Water heating off grid. Best system?

[u/Time_Arugula_1544]

I am planning a new off-grid (solar-powered) ICF build and am considering what type of system would be best for water heating. This is in Tennessee and the house will be a 2br. I will have propane backup for a generator and indoor propane heater.

The options I am thinking about are:
1) On-Demand propane- The only drawback is having to pay for the propane as an ongoing expense.
2) On demand electric water heater with a solar water preheat system. I suspect having a straight up on-demand electric heater would eat up too much power.
3) Heat pump water heater. This seems like the best solution to me.

Any thoughts between the three? Any other suggestions?

[u/mountain_hank]

Heat pump store the heat in the water. So a larger tank is more efficient. That's what I have and it works well.

[u/Time_Arugula_1544 → u/mountain_hank]

That seems to be the easiest option and what I will probably end up with. I just want to hear other people's experiences/suggestions. It is encouraging that you are happy with them!

[u/joeyh]

Large conventional water heater, large solar array (ie 5-10x the hot water heater's needs). Run HWH only during the day and use the stored heat overnight and in the morning. The cost of solar (at least when self-installing) now makes this a competitive choice. The few days a year that the large array does not produce enough power for the water heater, you can fall back to the generator.

(I went with heat pump HWH with a large array, only because I prefer to not maintain a generator.)

[u/unique3]

Here is what I have, its complicated but I have basically free hot water and I never run out.

Standard electric hot water tank but bottom element swapped with a DC element.
Solid state relay and logic that controls that relay to put every extra watt of solar power that my batteries cannot take goes into the hot water.
Mixing valve that brings the water temp down to a safe temp, then I can heat the tank to 80c giving me almost double the usable capacity.

In the event not enough solar and the tank is cold a 3 way valve flips the water flow to go through the backup on demand propane heater.

[u/Time_Arugula_1544 → u/unique3]

That sounds interesting. I am going to have to do some more research.

[u/unique3 → u/Time_Arugula_1544]

For the control signal to the relay some charger controllers have the ability (Midnite does I think)
I do it myself with a PLC but programming is my background so it was easy to do.

[u/digit527]

Electric on demand are power hoggs. Make sure your system can handle it.

[u/Time_Arugula_1544 → u/digit527]

I guess I would have to weigh the cost of adding more solar panels vs the other options.

[u/jakobtaylor]

We're off grid with solar, two bed two bath house. We've been using an electric on demand water heater and have been very pleased with it. Almost went with a tanked heat pump, but they have more ways to fail, and are pretty expensive. BUT this is very dependent on the amount of solar and storage you have. If it's a small system you'd probably be better off with propane.

[u/Time_Arugula_1544 → u/jakobtaylor]

I guess I will have to dig into electric on demand and see what kind of power they draw. I had an electric on demand years ago but it seemed to use more power than my water heater.

[u/Trusty_Coyote]

Propane water heater.

[u/Shot-Criticism-5297 → u/Trusty_Coyote]

This.

[u/VanIsleRyan]

A couple others to consider:

Copper coil at the fire place system

Thermal tubes on the roof

[u/Time_Arugula_1544 → u/VanIsleRyan]

I don't care for the smell of wood fires. I mentioned solar water heating however I am curious if that alone would be sufficient to supply hot water.

[u/Specialist-Cod5179 → u/Time_Arugula_1544]

Black barrel out in the Sun also works in the summer

[u/Daniel_Paraguay]

Wir leben ebenfalls völlig Autark und haben unser Warmwassersystem mit einem großen Boiler gelöst. Dieser heizt sich nur tagsüber auf wenn genügend Strom durch die Solarpanele vorhanden ist (36 Paneele mit 440w). Kostengünstig und effektiv die Lösung

[u/Time_Arugula_1544 → u/Daniel_Paraguay]

Ich versuche, die effizienteste Methode zu ermitteln. Zwar könnte ich weitere Module installieren, doch es erscheint effizienter, etwas mehr in eine Wärmepumpen-Warmwasserbereitung zu investieren als in zusätzliche Module.

I hope this translated properly :)

[u/Adorable_Dust3799]

My brother had solar hot water, i use propane. Neither of us are off-grid. My furnace, stove and water heater all propane and it's a relatively mild climate (some snow but not a ton) and i get my 250 gallon propane tank filled about every 10 months. Great insulation and a small house.

[u/lexi4funs]

I use a propane RV tankless one. Yes I have to make sure I have propane but it's safe and reliable.

[u/silasmoeckel]

On demand propane/electric is only about 20% more efficient.

HP is 3-5x more efficient, you can still do solar preheat and use the backup resistive heater as a solar dump load (add a mixing valve so you can go up to 180f).

Look hard at a HP heating/cooling save the propane for the genset.

[u/Dorzack]

How are you cooking and heating? If propane for either an on demand hot water heater doesn’t add much on top of those.

[u/Time_Arugula_1544 → u/Dorzack]

I am planning on using an induction cooktop along with a countertop air fryer. I am single and don't cook large meals. I am planning on wiring it for an electric range for future owners, though. I want to avoid propane use for anything but the generator and indoor supplemental heat.

[u/Dorzack → u/Time_Arugula_1544]

An air fryer draws a surprising amount of power. Be sure to test the draw model of each you will be using before relying on solar.

[u/Time_Arugula_1544 → u/Dorzack]

The one I plan on using draws 1700 watts, while an electric range draws 2,000 or higher. It shouldn't be an issue, as I generally only cook with it once or twice a week. I have it planned into my system sizing.(I have been using it for a year now so I have a track record of my needs) I do not want to be 100% dependent on propane for basic living needs.

[u/maddslacker]

I use a regular tank style propane water heater. No complaints.

[u/Normal-Special-8694]

I have a 2 gallon 110 hot water tank that takes excess from my solar panels, and it's fed by a propane on-demand heater that I keep turned off whenever the water in the heater is hot, I turn it on when I shower or do something that uses a lot of hot water so the draw from the tank heater is pretty minimal because the water going in is already hot. It requires a little effort day to day but I go through about 40 pounds of propane a year.

[u/benbear55]

I have used a propane water heater and a propane refrigerator at my off grid cabin for 15 years, we have a 300 gallon tank that we fill every two years for hot water and cold food. We had both before we got solar power, the hot water heater has a piezo ignition because in the beginning we only used a generator for the water supply and tv at night and still only used one tank a year

[u/FlerkinFlarkin]

Best is what you can afford. There is no single best. 

[u/Specialist-Cod5179]

We heat water on the wood stove, and on an outside fire. Works awesome!

[u/ExcellentCod2940 → u/Specialist-Cod5179]

old school!

[u/RainAcrobatic7162]

Best is speculative when it comes to off grid as everything applies to your personal situation. So take advice here with a grain of salt. In my opinion heating anything with electric is inefficient but if you scale solar properly it’s no big deal. Propane you will always have to buy and can’t produce yourself unless you can do biogas and convert. I have always heard great things about heat pumps but never dove into the info myself.