. If you need to run the element for 2 hours a day, that's a lot of hot water.
I figured, worst case scenario 🤷‍♂️

So I suppose the general consensus so far is run it how it comes and see how I like it before converting it to 12v?
 

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That would be my vote. I have 210 AH of LifePo battery (300W solar) and can do hot water. Needed to put hot water heater on a switch and add alternator charging to keep the batteries from deficit during trips.
 

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It would take around 141ah to heat water with the 110v element it comes with for 2 hours a day vs around 20ah using a 12v element...
There is something wrong with the math or the units you are using.

If you convert to watts/hour you will get a better idea of the energy required. It takes the roughly the same energy whether you are using a 120v heating element or a 12v heating element.
 

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Discussion Starter · #8 ·
There is something wrong with the math or the units you are using.

If you convert to watts/hour you will get a better idea of the energy required. It takes the roughly the same energy whether you are using a 120v heating element or a 12v heating element.
Really? I think I used the faroutride calculator last night.
 

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It looks like the Stiebel has a 2400 watt heating element. What is the wattage of the 12V Missouri Wind & Solar unit you are proposing? To get the numbers you are using it is likely in the 200W-400W range. Yes it will use fewer AH in 2 hours but it may take longer than 2 hours to get the same temperature at the lower wattage. On the other hand, it will probably take far less than 2 hours at 2400 watts to reach the desired temperature.
 

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Really? I think I used the faroutride calculator last night.
Either the calculator is wrong or you are using it wrong.

If the element has the same power it will use the same amount of energy.

It really goes beyond this because you will need the same amount of energy to heat a given amount of water from one temperature to another. This does not change when you change the voltage or power of the element. If the element is smaller it will just run longer and still consume the same amount of energy. The only difference will be small inverter and wiring losses, but those are likely to cancel each other out as the inverter will consumer some energy, but you will also have more wiring loses with 12V.

It takes the same amount of energy to achieve a given temperature rise in a given amount of water (or any substance). This is fundamental physics.
 

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Really? I think I used the faroutride calculator last night.
What he said above. Math may be correct but engineering is wrong because you are comparing apples and oranges and therefore assumptions don’t apply equally.

Inverter inefficiency of 10 to 20 percent is main difference you should encounter at most. If results are 141 versus 20 Amp-hours, you are definitely doing something wrong.
 

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MMXVI - L2H2 in IN
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Is there a 12v element that fits this? I can't tell if it's the standard size or proprietary?
73939
 

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I would think so but if not it’s certainly easy enough to make it fit with off the shelf pipe adapters
 

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I saw some metric numbers on a parts web site, if that's the case I think sticking with 120v would be the easiest route.
 

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There is no free lunch when it comes to heating water with 12 volts compared to 120 VAC...

To heat 2.5 gallons of water from 60F to 110F at 100% efficiency takes ...
(2.5 gal)(8.34 lb/gal)(1 BTU/lb-F)(110F - 60F) = 1040 BTU, or 305 watt-hrs whether you use 12 volts DC or 120 VAC.

For a 12 volt supply:
(305 watt-hrs) / (12 volts) = 25 amp-hr out of the 12 volt battery

For a 120 VAC supply
(305 watt-hrs) / (120volts) = 2.54 amp-hrs at 120 VAC, which is 25 amp-hrs at 12 volts

The only real difference is that with the 120VAC system you have to account for the inefficiency of the inverter, so with a 90% efficient inverter, the 25 amp-hrs becomes 27.8 amp-hrs.
So, with a 12 volt heating element, 25 amp-hrs to heat the water.
With a 120VAC heating element, about 28 amp-hrs to heat the water.

Seems like if you can find a 12 volt heating element, it gives you a bit more flexibility with no need to have the inverter involved. You can also play with heating element size - a 300 watt element will take about an hour to heat the water and draw 25 amps. A 600 watt element will take half an hour to heat the water and draw about 50 amps -- and so on.

edit: corrected the bad calc for the 600 watt heater just above.😞
Gary
 

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Hi Gary, I know you know what you meant but this could be confusing to some people if the amp-hours don't have a voltage specified.
That may have been the original misunderstanding.

So, with a 12 volt heating element, 25 amp-hrs to heat the water.
With a 120VAC heating element, about 28 amp-hrs to heat the water.

So, with a 12 volt heating element, 25 amp-hrs @12v or from the batteries to heat the water.
With a 120VAC heating element, about 28 amp-hrs @12v or from the batteries to heat the water.

28amp-hrs @ 12v = 336w
28amp-hrs @ 120v = 3360w
 

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Discussion Starter · #19 ·
Thank you all! I understand what went wrong now.

The Stiebel comes with a 1300watt heating element. It is arriving tomorrow, but on second thought I may just get a Rheem 2.5 gallon performance point-of-use, it's MUCH more compact. The casing of the Stiebel makes it a bit bulky. But I'll wait to see it in person.

Even if I'm not getting the same amazing deal at the Stiebel, the Rheem is still less than half the cost and easily swapped over to 12v.
 

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Thank you all! I understand what went wrong now.

The Stiebel comes with a 1300watt heating element. It is arriving tomorrow, but on second thought I may just get a Rheem 2.5 gallon performance point-of-use, it's MUCH more compact. The casing of the Stiebel makes it a bit bulky. But I'll wait to see it in person.

Even if I'm not getting the same amazing deal at the Stiebel, the Rheem is still less than half the cost and easily swapped over to 12v.
The Rheem 2.5 gallon is the one I put in. It has a drain on the bottom to “winterize”. Takes about 20mins to heat the water on the 120v.


73953




73954
 
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