I previously recommended the use of a controller for your water heater as a way of controlling power consumption. Now that I have the solar up and running, the Tesla app is a great way to monitor and check up on your electrical loads.

Now that I have the ability to do this, I am convinced that the Aquanta water heater controller doesn’t actually DO anything. Whether I am home or not, using hot water or not, the amount of electrical power that the Aquanta is claiming I am using to heat water doesn’t change, at about 15kwh per day. The only time the amount of power the Aquanta was claiming went down is when we put the system in “Away” mode- telling it to turn off the hot water while we were out of town. At that point, the Aquanta reported that we were using about 1 kwh per day. The only problem? The amount of power we were using for the entire household didn’t change, according to the Tesla app.

At this point, I began suspecting that the Aquanta is a random number generator. It reports to you how much money it is saving you, but isn’t actually doing anything. I decided to test it and see what was going on. I am going to make a serious accusation here, so let me state that I did all voltage readings using a FLUKE 115 True RMS multimeter.

I measured the voltage across the output of the Aquanta while it was in “efficient” mode with a green LED showing and got a reading of 249 VAC. The water heater uses power consumed of 4500 watts (4.5kw), so a little application of Ohm’s law results in 18 amps and 14 Ohms of resistance. We can directly use Ohm’s law because the water heater has no capacitance or inductance, it is a purely resistive load.

Then I put it into “Away” mode. The LED on the unit turned red, so I measured the output voltage and got 70 VAC. Another application of Ohm’s law using the same 14 Ohms we calculated before tells us that the water heater is now using 5 amps and 50 watts of power. Take that 50 watts of power and multiply it by 24 hours in a day, and you get about 1 kWH per day.

So it doesn’t shut off the water heater, it merely lowers the voltage from line voltage down to about 70 VAC. I am not sure when or how it does this, other than in vacation mode, but I don’t think I am saving any power.

I just don’t think that this is a good unit to use if you want to save power, so I am withdrawing my recommendation of the Aquanta unit. My recommendation is to either use an electro-mechanical water heater timer, or a smart pool pump relay.

Categories: Electric and Power

Mr Lintz · July 30, 2024 at 4:45 am

Why not simply turn off the power to the water heater?

Divemedic · July 30, 2024 at 6:07 am

The idea is to have the place run the same whether I am home or not.

wojtek · July 30, 2024 at 11:37 am

Because in the ideal world you’d like to have hot water when you return home. And it can take up to a couple of hours to heat it up after you turn the power back on. That still could work for longer vacations, because it’ll take you some time to bring in the luggage, unpack, check mail, turn other systems on as needed, make a cup of tea, etc. But when you go to work, or just leave the house for a couple of hours, it’s more of an inconvenience.

D · July 30, 2024 at 9:17 am

Do you think it’s a bad unit or are you thinking the company is trying to pull a fast one?

Divemedic · July 30, 2024 at 10:26 am

If it were a bad unit, I would think that it wouldn’t be reporting how much it’s saving you each month.

Noway2 · July 30, 2024 at 12:09 pm

I think you may have a math error in your calculation. You start out correct with your analysis with a measurement of 249 Volts (RMS) and using Ohms law where power = V^2 / R and the name plate power rating of 4,500 gives 249*249 / 4500 = 13.78 ohms, which we’ll call 14. Now, when you put the device in away mode, the voltage drops to 70. Again power = V^2 / R says power = 70*70/14 = 350 watts. Now power also equals voltage * current so we have 350, which does correspond to 5 amps, which would be 8.4 KWH for a 24 hour period.

However, there may be another factor in the water heater turning off upon reaching temperature set point and then turning back on at a lower point. I have to wonder if the temperature setpoint is changed or if it just drops the voltage, which means it will run longer to achieve set point but the overall amount of power consumed is the same; you’re heating the same water volume the same temperature delta, just at different rates.

I find your experiment interesting in light of the fact that we went a different direction in terms of trying to save on water and energy. Our water rates are rather high, making conservation of water a goal. We went with a propane on demand water heater with a recirculating system that, usually as it sometimes goes to sleep, gives us hot water at any faucet within 5 seconds (we have a long ranch style house and water from the heater would take a lot longer than 5 seconds without the recirculate). Obviously, there is a cost for the recirculate in terms of electricity and propane, but how does it compare to the costs of having an electric tank being kept warm?

I also did a little searching and it looks like a gallon of propane is equivalent to about 27KWH of electricity. Our electric rates run about 11.661 cents per KHW which means that a gallon of propane will cost about \$3.13, and I do believe ~3 is the last price I paid for propane, meaning they’re roughly equal in terms of fuel costs.

Divemedic · July 30, 2024 at 2:08 pm

How would this device change the thermostat setpoint, since the thermostat on an electric water heater is a mechanical thermostat that is a part of the water heater itself? As far as the total amount of energy it takes to heat the water, you are forgetting that the hot water itself is losing its heat to the environment at some unknown rate. This means that, if the power you are adding to the water is less than what is being lost to the environment, you will add that power indefinitely without increasing the temperature of the water. I think that is what is happening here.

Noway2 · July 30, 2024 at 4:42 pm

Good point on the mechanical thermostat, I wasn’t thinking along those lines and thinking more of a controller for the heater.

You may be on to something about the heat loss versus energy being supplied. Knowing a few things like the surface area of the tank, the outside temperature and water temperature (your delta T) you could probably get a rough idea of the energy transfer using either newtons law of cooling or for a more complicated answer use Fourier’s heat transfer equation.

wojtek · July 30, 2024 at 7:51 pm

Not without knowing the HT coefficient of the tank.

Wayne Johnson · July 30, 2024 at 1:33 pm

Measuring voltage doesn’t actually tell you that much. There are multiple ways to measure that.

Do you have the ability to measure current (amps)? If so, look at the amperage pulled from the breaker for a far more accurate picture of how much power it is consuming (measured amps * 240 volts).

Divemedic · July 30, 2024 at 2:13 pm

The nameplate on the water heater gives you the amount of power that is being consumed by the water heater at the design voltage. Since the water heater is a purely resistive load, the formula is pretty straightforward.

Since power is voltage times current according to Joule’s law, it follows that current is power divided by voltage. Therefore, 4500 watts divided by 249 volts equals about 14 ohms.
If we change voltage in a resistive circuit, the resistance remains unchanged. This makes it possible to calculate the new power consumed by multiplying that resistance times the new voltage.
All you need is two variables to calculate the third, missing variable.

Wayne Johnson · July 30, 2024 at 2:43 pm

You are correct on the power being consumed by the water heater. The problem is that does not tell you how the voltage is being dropped.

There are only a few ways to drop the voltage to the water heater from 240V to 70V:

1 – They can use a big resistor (about 35 ohms). This would end up heating up part of the house. It would reduce power to about 1300 watts total (from 4500). Under 1/3 of that power would actually be spent heating your water.

2 – They can use an inductor. This would dramatically reduce the power wasted, but it would also result in a pretty horrible power factor.

3 – They can use a capacitor. This would dramatically reduce the power wasted, but (just like the inductor) it would result in a pretty horrible power factor.

4 – They can reduce the duty cycle (turn the water heater on and off quickly to approximate lower voltage). This is the highest-tech version, but it requires a couple of high-power switching devices (probably either SCRs or TRIACs).

Honestly, you are less concerned with how much power is saved by the water heater and more concerned with the change to the total draw from the house, hence my suggestion to measure current at the breaker.

Divemedic · July 30, 2024 at 3:06 pm

There are other ways to reduce voltage. You can use high current switching diodes, Zener diodes, MOSFETs, those sorts of things. Perhaps they have a relay that only opens one leg.

I would be curious to see how they do it, and once I uninstall this unit, I may take it apart to see how they do it.

Wayne Johnson · July 30, 2024 at 6:03 pm

Does the run from your panel to this device have a neutral wire? If it only has 2 hot wires and a ground, opening either leg turns it completely off (and so doesn’t work to reduce voltage).

Regardless, it is important to include the device in your power comparison, since its losses partially offset the energy savings. In some ways, energy lost there is worse, since it does not heat your water and needs to be removed by your AC.

How much heat does the unit generate when it is reducing power (does it have a fan and/or exposed heatsinks)? If it generates a lot of heat, that hints that it is less efficient than it is advertising…

Divemedic · July 30, 2024 at 7:19 pm

No neutral, but remember that it depends on how the circuit is being broken as to what voltage is there. For example, if a half wave rectifier is being used to to control voltage, the voltage would be divided by the square root of 2 (or it may be the square root of 3, I don’t remember.)

Wayne Johnson · August 2, 2024 at 2:25 pm

They had better not be using a half-wave rectifier feeding that large a load. That would introduce a significant DC offset to the current draw from your utility transformer, which has a bad habit of saturating the core and causing them to overheat…

Looking at the website for the device in question, I am a bit surprised you are measuring a lower voltage. It looks like a 4″x4″ plastic enclosure with a few electronics in it, meaning it cannot dissipate any significant heat. The only thing it can really do is switch the load on and off based on that thermal design.

I am going to bet that it contains one or more TRIACs or SCRs along with a little bit of control circuitry.

I strongly suspect that the data being claimed by the app is … optimistic … at best.

Another interesting thing to check would be the frequency (Hz) being delivered to the hot-water-heater when the device is reducing power. That would tell us quite a bit about how it operates…

Divemedic · August 2, 2024 at 5:53 pm

I didn’t think of measuring freq. I will see about that.

Dan D. · July 30, 2024 at 6:06 pm

Definitely on the triac, my thought exactly. They’re essentially free these days and you don’t get the hum from a heating element in water like the old wall dimmers of the 80s did to incandescent lamps.

You’re right that measuring clipped AC voltage won’t tell the whole story, DM needs an integrating power meter for a solid analysis.

Divemedic · July 30, 2024 at 7:20 pm

That’s why I am using a true RMS meter. It calculates that. The need for true-RMS meters has grown as the possibility of non-sinusoidal waves in circuits has greatly increased in recent years. Some examples:

Variable-speed motor drives
Electronic ballasts
Computers
HVAC
Solid-state environments
In these environments, current occurs in short pulses rather than the smooth sine wave drawn by a standard induction motor. The current wave shape can have a dramatic effect on a current clamp reading and will cause clamp amp meters to be incorrect. In addition, a true-RMS meter is the better choice for taking measurements on power lines where ac characteristics are unknown.

Reference: Digital Multimeter Principles by Glen A. Mazur, American Technical Publishers.

Aesop · July 30, 2024 at 1:44 pm

The multimeter isn’t lying. So obviously the company is.

WallPhone · July 30, 2024 at 5:10 pm

These videos are perhaps required watching for people who want to tweak their heater efficiency: https://youtu.be/Bm7L-2J52GU

There’s a “part two”, but I’ll respect my single link per comment limit and let you find that.

JC · July 30, 2024 at 5:54 pm

I didn’t catch in your original post if you were using a trad resevoir type water heater or a demand type. That is, are you keeping a barrel of water hot (constant demand) or, as the name implies, using hot water only when you’re using hot water?
(My VOM is an Amprobe RS3, with a needle and a dial to change the scales of measurement. I like it, I trust it, AND I have a spare.)

Divemedic · July 30, 2024 at 7:15 pm

Fluke makes some of the best meters, and the only RMS meters that I am aware of. When I did electrical stuff full time, I also had a Fluke Scopemeter and a Tektronics O-scope.

Rick T · July 30, 2024 at 9:07 pm

15 kw-hr a day is a LOT of energy, a bit over 50,000 BTUs, and a BTU is defined as the energy to raise the temperature of 1 pound of water 1 degree F. Your water heater is supposed to be consuming enough energy to heat 2,500lbs of water 20F every day.

Are you using over 300 gallon of hot water each day?

Divemedic · July 30, 2024 at 9:23 pm

No. I don’t think that’s accurate. Yesterday the entire house used a total of 43 kwh, and the Aquanta is claiming that 14.3 kwh of that was for hot water, with the other 28.7 kwh running two air conditioners, the stove, lighting, the washer, the dryer, and every other load in the house combined. I can’t believe that a third of my electrical usage is heating water.

Noway2 · July 30, 2024 at 9:55 pm

If you trust the power wall, and yes, when you have two meters d which is correct is a question, shut the water heater off for a day and see what it registers as the difference. That should give you an idea of what you’re using for water usage. I too have a hard time believing that 1/3rd if your usage is water heating in summer, in Florida. The ground water temperature is not likely to be that large relative to the other loads like cooling.

Divemedic · July 31, 2024 at 6:36 am

I trust the powerwall because it agrees with the readings from the power company meter can.
It’s a “tell me three times” system with one disagreeing with the other two.

Tom762 · July 31, 2024 at 7:59 pm

The Fluke true rms meters have a min max function that might give you more information. Hook the leads in parallel with the load and see what happens over the course of a few hours. Check it on both sides of the controller and it may show you if it actually doing something or not. What ever mode of function, it will show a difference, if there is one. Fluke makes some truly awesome products!

Get Freight · August 1, 2024 at 1:19 am

Looking at ohms, volts, etc. Still does not explain (or maybe I missed it) the technique it uses to minimize power use.
The tank has ~ 60 gallons of water, heated to say 100F. A normally operating tank will reheat sitting water once it cools to, let’s say 90F back to 100F. The energy required to heat 60 gallons 10 degrees is consistent for our purposes. Call it 4998 btu
So, if while I’m in normal.mode I heat 60 gallons. The system cycles every 10 degree loss and uses a total of 4998 btu. Thermometer stays the same.so.it cycles once an hour to maintain temp. 24 hours = 19952 btu’s total
We’ll round to 20K BTU
In second scenario, same exact equipment but we turn off at the beginning of the 24 hours and keep ot off until the end of the day. Where I am tap water is around 40 degrees. So, wr have to heat 60 gallons ~60 degrees to get back to temperature of 100F
Would be 29,998 btu used. We’ll round to 30K BTU used

So, maintenance to.hold at temp.roughly.in the 24 hrs uses 20K BTU. Letting the temp drop to the environmental average (~40degrees) the heat water all.at.once back to 100F at.the end.of.the day takes 30K BTU.
You can ay with the sequence and timing but basically maintaining the temp takes.less energy based on those approach.
The only way to save usage is extended periods (multiple days) of no heat and heating when ready to be used. Which is.not useful. Extra insulation to retain captured heat seems a better strategy. But not by.much.

Rick T · August 1, 2024 at 5:54 pm

30K BTUs is 8.8 KW-hr for the reheat, but this is in Florida so even groundwater will be coming a lot higher than 40F. If the heater is in a utility space it is going to be 80F or higher during the day now, reducing the ‘reheat once a day’ requirement even more.

This Aquanta box seems to be plucking it’s numbers out of the air and hoping you never cross-check its claims.

Anonymous · August 1, 2024 at 11:08 am

Since some moron of an architect decided putting 2 50 gal water heaters up in the attic was a clever idea, we swapped them for a demand hot water system.
After 8 years, we freaking still love it.
Only drawback is you must have water conditioning & they run on NatGas.
I kinda like not having 100 gallons of hot water sitting above the ceiling too!
CC

Load Shedding – Area Ocho · July 30, 2024 at 4:29 am

[…] IFTTT. For the water heater, you can get a smart water heater controller like this one (EDIT: there is an updated post on this water heater controller. Read before you buy), and use IFTTT to integrate it to your backup system, or you can use a smart […]