Electric tankless water heater using a lot of kw hours

[WILLY TINKLER]

Ok I need someone to answer this question on how much an electric tankless hot water heater uses. If a tankless heater is rated at 24kw, does it mean everytime the hot water is turned on the heater will be drawing 24000 watts of power from the circuit panel? I am no math genius at formulas but if this is the case, just a 10 minute hot shower with the tankless heater will be using up maybe 4kw hours of electricity which is maybe $.50- $.70 added to your electric bill for each shower? Not to mention all the hot water usage through the days of the month....dishes, washer etc. Or is it if the hot water heater is set at lower temperature not all the wattage is drawn? Please advise.

 

[Dana]

Every time the water is turned on the tankless applies just as much power is needed to raise the temperature of the water to the output setpoint temperature. At 0.5 gpm through the tankless it's only using 1/5 as much power as when it's running 2.5 gpm through the tankless.

Electric tankless units still represent a HUGE intermittent power load compared to most other power use in the house. A standard electric, tank draws 4.5 kw for many minutes, even though when the flow is turned on to max it's flow rate out of the tank fpr several minutes heat might be leaving the tank at a rate equivalent to 30-50kw, the power intput to the tank is never more than 4.5kw. With a tankless there is no stored energy to work down, so it needs as much power as it takes, RIGHT now, or else it's temperature drops off. This choppy short but high power draws can be a bit abusive to the distribution grid infrastructure (including the transformer feeding your house) and the power quality to other loads, so they're not a universally great idea.

It takes some pretty beefy wiring to run them too- a 24,000W tankless has to have wiring & breaker capable of supporting 100A @ 240VAC for sustained period of time.

The only good reason to install one would be if there are no other viable alternatives (severe space limitations, etc.) The standby losses of an electric tankless are generally less than 2% of the total power input, but an electric tank's losses is less than 8%, which is still pretty good if you have the space.

 

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[Master Plumber Mark]

Every time the water is turned on the tankless applies just as much power is needed to raise the temperature of the water to the output setpoint temperature. At 0.5 gpm through the tankless it's only using 1/5 as much power as when it's running 2.5 gpm through the tankless.

Electric tankless units still represent a HUGE intermittent power load compared to most other power use in the house. A standard electric, tank draws 4.5 kw for many minutes, even though when the flow is turned on to max it's flow rate out of the tank fpr several minutes heat might be leaving the tank at a rate equivalent to 30-50kw, the power intput to the tank is never more than 4.5kw. With a tankless there is no stored energy to work down, so it needs as much power as it takes, RIGHT now, or else it's temperature drops off. This choppy short but high power draws can be a bit abusive to the distribution grid infrastructure (including the transformer feeding your house) and the power quality to other loads, so they're not a universally great idea.

It takes some pretty beefy wiring to run them too- a 24,000W tankless has to have wiring & breaker capable of supporting 100A @ 240VAC for sustained period of time.

The only good reason to install one would be if there are no other viable alternatives (severe space limitations, etc.) The standby losses of an electric tankless are generally less than 2% of the total power input, but an electric tank's losses is less than 8%, which is still pretty good if you have the space.

A Very good explanation.... Basically Electric Tankless heaters SUCK...
Their are stories about these units coming on and all the lights in all the homes near by begin to go dim because the transformer that supplies the general area cannot handle the load that is going to that one single tankless heater..
so all the neighbors know when you are taking a shower LOL...

Stay away from them....

 

[WILLY TINKLER]

So if these electric tankless heaters suck why are they even made? It seems they are a big waste of energy if they suck out so much energy it knocks out an entire block. Is gas powered heaters any better? Maybe they suck up 100 times more natural gas when you turn them on running up your gas bill sky guy compared to a tank gas heater?

 

[Reach4]

I don't think tankless is good for most people. However you ask some questions.

Electric tankless can be good for people with adequate electric service under some circumstances. Some places have expensive electricity and do not have even moderately priced gas. If you were paying $0.35 per KWH for electricity, that would probably overcome the downsides of dealing with the complexity. An electric tankless water heater will save electricity over a tank unit, but takes more maintenance. Some California rates can get high if you go much above par (baseline). https://www.sdge.com/high-usage-charge

A tankless is smaller. A tankless could be good when the need for hot water is occasional but for many gallons when needed.

Some people don't have natural gas available.

 

[Dana]

So if these electric tankless heaters suck why are they even made? It seems they are a big waste of energy if they suck out so much energy it knocks out an entire block. Is gas powered heaters any better? Maybe they suck up 100 times more natural gas when you turn them on running up your gas bill sky guy compared to a tank gas heater?

They're not a big waste of energy- on energy use they're just fine. They just have lousy hot water delivery and abuse the power grid.

That grid-abuse doesn't "knock out" an entire block, but it can sometimes affect the power quality on an entire block when in use, in much the same way as your neighbor's ridiculously oversized 12 ton air conditioner might. The lights may flicker during the start up surge, but don't necessarily go out.

They're made because people with severe space constraints actually need them. Other people buying them (wrongfully) assume that the energy savings of have near zero standby loss means they're "greener" than a tank type electric water heater. But the additional infrastructure requirements for being able to support a whole neighborhood of tankless electric water heaters is more than what it takes to support a neighborhood of electric car chargers.

A neighborhood can be an issue too, but car charger loads can be scheduled, and controlled by on-board smarts to temporarily turn off when the grid voltage drops too low. A tankless water heater can't be set up to turn off for 5-10 minutes and still provide adequate hot water service, but a pause for a car charger isn't a big deal.

 

[Master Plumber Mark]

They're not a big waste of energy- on energy use they're just fine. They just have lousy hot water delivery and abuse the power grid.

That grid-abuse doesn't "knock out" an entire block, but it can sometimes affect the power quality on an entire block when in use, in much the same way as your neighbor's ridiculously oversized 12 ton air conditioner might. The lights may flicker during the start up surge, but don't necessarily go out.

They're made because people with severe space constraints actually need them. Other people buying them (wrongfully) assume that the energy savings of have near zero standby loss mean's they're "greener" than a tank type electric water heater. But the additional infrastructure requirements for being able to support a whole neighborhood of tankless electric water heaters is more than what it takes to support a neighborhood of electric car chargers.

A neighborhood can be an issue too, but car charger loads can be scheduled, and controlled by on-board smarts to temporarily turn off when the grid voltage drops too low. A tankless water heater can't be set up to turn off for 5-10 minutes and still provide adequate hot water service, but a pause for a car charger isn't a big deal.

very good info.... but you forgot to mention that the electirc tankless also needs constant maintaince to keep the coils from limeing up ..... which they certainly will

In our area the gas tankless seem to be working adequatly, even though all the money you save with one
goes totally back into the de-limeing maintaince every year

 

[Jadnashua]

It takes a certain amount of watts to raise a certain amount of water a certain temperature...doesn't matter if it's trying to do it all at once in a tankless situation, or over time in a standard tank. The biggest difference is in the peak load and the flexibility. In theory, a tankless could provide a continuous stream of hot water...a tank will likely not be able to keep up. But, if a tankless was asked to provide say 10gpm, it couldn't at least with a normal temperature rise, whereas, since you've stored that hot water, a tank would have no problems at all as long as you didn't ask it for more than it had stored up. A typical tank, no required maintenance. A typical tankless, at least annual or you'll notice degradation of operation.

They can make sense for a small public restroom that sees heavy use where there's no space for a tank, the volume is small but constant where a tank would take up room and not be able to keep up unless larger.

If you're space limited, have no other power source, and accept their limitations, they can provide hot water for you, so they do have a purpose. For those growing up with a large tank, it can take some accommodation to its limitations and maybe a change in lifestyle. If you live where the incoming water is warm all year, they can work better than if your incoming water is frigid with fewer limitations.

Expect utility companies to start to charge for your peak demand capability if they don't already, even if your actual average load isn't very high. The whole grid works better when the load is fairly constant. Sharp spikes can mess up the network. Tankless electrical, if widely adopted, could disrupt things.

 

[Dana]

Make that "It takes a certain amount of KILOWATT HOURS to raise a certain amount of water a certain temperature..." , not a certain amount of watts.

A watt is a unit of power, which is an energy rate, not a quantity of energy.

Rates matter here. The quick & dirty analysis for a tankless in this situation goes something like this:

Deep subsoil temps in Robertson County TX run in the low to mid 70sF, but wintertime incoming water temps are in the mid-50s F. A flow of 1gpm is about 500lbs per hour, and showers are typically 2-2.5gpm, at about 105F out of the shower head.

So a 2.5gpm = 1250 lb/hr, x 50F rise is 62,500 BTU/hr. That's an energy rate too, in British unit conventions. Converting BTU/hr to watts (a metric unit) is to divide by 3.412, so a mid-winter shower in Robertson County needs a minimum of 62,500/3.412= 18,317 BTU/hr.

That's 18.3 kw of instantaneous power. With a 24 kw tankless there would be less than 6kw of overhead- better hope nobody turns on the washing machine or even washes their hands while someone is in the shower! Ultra low-flow showers are one solution. Filling at bathtub with 110F water is 10% more temperature rise, and would max out a 24kw tankless at 3 gpm with 55F incoming water. It'll take more than 10 minutes to fill a 35 gallon bathtub in winter with a 24kw tankless, whereas a 50 gallon electric tank could fill it in under 5, assuming you have sufficient water pressure and reasonably sized plumbing runs.