36KW water heater? How could you even...

[Sam Cho TX]

So while discussing water heater for an off grid cabin, instant water heater came up... and then I saw the power requirements:

Rheem Performance 36 kw Self-Modulating 7.03 GPM Tankless Electric Water Heater RETEX-36 - The Home Depot

Get hot water for your space with this Performance Self-Modulating Tankless Electric Water Heater. This energy-efficient tankless water heater delivers an endless hot water supply and consistent temperature

www.homedepot.com

36KW...

 

[zanydroid]

That's exactly why nobody electrifying their home today or trying to use it off-grid would use one. On top of heat pump being >3x more efficient.

(Tankless is great for a fossil fuel fired system)

 

[Supervstech]

Yup…

That’s a lot of BIG wires…

Tankless is fine for a shower head or a sink faucet… slow output… but big enough to fill a tub? Um… no.

And yet, I have installed them.

It’s insane.

 

[DaBeaver]

I plan to install one in a place I’m currently building, total efficiency is notably better than a normal tank-having water heater but worse than a heat-pump water heater. The main draws to me are space savings and aesthetics, the house will have 400A service and 14kW of PV.

 

[hwy17]

This is the kind of excess that net metering programs incentivize.

 

[timselectric]

Tankless is not for off grid.
Only if you are grid-tied.

 

[sunsurfer]

Im amazed at some of these off-grid setups that could almost run that. Back in the day this was not how it was done. I had a smaller budget so I down sized my life style, but at least my systems are very redundant and simple. I'm actually building our main house around zero power if needed and minimal power while times are good.

 

[Balrog006]

I plan to install one in a place I’m currently building, total efficiency is notably better than a normal tank-having water heater but worse than a heat-pump water heater. The main draws to me are space savings and aesthetics, the house will have 400A service and 14kW of PV.

That thing will eat up your 14kW of PV and still be hungry, and your batteries don’t stand a chance…

 

[daklein]

Wow that's 150A at 240v. Does it use several circuits? Hah, it does, 4x 40A, that'd be #8 copper, please remember to budget for that.

 

[Alkaline]

You would need..

two, umm no like four, or may be 5 ...Hell you would need to rob a bank to get the money for the inverters and batts to power that thing, 4x EG4 18K or like 8 of the 6000ex.

A shipping container worth of cells!

 

[Supervstech]

I plan to install one in a place I’m currently building, total efficiency is notably better than a normal tank-having water heater but worse than a heat-pump water heater. The main draws to me are space savings and aesthetics, the house will have 400A service and 14kW of PV.

Total efficiency can be better than a tank style heater IF you don’t use water often, or you are away frequently… as it won’t use any energy sitting there, only while being used. However… if you have any type of tiered energy use rate, or demand fees… yeah, his beast will cripple your energy bill…

 

[daklein]

I guess a tankless electric is about the cheapest to install. A coozie should help a regular water heater. Mineral wool batt insulation and an old boat bimini cover. One day I'll set up temperature logging to measure the standby loss. In the mean time, I should just write down the tank temp and time for a few evenings to mornings.

 

[AntronX]

My 50 gal tank heater uses 3.6 kWh per day. 67% of that (2.4 kWh) goes to heat loss. A tankless would save me 876 kWh or $140 per year.

 

[DaBeaver]

That thing will eat up your 14kW of PV and still be hungry, and your batteries don’t stand a chance…

Lots of hand-wringing here, the electric tankless heater will not be on a panel that’s backed up by batteries or solar. Statements that a massive inverter array would be needed to power it are true, statements that a massive PV array would be needed are mostly not true, total energy used by a tankless system is lower than a standard tanked hot water heater so a smaller PV array would suffice vs a standard hot water heater. The power is the issue with these things not the energy. I have 1:1 net metering and no TOU. The main drawbacks in my assessment are lower total efficiency than a heat pump hot water heater and the extra thinking I had to do (which I crowd-sourced here, thank you everyone who helped!) to decide on the panel configuration for the house.

 

[740GLE]

Lots of hand-wringing here, the electric tankless heater will not be on a panel that’s backed up by batteries or solar. Statements that a massive inverter array would be needed to power it are true, statements that a massive PV array would be needed are mostly not true, total energy used by a tankless system is lower than a standard tanked hot water heater so a smaller PV array would suffice vs a standard hot water heater. The power is the issue with these things not the energy. I have 1:1 net metering and no TOU. The main drawbacks in my assessment are lower total efficiency than a heat pump hot water heater and the extra thinking I had to do (which I crowd-sourced here, thank you everyone who helped!) to decide on the panel configuration for the house.

Says you, if I had that installed, I’d use it to fill up my pool with hot water just to test it!

 

[hwy17]

Some users are running 750ah+ banks so they could power a 36kw DC whole house on demand water heater at <1C. Producing a DC model would be a trivial change to the element, with some beefy switching parts. Also would have to locate the water heater near the battery bank and learn what the heck a kcmil is for the cables it would need.

 

[Partimewages]

Just remember with a heat pump water heater you are just moving the heat from around the tank to inside the tank. In the summer it's to your benefit by cooling the surrounding areas a little like free cooling. In the winter you are losing heat in the surrounding areas. You have to make up for this somehow.

 

[740GLE]

Just remember with a heat pump water heater you are just moving the heat from around the tank to inside the tank. In the summer it's to your benefit by cooling the surrounding areas a little like free cooling. In the winter you are losing heat in the surrounding areas. You have to make up for this somehow.

Our basement seems to be holding at 59f, the heat pump water heat is trying it’s best to drop it more. Perfect root cellar temp. Still getting about 4gal of water out of the air (garage under + walk out basement) per week.

 

[Supervstech]

Just remember with a heat pump water heater you are just moving the heat from around the tank to inside the tank. In the summer it's to your benefit by cooling the surrounding areas a little like free cooling. In the winter you are losing heat in the surrounding areas. You have to make up for this somehow.

True. However, with a basement install, the ground temp holds pretty well year round...

 

[AntronX]

Also would have to locate the water heater near the battery bank and learn what the heck a kcmil is for the cables it would need.

You can place DC wires inside water pipe feeding point of use 48v DC water heater. Wires can be much thinner since they will be immersed in water and resistive loss will directly go towards heating water. PVC wire insulation should handle it fine. Edit: also a good way to give your electrician (and inspector) a heart attack.