How much does a water heater REALLY use?

[MushCreek]

Our power was out for several days after Hurricane Zeta. I have a 6500 watt generator, and we were able to keep the food cold and heat food up. I want to hook the thing up to the whole house with a transfer switch or whatever to be safe and code-compliant. I know it won't run everything, but I can run a few things like we did this time, without having extension cords all over the place.

What about the electric water heater? It's on a 30A breaker. The genny has a 30A 240V twist-and-lock outlet. Does that mean it will (theoretically) run the water heater as long as nothing else is on? My confusion comes from reading the specs on the WH. Top element= 4500 W. Bottom element= 4500W. BUT, it also says the total is 4500W! Last I knew, 4500 X 2 = 9000W. Does that mean that it never runs both elements at the same time?

I could always disconnect one element when running in a emergency situation like this, that way being sure to only draw 4500W. I'm confused, though, why it says the total load is only 4500W. Ideas?

 

[The Cobbler]

Water heaters only use 1 element at a time. Te top element comes on to heat water in the top of the tank,so you have hot water quicker, then switches to the bottom element to heat more water

 

[wyliesdiesels]

What he said above

 

[nadogail]

If your waterheater is well insulated, the insulation will hold the hot water near the temperature for considerable time. As the water cools, the elements should come on to return the water in the tank to temperature. Any hot water drawn from the tank for bathing, etc. will be replaced by cold water and that water will need to be heated.

Think of a water heater as being somewhat like a refrigerator, it really only needs to run if somebody takes things out or adds something.

 

[Will S.]

I have am Emporia Vue energy monitor in my breaker oanel, and today while wife was doing laundry, I checked it. Water heater was drawing 38 amps.

 

[wyliesdiesels]

I have am Emporia Vue energy monitor in my breaker oanel, and today while wife was doing laundry, I checked it. Water heater was drawing 38 amps.

Thats a large water heater. 9,000w element

 

[MushCreek]

So the bottom line is- my 30A 240V genny should handle it? If only one element comes on at a time, it would be under 20A.

 

[Will S.]

Thats a large water heater. 9,000w element

Yes, so I'm sure it is dual element, and she had the dishwasher going at the same time as the laundry.

 

[Bert_]

4500 is standard for residential size heaters. The Marathons are 3800 usually. Top element gets priority, never both at once.

 

[sberry]

4500 is a standard heater, 5500 is high recharge.

 

[MushCreek]

I think I'll buy something like the Emporia Vue. I've thought about getting one anyway. That will make it a lot easier to decide what to run if were on the generator. It's a pain to have to find the wattage rating on each appliance, then add it all up. I'll get the Vue, and compile a list of the various devices and what they actually draw. For example, my mini-splits are on 15A breakers, and supposedly draw about 10A, but I'm not really sure.

 

[Will S.]

I previously posted that my water heater was drawing 38 amps, and even as I posted, I thought that was wrong, but that's what the E/V showed. So here's the rest of the story:
copy/paste from the Vue manual:

Step 8 (continued): A note
about multi-pole breakers
If you wish to monitor 2- or 3-pole breakers,
you can either use one sensor on each pole,
or you can use a single sensor. To use a
single sensor, clasp the clamp around either
one of the hot leads coming off the breaker (it
doesn’t matter which). You’ll then be able to
input a circuit multiplier in the app to double
or triple the reading by entering a “2” or “3.”
We don't recommend multiplers for
unbalanced loads, such as subpanels.

So I did that on one 240v leg to the water heater, and used a "2" multiplier in the setup. The display reads 37 amps vs the actual 18.** that my clamp-on digital meter shows.

 

[jkeyser14]

Mine draws close to 6kw.

 

[Bert_]

So I did that on one 240v leg to the water heater, and used a "2" multiplier in the setup. The display reads 37 amps vs the actual 18.** that my clamp-on digital meter shows.

18A on 240v is 4500w. You don't multiply by 2. Not sure what your monitor thing is trying to tell you but it's not right in this instance.

Unless it can only be set up to read 120v loads, then a multiplier of 2 would make sense. It's still only 4500w so something is missing.

 

[Will S.]

18A on 240v is 4500w. You don't multiply by 2. Not sure what your monitor thing is trying to tell you but it's not right in this instance.

Unless it can only be set up to read 120v loads, then a multiplier of 2 would make sense. It's still only 4500w so something is missing.

I agree, and yet this is what the installation manual says to do. Here's a link. See step 8:

https://emporiaenergy.com/Gen%202%20Emporia%20Vue%20Installation%20Guide.pdf

on edit:
This gets more interesting. There is at least on other company that has been around for many years, making a product called TED... The Energy Detective. The same principal as E.V. Here is a copy/paste from THEIR instruction manual:

5. Turn off the breaker for the circuit to be measured.
6. Loosen screw and remove wire from circuit breaker.
7. Slide CT onto wire. Orientation of CT does not matter.
8. Reinsert wire into breaker and tighten screw.
9. (IMPORTANT) Determine the type load to be measured.
a. In the case of 2-phase or 3-phase loads that are not balanced,
you will need to install a CT over each power wire. For example:
a subpanel. You would need to install one CT over each phase
wire.
b. In the case of a balanced load, (for example: a motor or
heater), you can simply install one CT over any of the power
wires and apply a multiplier of 2 or 3 (select ‘3’ if 3-phase)
in the Footprints Spyder setup.

Bert, if a water heater draws 18 amps from L1 (measured with a clamp-on ammeter) and also 18 amps through L2, then we're paying the utility company for 36 amps per hour. So for the purposes of displaying amp (watts or kw), the Vue would be correct to double the measured current, if only measuring one leg of a 240 load. So technically, they are correct. Correct?

 

[Bert_]

I agree, and yet this is what the installation manual says to do. Here's a link. See step 8:

https://emporiaenergy.com/Gen%202%20Emporia%20Vue%20Installation%20Guide.pdf

on edit:
This gets more interesting. There is at least on other company that has been around for many years, making a product called TED... The Energy Detective. The same principal as E.V. Here is a copy/paste from THEIR instruction manual:

5. Turn off the breaker for the circuit to be measured.
6. Loosen screw and remove wire from circuit breaker.
7. Slide CT onto wire. Orientation of CT does not matter.
8. Reinsert wire into breaker and tighten screw.
9. (IMPORTANT) Determine the type load to be measured.
a. In the case of 2-phase or 3-phase loads that are not balanced,
you will need to install a CT over each power wire. For example:
a subpanel. You would need to install one CT over each phase
wire.
b. In the case of a balanced load, (for example: a motor or
heater), you can simply install one CT over any of the power
wires and apply a multiplier of 2 or 3 (select ‘3’ if 3-phase)
in the Footprints Spyder setup.

Bert, if a water heater draws 18 amps from L1 (measured with a clamp-on ammeter) and also 18 amps through L2, then we're paying the utility company for 36 amps per hour. So for the purposes of displaying amp (watts or kw), the Vue would be correct to double the measured current, if only measuring one leg of a 240 load. So technically, they are correct. Correct?

18A is 18A. You don't add each line together.

Even on 120v the neutral will read the same amps as the hot. You wouldn't think of adding them together would you?

 

[Will S.]

So then the explanation of why they all say to double the reading if only using 1 CT, would revert back to your question of is there a voltage setting? 18A at 240 vs 36A at 120v.

Both the same wattage, and they are billing in kwh. Correct?

 

[wyliesdiesels]

I previously posted that my water heater was drawing 38 amps, and even as I posted, I thought that was wrong, but that's what the E/V showed. So here's the rest of the story:
copy/paste from the Vue manual:

Step 8 (continued): A note
about multi-pole breakers
If you wish to monitor 2- or 3-pole breakers,
you can either use one sensor on each pole,
or you can use a single sensor. To use a
single sensor, clasp the clamp around either
one of the hot leads coming off the breaker (it
doesn’t matter which). You’ll then be able to
input a circuit multiplier in the app to double
or triple the reading by entering a “2” or “3.”
We don't recommend multipliers for
unbalanced loads, such as subpanels.

So I did that on one 240v leg to the water heater, and used a "2" multiplier in the setup. The display reads 37 amps vs the actual 18.** that my clamp-on digital meter shows.

For a 240v circuit, you don't add up the current on each leg. It's an 18a 240v circuit. That's just under 4,500w....

 

[jlv03]

I wonder if these smart usage meters are needing the X2 setting because they don't know the voltage?

240VAC x 18A = 4320W

120VAC x 18A x 2 = 4320W

If you need hot water and don't have a generator with enough output, an unusual (and slow!) method to get an electric water heater to do something is to connect it to 120VAC. Your 4kW element at 240VAC will be 1kW at 120VAC - not great, but better than a cold shower.

 

[wyliesdiesels]

I wonder if these smart usage meters are needing the X2 setting because they don't know the voltage?

240VAC x 18A = 4320W

120VAC x 18A x 2 = 4320W

If you need hot water and don't have a generator with enough output, an unusual (and slow!) method to get an electric water heater to do something is to connect it to 120VAC. Your 4kW element at 240VAC will be 1kW at 120VAC - not great, but better than a cold shower.

for a resistive load like a water heater, it wouldnt be the same current or wattage. it would be 1/2 the current and 1/4 the wattage...

 

[Will S.]

So I called Emporia Vue this morning, and mentioned this thread, and that when their install instructions are followed, the displayed current draw is incorrect (only on circuits using a single CT and setting 2.0 multiplier). BUT, if Watts or Dollars are selected to be displayed, it's correct.

It's a known software issue; the're working on it, and an app update is forthcoming.

 

[sberry]

for a resistive load like a water heater, it wouldnt be the same current or wattage. it would be 1/2 the current and 1/4 the wattage...

Yas, Si.