Hello All, I'm new to the board and read the entire post on Indoor or Outdoor Tankless Water Heaters. I have a little info which may be helpful and some food for thought.
I really love the Tear Drops and am fascinated by how creative the makers are. Unfortunately I haven't put one together myself as I have a Cabover Camper but I still share many of the issues as all of you limited space, How to shower and make hot water when off the grid so hopefully you'll tolerate my input.
First a few comments on prior threads. A couple people brought up the 300 & 600 watt 12V heaters. While there is no doubt they will heat the water eventually they are slow to recover just not enough energy to bring up the water temp. The other down side is 300 watts at 12 volts equals 25 amps so your deep cycle Battery depending on how new it is gets eaten real quick and will be needing a recharge in a couple hours. 600 watts @ 12 volts is 50 amps. Just sayin!
Second I saw several posts regarding the 6 gallon 120 Volt AC water heaters. In my camper when on the grid I slid a small .6 cuft microwave in and it takes 900 watts @ full power or 7.5 amps. That's half of a 15amp service. I managed to score on Craigslist a 6 Gallon water heater from a boat for $35.00 it runs on 120 volts.
I wanted to be able to run everything in my camper on a single 15 amp line so that meant the existing 1500 Watt element was just to much. I started looking for an "
inexpensive" alternate. I went to the hardware store and found the replacement elements with the std screw thread where about $9.00 and up. But the elements where still energy hogs at 1000 Watts or more. I did have the option of getting 110/120 Volt or 220/240 Volt elements. I was hoping for something around 700 watts. After some research I found an option at Camping World a replacement/upgrade element for the 6 Gal RV water heaters that was 700 Watts. Only problem is was also $70.00. Can you spell RIP-OFF!! Then my engineering side kicked in Why pay 70 bucks when you can get the same for $9.00? So back to the hardware store.
They had 240 volt elements with a 1500 Watt rating, if I run it at 120 Volts I should get my ~700 watts, Yea!! and at $9.00 if it doesn't work I haven't spent a fortune either.
Turns out it works better than I planned the element is rated at 240 Volts in most of the US we have 110/220 Volt power. I hooked up the element to 110 volts and measured it with a clamp on AC ampere meter and it draws less than 4 amps!!! And better yet it takes about 30 min to heat the tank!! Double YEA!!
Did I mention the marine water heater also has a heat exchanger.
You say what's a heat exchanger? Well on boats they use the radiator water to heat the hot water so it has a coil that raps through the water tank and when the engine is running you have all the hot water you need. I'm talkin 180 Degrees!!!

So in addition to the 120 Volt heating element I plumbed water hoses into the camper to the water heater from the motor and after 5 to 10 min. at idol I have VERY HOT Water

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So for you creative soles may I suggest a heat exchanger mounted in your car as a possible option that hadn't been brought up here. I'll draw up a simple exchanger for the car and posted it under another post if anyone is interested. Let me know.
Thanx for reading my post!
- Phil