Hot Water Heater Wattage, can it run on less?

[myles]

Hey guys, my question is simple but I can't seem to find the answer.

If my hot water tank has two 3000W elements on 220v, I have a solark 15k so i want the smart load feature to heat the water tank when excess solar is available.

if only 3000w or less or more is available it should still heat the water with the elements just slower correct?

More simply put, do water heater 3000w elements require 3000w to operate, or can they operate with less input just get less hot?

 

[schmism]

They are just restive elements. The amount of heat they put out is directly proportional to the amount of energy you put in them. Burning off the extra solar may result in no noticeable temp difference in the water at low energy amounts.

 

[myles]

perfect, thank you for that! based on my calculations I have 4000-5000 extra watts of solar at peak times during the day, I have this tank in series with my indirect hot water tank (fed from propane boiler)

so every bit of heat rise prior to propane will help cut down on propane usage

 

[400bird]

To clarify, those elements are resistive. Yes they burn off the energy at the rate you give it. But there's solid math there. The resistance of the element is fixed as is the voltage (AC line voltage, likely 240 vac) so you're only going to have on/off control at 3000 watts. You can't make them burn 1800 watts without changing the voltage.
It's just ohms law.

You could rewire them at 120vac. That would cut the wattage down by 1/4. But that's really your only option. Short of replacing an element with one of a different resistance.

 

[efficientPV]

One option would be to put the heaters in series if this is just a pre heat tank. I have a 40 gallon tank in my garage just for laundry and I have the two in parallel because of low voltage. The upper 15 gallons heats to over 120F easily and the lower section acts as pre heater. Tall tanks easily stratify and is the way to go with solar.

 

[robby]

Hey guys, my question is simple but I can't seem to find the answer.

If my hot water tank has two 3000W elements on 220v, I have a solark 15k so i want the smart load feature to heat the water tank when excess solar is available.

if only 3000w or less or more is available it should still heat the water with the elements just slower correct?

Depends. I plan on making a seperate Post on using the Smart Load. The main thing is that it is always going to deliver the 6000W to the heater as long as the smart load requirements are met. The extra 3KW that the PV is not producing will either come from the Battery/Grid.

More simply put, do water heater 3000w elements require 3000w to operate, or can they operate with less input just get less hot?

They always require 3000W if they are run on AC voltage from an Inverter.
The only way you can scale back the power is if you are running them directly from DC voltage from PV panels.
This method comes with it's own set of problems.

 

[wattmatters]

if only 3000w or less or more is available it should still heat the water with the elements just slower correct?

No. If you are supplying a heating element with regular 220 V AC power then a 3 kW element will draw 3 kW. It will be on or off, no in-between.

The only thing which can vary the power draw is changing the nature of the power supply, either varying the AC voltage and/or the AC waveform.

If your system is unable to supply 3 kW, it will overload.

If your system is capable of blending PV output with grid power, then it will use all the available PV power and supplement what it needs with grid power. If your system uses batteries then it will use those to supplement the power demand.

But the element will draw 3 kW.

There are however smart diverters which are capable of modifying the AC power supply presented to the water heating element. They work using various methods such as burst fire or PWM control. In these cases then the power the heating element operates at can indeed be controlled anywhere between nothing and its power rating.

I have such a device for my hot water storage tank. Mine is a single 3.6 kW heating element in a 315 litre tank (we are 240 V AC here). The smart diverter monitors how much excess solar PV (IOW what is being exported to the grid) and it adjusts the amount of power being sent to the water heating element so as to avoid importing power from the grid. If by the end of the day the water heater's thermostat switch has not opened (indicating it has not completed a heating cycle), then the smart diverter waits until my off-peak energy is available late at night to perform a boost completing the heating cycle then.

I've not had it in operation for a full year but I expect the 90% of our hot water power will be from our solar PV, with 10% being off-peak grid supplemental.

If using a basic timer controller, or an integrated inverter control which monitors PV output (or grid exports) to determine when to switch the water heater on, then it can help to use a lower power heating element as this means the solar PV can begin diversion earlier and finish later. The heating cycle does take longer as well but in general you will squeeze a bit more from the PV supply this way and require a bit less supplemental energy. The exact maths is obviously going to vary depending on your specific hot water consumption, PV output, location etc etc.

Not sure what sort of elements are available in the USA but here we can choose from 1.8, 2.4, 3.6 and 4.8 kW heating elements.

 

[Spunky]

I wired mine in series too,
I pulled the wire out of the T4 terminal and the blue wire next it then wire nutted them together.
That pulls about 2400 watts. I have the SmartLoad set to start when my batteries get to 99% SOC.


I wish the SmartLoad was smart enough to not take from my batteries when a cloud passes (one storage to another).

 

[wattmatters]

The only way you can scale back the power is if you are running them directly from DC voltage from PV panels.

Or by modifying the AC voltage and/or waveform. This is what smart PV diverters do (for resistive loads only). I have one and it varies the power sent to the heating element based on available excess PV output.

 

[Ampster]

I made the investment in a Heat Pump Water Heater. It uses about a quarter of the energy to heat the same amount of water that a resistive element does. The other thing that they do is they can be programmed to heat your water when rates are low or in the afternoons when I typically am sending power back to the grid. In California with high rates, I recovered the additional cost within three years and after that the savings keep coming. They may not work in every environment but a HPWH worked for me. It is a different equation off grid but having a load of 500-800 Watts would be a whole lot easier off grid than a 3000-6000 Watt resistive element.

 

[Mike 134]

They are just restive elements. The amount of heat they put out is directly proportional to the amount of energy you put in them. Burning off the extra solar may result in no noticeable temp difference in the water at low energy amounts.

This is the correct answer regarding the hot water elements only. Being a resistor of fixed ohms the less voltage you give them the less heat they create (ohm's law). Any value between 1 and 240 volts will work for example, giving you no heating at 1 volt to the full 3000watts at 240V. Your hot water tank controls, however, will need the voltage they were designed for. The rest of your system supplying the voltage to the hot water tank resistors (heating elements) I'll let others offer their advice.

 

[Ampster]

This is the correct answer regarding the hot water elements only. Being a resistor of fixed ohms the less voltage you give them the less heat they create (ohm's law). Any value between 1 and 240 volts will work for example, giving you no heating at 1 volt to the full 3000watts at 240V. Your hot water tank controls, however, will need the voltage they were designed for. The rest of your system supplying the voltage to the hot water tank resistors (heating elements) I'll let others offer their advice.

Great theory but by the time I figured out a way to vary the voltage, a HPWH is looking pretty simple. The other thing I did not mention is you can control it remotely so if you go on vacation, while you are waiting for your flight or stopping for coffee, you can remotely turn it off for the duration of your vacation.

 

[LydMekk]

Simple solution: put a cheap timer on the outlet where the tank is plugged in and atleast force it to only heat during the hours you want it to. F.ex. during daytime with sun out instead of nighttime. The water heaters do not need to be run 24/7.

 

[Rednecktek]

So just as a thought experiment, and a cheap way to fid out, what would happen if you connected a regular heating element to a DC source? Would it get warm? If you connected it to, say for example, a basic old-school 6a 12v battery charger, would it just get to 72w of heating or would the battery charger explode?

It might be a fun, and relatively cheap, experiment to run.

Also, since Wind and Hydro both call for diversion loads to prevent overspeed, could you just use one of those as an element control?

Ponderous man, really ponderous...

 

[740GLE]

While I also love our hybrid hot water heater, sounds like the OP is using excess PV generation for an existing water heater, vs wanting to buy a new HWH.

 

[Ampster]

Simple solution: put a cheap timer on the outlet

Good thought but most water heaters are hard wired. It is still a resistive element and the savings would be hard to figure if the consumption of hot water was the same every week.

 

[Ampster]

So just as a thought experiment, and a cheap way to fid out, what would happen if you connected a regular heating element to a DC source?

I did use a water heater element as a load when testing LFP batteries. As noted above the higher the voltage, ie when testing several cells in series, the hotter it got. At one point I had to put it in a glass of water or insulate it when the voltage got over 12 or 24 volts. Unless your DC source is current limited it could trip a breaker. Ohms law is quickly obvious as the voltage is increased the current also ramps up.

 

[Ampster]

sounds like the OP is using excess PV generation for an existing water heater, vs wanting to buy a new HWH.

I know. I was suckered into the discussion by the title phrase, "...can it run on less?". To a HPWH fanboy like me that is a teaser headline that begs a response. I was not trying to change the OPs decision, I was trying to inform other readers that conservation always has the best long term payback.

 

[Rednecktek]

Unless your DC source is current limited it could trip a breaker. Ohms law is quickly obvious as the voltage is increased the current also ramps up.

Which is another question I've always wondered about. How DO you current limit a source? Why doesn't a dead LFP battery blow up a 20a SCC trying to draw 100a of recharge? I know fuses are either on or off, but how would one limit the current to a load that can draw WAY more than can be provided?

Either way, it'd be a fun experiment and since any power produced over 100% charge + load is just wasted, a shunt controller heating water would be handy for a lot of folks I think.

 

[realpinochet]

I made the investment in a Heat Pump Water Heater. It uses about a quarter of the energy to heat the same amount of water that a resistive element does. The other thing that they do is they can be programmed to heat your water when rates are low or in the afternoons when I typically am sending power back to the grid. In California with high rates, I recovered the additional cost within three years and after that the savings keep coming. They may not work in every environment but a HPWH worked for me. It is a different equation off grid but having a load of 500-800 Watts would be a whole lot easier off grid than a 3000-6000 Watt resistive element.

Thanks for that input, I am looking at heat pump water heater vs on demand. I'm also looking at heat pump AC units would you have any experience with them vs the old school compressor types?