Discharge rate?

[CalvinH]

In working through my whole house design, I've got a question for this group. My home has 2 28kw electric, tankless water heaters - These things really draw a lot of current when the tub is being filled . What level of battery/inverter is appropriate for the short term high demand requirements? I was thinking that 2 of the Sol Ark 15kw units might be needed, but I don't know if that would be enough for these things. Also, if that would work, can I build one or two battery banks to provide the peak power demand for my home (max 1 day usage for past year has been 95 kw). I would hate to have to switch to a tank system.

 

[MurphyGuy]

In working through my whole house design, I've got a question for this group. My home has 2 28kw electric, tankless water heaters - These things really draw a lot of current when the tub is being filled . What level of battery/inverter is appropriate for the short term high demand requirements? I was thinking that 2 of the Sol Ark 15kw units might be needed, but I don't know if that would be enough for these things. Also, if that would work, can I build one or two battery banks to provide the peak power demand for my home (max 1 day usage for past year has been 95 kw). I would hate to have to switch to a tank system.

You're going to need to stack a bunch of inverters together and you're probably looking at north of $100k between batteries and inverters to do it.

Powering two 28kW heaters is not going to be easy, and if you think the inverters are a problem, wait until you set up the battery bank to run those heaters.

A wood fire would be much better, natural gas, etc.

The other thing you could do is put in a large hot water storage tank so you can spread the energy out instead of trying to dump it all into the water at once.

 

[timselectric]

Agreed
Instant heating of any kind is not good for inverter sizing.

 

[CalvinH]

Thanks for the responses. This is my biggest draw/concern. Perhaps I need to rethink the water heater altogether. Natural gas is not an option and I was hoping to avoid switching to propane units.

 

[brum]

I would hate to have to switch to a tank system.

Either prepare for a pretty high cost of the system or go for a tank heater and heat pump.

 

[Texican]

leave those circuits on the grid. I left my electric water heater on the grid and it only runs at 4500W, but when the input water is really cold it can run for an hour to reheat.

 

[CalvinH]

One of the reasons for pursuing this is to be able to operate fully if the SHTF. I see that the cost to do water heating by solar/batteries using the tankless electrics would be a big cost. This is not about saving the ~$150 I pay monthly today, although that is a bonus.

 

[Texican]

One of the reasons for pursuing this is to be able to operate fully if the SHTF. I see that the cost to do water heating by solar/batteries using the tankless electrics would be a big cost. This is not about saving the ~$150 I pay monthly today, although that is a bonus.

as my water heater I left on the grid, I have a 12kw dual fuel gen that could be used every day for an hour to let the standard electric water heater do its thing. cost about 1200.
if you stay with the elec tankless you are indeed stuck in shtf situation

 

[sunshine_eggo]

Instant electric hot water heaters are essentially the worst case for any power system. If your concern is SHTF, get a SMALL system with a tank. You really need to leave the 56kW on the grid. A second backup system will cost you a tiny fraction of the hardware needed to power 56kW.

 

[brum]

What do you use for heating your home during the cold months?

 

[Solar Guppy]

One of the reasons for pursuing this is to be able to operate fully if the SHTF.

Then get a standard 40 gallon electric tank heater to feed the tankless and keep them powered off until SHTF. And as needed, turn off the breakers for tankless.

 

[Texican]

Then get a standard 40 gallon electric tank heater to feed the tankless and keep them powered off until SHTF. And as needed, turn off the breakers for tankless.

thats the best idea for grid down.
also for shtf the tank holds 40 gal of water you can get to flush toilets. Im in N Texas, ask me how I know...

can a tankless feed thru water when no power to it ?
if not then a bypass valve will let the 40 gal tank feed the system

 

[MisterSandals]

One of the reasons for pursuing this is to be able to operate fully if the SHTF.

Are you assuming that there will be utility water pressure in SHTF?
Having clean drinking water is on/near the top of the list. Hot water for showers maybe on page 2.

 

[CalvinH]

Home heat is currently propane + wood burning fireplaces. A few years ago we were without electricity for a week in the winter - we made it, but it was like camping inside our own home! The power company has moved a lot of the lines underground so I am hoping that we don't see a repeat.

I'll look into the prospect of putting in a bypass to a tank-based water heater.

 

[CalvinH]

Are you assuming that there will be utility water pressure in SHTF?
Having clean drinking water is on/near the top of the list. Hot water for showers maybe on page 2.

I have a well, so I am looking at the solar/battery/inverter system to support that, too.
I am reliant on the propane company to deliver that and the grid for electricity, that's it.

 

[MisterSandals]

I have a well, so I am looking at the solar/battery/inverter system to support that, too.

Very good. Not trivial from what i've seen here. Plan early for this need.

 

[burgerking]

2 x 28kw water heater . . .

you will need at x3 solar panel, x5 in battery storage
this is a big system ... cost a fortune

 

[SamDeleted]

One of the reasons for pursuing this is to be able to operate fully if the SHTF. I see that the cost to do water heating by solar/batteries using the tankless electrics would be a big cost. This is not about saving the ~$150 I pay monthly today, although that is a bonus.

Solar hot water panels are a great option

 

[Solar Guppy]

Solar hot water panels are a great option

Solar hot water requires a standard tank for storage, the OP has instant hot water heaters, hence the 28kw load, which would not help. I've had Solar hot water before and wouldn't do it again.

 

[TomC4306]

Are you assuming that there will be utility water pressure in SHTF?
Having clean drinking water is on/near the top of the list. Hot water for showers maybe on page 2.

Right ?. The 24k or so chlorinated gallons in my pool. Closed and covered for 8 months of the year.