Backing up an On-Demand / Tankless (electric) water heater with Powerwall(s)

[YRide]

Wondering if anyone has used a Tesla PV/ESS setup with on-demand / tankless (specifically electric) water heaters?

TL;DR...could you split a 36kW tankless electric water heater across both your backed up load center and non-backed up (main panel) to have some hot water in the event of a power outage...but not sacrifice a majority of your backed up load center to heating water (or have to get an excessive amount of ESS).

Looking at specs…PW+ is

And Powerwall2 is

So for a Rheem 36kW (Find product information, documents, videos, and installation assistance for the Rheem Professional Classic Series: RTEX-36), which requires 4x40A (36kW) when operating at 8gpm flow it would seem powewalls cannot cut it.

First, based on above specs...you'd need like 2xPW+ (2x9.6kW) and 4xPW2 (4x5kW) in parallel giving you 39.2kW output if you were off-grid (and that appears it would also require full sun from the PV)...if I'm doing this correctly.

Additionally, it would seem impractical in that I think Tesla limits the backed up Load Center for your house to either 200A or 225A. This would mean taking 160A out of your load calc for backing up the house, leaving you with not very much headroom for all other electrical lightning, appliances, EVSE, etc. Also, does this get considered a "continuous" load by the NEC? In which case wouldn't you have to uprate by 125%, so actually you would need to use 200A for the load calc on the back'd up load center? So there goes all your power!

It got me thinking...since the tankless 36kW is really just 4 parallel heating elements, 9kW each...could you in theory run 3x of the 40A breakers off your main panel which is not backed up and will only get power from the grid and not your PV or ESS (lets assume in this case you have a 300A or 400A service from utility)...and then only run a single 40A breaker off the backed up load center, running a 9kW element.

1) is this an acceptable config? Do the 4x breakers and circuits for the 4 heating elements have to run off the same load center for any reason? Or can you split them 3 and 1?

2) will a tankless electric water heater function should only one of the circuits (40A, 9kW) be operational? Thinking the power goes out, so you lose grid power (and 3x40A, three of the four heating elements)? Will it just basically downgrade the WH to a single 40A/9kW heater...which should be able to handle ~2.0gpm with a decent temperature rise...which is enough to take a hot shower.

This would see like a reasonable compromise. Most of the time you'll have the full 160A / 36kW available, but without having to sacrifice a major amount of Amps for your backed up load center. But, you will still have a hot shower should you lose power.

Havent done any of the financial calcs to see if this is even worth it (natural gas is pretty cheap). But in an earthquake, you might lose case...with a limp along tankless electric + ESS + PV, you could manage indefinitely. Also, I believe electric tankless have a the least maintenance and highest efficiency.

 

[power.saver]

Wondering if anyone has used a Tesla PV/ESS setup with on-demand / tankless (specifically electric) water heaters?

TL;DR...could you split a 36kW tankless electric water heater across both your backed up load center and non-backed up (main panel) to have some hot water in the event of a power outage...but not sacrifice a majority of your backed up load center to heating water (or have to get an excessive amount of ESS).

Looking at specs…PW+ is

View attachment 897763

And Powerwall2 is

View attachment 897764

So for a Rheem 36kW (Find product information, documents, videos, and installation assistance for the Rheem Professional Classic Series: RTEX-36), which requires 4x40A (36kW) when operating at 8gpm flow it would seem powewalls cannot cut it.

First, based on above specs...you'd need like 2xPW+ (2x9.6kW) and 4xPW2 (4x5kW) in parallel giving you 39.2kW output if you were off-grid (and that appears it would also require full sun from the PV)...if I'm doing this correctly.

Additionally, it would seem impractical in that I think Tesla limits the backed up Load Center for your house to either 200A or 225A. This would mean taking 160A out of your load calc for backing up the house, leaving you with not very much headroom for all other electrical lightning, appliances, EVSE, etc. Also, does this get considered a "continuous" load by the NEC? In which case wouldn't you have to uprate by 125%, so actually you would need to use 200A for the load calc on the back'd up load center? So there goes all your power!

It got me thinking...since the tankless 36kW is really just 4 parallel heating elements, 9kW each...could you in theory run 3x of the 40A breakers off your main panel which is not backed up and will only get power from the grid and not your PV or ESS (lets assume in this case you have a 300A or 400A service from utility)...and then only run a single 40A breaker off the backed up load center, running a 9kW element.

1) is this an acceptable config? Do the 4x breakers and circuits for the 4 heating elements have to run off the same load center for any reason? Or can you split them 3 and 1?

2) will a tankless electric water heater function should only one of the circuits (40A, 9kW) be operational? Thinking the power goes out, so you lose grid power (and 3x40A, three of the four heating elements)? Will it just basically downgrade the WH to a single 40A/9kW heater...which should be able to handle ~2.0gpm with a decent temperature rise...which is enough to take a hot shower.

This would see like a reasonable compromise. Most of the time you'll have the full 160A / 36kW available, but without having to sacrifice a major amount of Amps for your backed up load center. But, you will still have a hot shower should you lose power.

Havent done any of the financial calcs to see if this is even worth it (natural gas is pretty cheap). But in an earthquake, you might lose case...with a limp along tankless electric + ESS + PV, you could manage indefinitely. Also, I believe electric tankless have a the least maintenance and highest efficiency.

Sure, you can have some of the circuits backed up and other not. But have you tried this (turn 3 of your 4 breakers off) and see if it will even warm up the water?

 

[wwhitney]

Tankless electric water heaters are generally a bad idea for residential services, due to the large power demand. And backing one up with PWs is a terrible idea, for the same reason.

Cheers, Wayne

 

[jboy210]

Tankless electric water heaters are generally a bad idea for residential services, due to the large power demand. And backing one up with PWs is a terrible idea, for the same reason.

Cheers, Wayne

I have wondered about this. I have tankless, but it is natural gas. As long as I have the power to get the burner going we have hot water. But I can't imagine heating that much water that fast with electricity. I believe the water heater consumes from 150,000 to 200,000 BTUs per hour.

 

[wwhitney]

I believe the water heater consumes from 150,000 to 200,000 BTUs per hour.

1 BTU/hr = 0.293 W. So if you have a 90% efficient 200,000 BTU/hr gas tankless, the equivalent electric tankless (100% efficient) would need 53 kW, or 220A @ 240V.

Cheers, Wayne

 

[mongo]

I have wondered about this. I have tankless, but it is natural gas. As long as I have the power to get the burner going we have hot water. But I can't imagine heating that much water that fast with electricity. I believe the water heater consumes from 150,000 to 200,000 BTUs per hour.

For electric: 55 degree incoming 120 degree outgoing= 65 degree rise @ 4 GPM @ 8 lbs/ gallon = 2,080 BTU per mimute * 60 min/hr = 125k BTU/hr / 3412 BTU/kWh = 36.6 kW / 240 = 152A

 

[YRide]

Tankless electric water heaters are generally a bad idea for residential services, due to the large power demand. And backing one up with PWs is a terrible idea, for the same reason.

Cheers, Wayne

Could you elaborate on why a bad idea? I have a new transformer right on pole outside, 30ft over the air drop…PGE said $0 to upgrade to 400A service. So it’s just the electrician install and new panel, and new service entrance.

I get fully backing up a tankless electric is bad idea via ESS (not even sure it’s even possible with < 6 PW per my original post)…

But it’s not like im planning to be living off the grid.

Still looking to see if someone knows the answers to what is possible (even if remains a “bad” idea):

1) do all 4 circuits need to come off the same panel for someone reason? Or is it ok that they are split 3 off main and 1 off backed up load center?

2) can a 36kW rheem (4x40a/9kW elements) even operate with only one 9kW element on? Like if you flipped 3 breakers, will the controller allow that and operate at 9kW, or shut down because it detects something wrong?

I believe 9kW @ 2gpm (shower) is sufficient im an emergency mode (storm / tree branch takes out electricity for one night…earthquake takes out gas for several days, etc).

 

[wwhitney]

Could you elaborate on why a bad idea?

The main upside is you don't need space for a tank. [Heat loss on an electric tank is pretty low.] The downside is the large power draw, the expense of the extra service conductors and branch circuits, maybe some voltage drop when the unit is running.

I'd suggest a tank heat pump water heater. That will actually save energy over a tank electric or tankless electric.

1) do all 4 circuits need to come off the same panel for someone reason? Or is it ok that they are split 3 off main and 1 off backed up load center?

I think a common disconnect would be required. Typically that would be the 4 breakers all in one panel.

So I'd suggest running 4 circuits from the non-backed up panel, and 1 circuit from the backed up panel. Then you could have a manual transfer switch (which can just be a small panel with a sliding interlock bolted to the cover) to change one of the branch circuits from the regular non-backed up source to the backed up source.

I believe 9kW @ 2gpm (shower) is sufficient im an emergency mode (storm / tree branch takes out electricity for one night…earthquake takes out gas for several days, etc).

Per mongo's calculation, 9 kW gives you 65 gallons * deg F /min rise. So with a 2 gpm shower, it could heat 55 deg water to 87. You could probably get a reasonable 1.2 gpm shower.

Cheers, Wayne

 

[jboy210]

...

Per mongo's calculation, 9 kW gives you 65 gallons * deg F /min rise. So with a 2 gpm shower, it could heat 55 deg water to 87. You could probably get a reasonable 1.2 gpm shower.

Cheers, Wayne

87-degree F shower! Brrr

 

[BGbreeder]

Every tankless model that I have worked with uses the elements, gas or electrical, at full capacity until the output water is at temperature.

In practice what that means for @YRide is that the minimum flow (2gpm) would try to engage all four elements. At 2gpm, one element would heat enough water for you to be happy. At higher draws, your going to get lukewarm water.

I have trouble imagining a wiring setup inside your tankless that would/could support a 3+1 configuration. (And I can think of a slew of reasons why it would be likely to be neither practical nor safe.)

I'm with @wwhitney; an electrical tankless water heater on ESS is just not a good idea. Get an electrically heated oversized water heater with a circulation pump, a mixing valve, and a programmable controller so that you can soak up excess power during high solar output, or low cost grid rates. Make it a heat pump water heater for even better efficiency.

Given the lag in getting warm water to end use points, tankless water heaters force the user to end up running faucets and showers for awhile, wasting water. I think in water limited regions, like San Jose, CA, they are an especially poor choice.

All the best,

BG

 

[YRide]

Appreciate all the insights.

Prob will just stick with tankless gas for now, which is already installed. Once we have the ESS, that should provide relevant backup in most outage situations (except the “big one”…knock on wood).

@BGbreeder FWIW, I’ve experienced almost no lag on this system…pretty quick to temp. House is compact, so that helps I assume.

 

[msm859]

So my set up is a little more complicated. I have a gas boiler with radiant heat that also is a tankless water heater. In the winter the water is preheated to @80 degrees by a heat pump water heater with an 80 gallon tank. If I have a lot of company I can always raise that to make sure I have enough hot water. In the summer (actually @ 1/2 year) I turn off the boiler and just have the heat pump water heater provide all my DHW. I have 3 way valves that will bypass the boiler. I also have a 2 gallon electric heater right next to the kitchen sink (in an empty cavity) that gives me instant hot water.

 

[Charlesz]

I’m running a 24kw Stiebel eltron tankless water heater. It requires 2 x 50amp breaker. At first when I installed it I have 1 faulty breaker when I turn the hot water on it wont get hot enough and after a few minutes of running it the internal safety breaker trips. So Id recommend running the breaker size and amount according to the specs.

Now as far as energy consumption. When I turn on the shower it can get as high as 18kw consumption looking at the tesla solar app. I wouldn’t recommend running it when it’s powered by the power walls. Because it will drain it very fast.

My current set up is that I have both electric water heater and oil burner. If incase I have an power outage I’d turn off the electric water heater / heat pump and turn on the oil burner and the 2 Pw should last me 7+ days on solar energy + stored energy.

 

[jboy210]

...

Given the lag in getting warm water to end use points, tankless water heaters force the user to end up running faucets and showers for awhile, wasting water. I think in water limited regions, like San Jose, CA, they are an especially poor choice.

Why would the loss of water be any worse than with any other water heater? The length of the pipe run from the water heater to the fixture is the same regardless of the water heater type. And a tankless produces hot water in a few seconds (10-15 on our spec sheet).

 

[Patrick66]

In the winter the water is preheated to @80 degrees by a heat pump water heater with an 80 gallon tank. ... In the summer (actually @ 1/2 year) I turn off the boiler and just have the heat pump water heater provide all my DHW.

Hands-down the best approach. Does the boiler run ok with tempered inlet water?

Somewhat curious why you don't run the heat pump water heater to utilization temperatures and just disable resistance heat, using the boiler downstream to bump up temperature if/when needed.

 

[fresnoboy]

Hands-down the best approach. Does the boiler run ok with tempered inlet water?

Somewhat curious why you don't run the heat pump water heater to utilization temperatures and just disable resistance heat, using the boiler downstream to bump up temperature if/when needed.

I don't understand this. If you have a gas boiler, why not have it do all the DHW and bypass the heat pump water heater completely? If it's a modulating burner it's already super efficient, or are you using the heat pump heater just as a storage tank?

 

[wwhitney]

I don't understand this. If you have a gas boiler, why not have it do all the DHW and bypass the heat pump water heater completely?

Because electricity > fossil fuels?

Cheers, Wayne

 

[fresnoboy]

Because electricity > fossil fuels?

Cheers, Wayne

Not at $0.55/kWh. Just think about how painful it would be to heat a pool with electricity... We need much cheaper electric power if we want to remove nat gas from the ecosystem.

 

[wwhitney]

Not at $0.55/kWh

There's more to the energy market than just dollars and cents. I.e. we are having a market failure due to the unpriced externalities.

Cheers, Wayne

 

[msm859]

I don't understand this. If you have a gas boiler, why not have it do all the DHW and bypass the heat pump water heater completely? If it's a modulating burner it's already super efficient, or are you using the heat pump heater just as a storage tank?

I want to reduce my carbon footprint. I also have solar -and am going to add more. I have 4 bathrooms in my house. One of the issues with a tankless water heater is capacity. By "preheating" the water I basically doubled my potential output. Another concern they say it takes a little longer for the hot water to reach distances - i.e my kitchen which is why I added the electric 2 gallon tank @ 30" line to the faucet . I also wanted to reduce my water waste. In the upstairs masterbedroom I have a recirculating pump that uses the same loop and runs until the water is hot. It turns on by a motion detector that is near my bed that I waive my hand to activate before I get up. Trying to save water.