Electric Hot Water Options

[matthewlord]

New here and I did do a little looking around for a discussion but didn't see much. We are building an off-grid cabin and as I'm designing our solar electric system I'm struggling with the best option for heating our domestic hot water. I do not want to use a specific thermal system and I don't have a location for a big tank. Our plans call for an instantaneous electric water heater but the power needed to run the unit seems extreme. I think I'm being pushed towards the older style of inefficient tank heaters. We really only have a crawl space for this if we don't use instantaneous. How have you all dealt with this? I don't mind investing in a large battery system. Anyone have experience with the mini tank point of use systems?

 

[Supervstech]

Electeic Instantaneous water heaters are extreme energy hogs... even if you put one on each sink and shower, the small ones pull tons of watts.
A tank style heat pump water heater is the most efficient electric type, direct solar would be better...

 

[saen]

Take hybrid panels

DualSun SPRING

DualSun designs and manufactures the world’s first certified hybrid solar panel (PVT): the SPRING 2-in-1 panel.

dualsun.com

work good in temp over freezing with direct heating
then with combination with
Heat pump water heater

 

[Texican]

what about using a tankless instant heater running on propane ?

 

[matthewlord]

what about using a tankless instant heater running on propane ?

I honestly think that is where I'm going to end up. For a bit I got excited about having a zero fossil fuel house.

 

[JimP]

Maybe they are too big and too expensive but the heat pump type water heaters are much more efficient.

 

[saen]

Maybe they are too big and too expensive but the heat pump type water heaters are much more efficient.

Ariston NUOS EVO A+ WH 80

Width (cm) 51
Height (cm) 171
Depth (cm) 54
Content (l) 80
1,052.05 € (VAT incl.)

lot more brand and prices

 

[OzSolar]

Our plans call for an instantaneous electric water heater but the power needed to run the unit seems extreme. I think I'm being pushed towards the older style of inefficient tank heaters. We really only have a crawl space for this if we don't use instantaneous. How have you all dealt with this?

I'll offer that tanked and instantaneous electric resistance style water heaters are essentially the same efficiency. They both will use the same amount of energy (kWh) to heat water but the tankless will take a lot more power (kW) to do it.

Yes, there's a bit of standby loss in the tanked style but the losses are nothing compared to added costs of your system to go to a tankless electric.

You've not mentioned how many occupants but there's plenty of smaller tanked options ("low boy") that will provide plenty of hot water for a few people and could handle more than that if folks were smart about timing thier showers.

 

[Frick]

I have a small tankless propane unit, works great. Smaller than a briefcase Only needs 2 D batteries That last 6 months.
problem is only the function of temperature relating to usage. Only the very fancy ones have controlled output temps, every other one has a gas valve, so a shower head using 2gpm is great at 140 degrees, but use the sink and it’s 1gpm so the temp nearly doubles and boils the water.

the better setup is a tank electric unit With these and direct panel to element heating.

DERNORD 36V 1200W Stainless Steel Heating Element Immersion Heater Submersible Water Heater Element with 1 Inch NPSM Fitting Double U Type​

i run an Eletrodacus bms which includes direct DC diversion when the battery is full. I bought 60 cell panels used that are 36 volt, which it ties directly to the battery for charging, then diverts to these. They are 250 watt panels, and I wore 2 in parallel to each Electrodacus relay, then tie the output of two relays together to one heating element. My tests showed an average wattage of 200 each delivered to the element so a normal 800, two elements per tank and that’s 1600 watts of heating. Not the fastest so I have planned (so much to do!) 2 tanks of 40 gal each.

the propane is the backup and used until I get the full system online. And on a tank that is double bbq sized we get 4 long showers a day for over a month.

I honestly think that is where I'm going to end up. For a bit I got excited about having a zero fossil fuel house.

 

[efficientPV]

I heat all my hot water at camp with excess PV. Small tanks tend to be as expensive as big ones. I have found you can get BOSCH ES4 4 gallon water heaters on ebay for $48 shipped. I just got one, it was brand new and just an open box someone had brought back. These have good insulation. I also have a 40 gallon tank for my laundry. These tall tanks easily stratify and the top 15 gallons heats easily and the lower section is preheat. Here is a camp setup with only 600W of panels and it has enough excess to heat water. It operates in parallel with charge controller, but uses no power from controller or batteries. They are always left fully charged. Nobody else knows how to heat water on this board.

 

[mgnimike]

I have an old style electric water heater & 30 gallon tank. The heating element is rated for 240 volt, 4500 watts. Heating elements are just a resistance. I connect the heating element to a bunch of solar panels directly, no batteries, no controller, but keep the thermostat connection. The solar panels produce any where between 50volt and 90volt in full sun. Max of about 700watt. We have about 5 hours of sun max per day because of trees. This heats the water, but it has never gotten hot enough to trip the 125 degree thermostat. YMMV and I'm only telling you what I have, not recommending it, as it may be dangerous.

 

[OzSolar]

I love all these different suggestions that people are throwing out! Great example of what a forum is about.

It would fun to invent a scoring rubric that was specific for the individual application.

Initial cost : low - medium - high
Foolproof : yes - no
Locally available today: yes - no
Ease of installation : low - medium - high
Ease of Use : low - medium - high
Ease of repair - easy - medium - only the original mad scientist installer can fix it
Total cost of ownership : low - medium - high
Impact on Off Grid System: low - medium - high
Impact on Grid Tie System: low - medium - high
What else?

 

[Steve_S]

Unmatched Savings & so Much More​

The Rheem® ProTerra® Hybrid Electric Water Heater draws heat from the air around it to deliver unmatched savings and works like a standard electric water heater when the air is too cold. Either way, you enjoy worry-free hot water.

ProTerra - Rheem Manufacturing Company

Global Leader and Manufacturer of Heating, Cooling, and Water Heating Products

www.rheem.com

Good article with several links:

Rheem Releases 120V Plug-In Heat Pump Water Heater That Can Be Plugged Into Typical Outlet

They’re finally here! Rheem released its 120V ProTerra(R) Plug-in Heat Pump water heater (HPWH) in July and everyone is excited!

cleantechnica.com

Various Rebates are also applicable in USA, also through the IRA initiative.

 

[saen]

Running AC water heater on DC power is dangerous!
DC spark can weld the safety mechanism and create a dangerous situation.

 

[Steve_S]

Running AC water heater on DC power is dangerous!
DC spark can weld the safety mechanism and create a dangerous situation.

You do NOT actually run off DC, you use an Inverter to give you VAC ! Only a total git would try to run an AC device using DC Power.

 

[Supervstech]

You do NOT actually run off DC, you use an Inverter to give you VAC ! Only a total git would try to run an AC device using DC Power.

Two above responses state they directly connect solar to the heater. I guess it might be safe with only a few hundred watts going through the thermostat, but if excess watts are transferred, dc arc can destroy and weld itself, or simply start a fire...

 

[Frick]

To be clear, I use dc to the water heater elements, I do not use the temp switch that comes on a AC element, but a digital temp controller and a SSR to shut it off when it reaches temp.
I also use low voltage ( parallel panels at 20-32vdc typically). I found the 36v elements were just a right resistance match for my panels and since my entire system is based on matched equipment with compatible voltages ( Electrodacus) it made sense For me.

 

[SamDeleted]

I honestly think that is where I'm going to end up. For a bit I got excited about having a zero fossil fuel house.

What about bio gas ? Easier than it looks. House wives in third world countries like India do it all time , a 1,000L IBC, some plumbing bits, a cabbage and a bag of horse poo...

BOOM (hopefully not literally) , you've got gas

Bio gas running one of these

 

[SamDeleted]

Or do it the old fashioned way, light a fire, boil a pot of water ... Have you considered a rocket stove ? They can boil half a gallon of water in minutes with just a candy wrapper and some twigs, 100% sustainable & free

 

[SamDeleted]