re: Tank or Tankless Hot Water Heater

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Not sure why it wouldn't be 1/2 but I had hot water during the Great Freeze and DGAF about how I got it.

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It has to do with RMS or "root-mean-squared" voltage from an AC power source. The load does not see the peak voltages over and over again. It sees the RMS voltage and when that voltage is reduced, it follows a specific formula. More info here:

LINK

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I had a tankless electric and you're SOL if needed hot water on anything but a large generator. iirc it was a small one and it drew around 50amps. Goes to show just how much energy is needed to heat water.

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Any time electricity is used for resistive heat, tons of power is used. A resistor heats up as electricity is passed through it and a lot of juice is required to generate that heat. 50A being pulled in that instance is not surprising and for most homes would require a new panel and potentially a new service feeder.

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Seems like when I looked up info on it after the fact, a website said running 120 on a 240 water heater would use 1/4th of the power and thus take 4x as long to heat the water. Not sure why it wouldn't be 1/2 but I had hot water during the Great Freeze and DGAF about how I got it.

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120V @ 15A is about 1800W. A typical 240V electric water heater element runs around 4500W with some up to 5500W. That's a big difference in thermal output. That does pale in comparison to a larger electric tankless unit though, I installed a 36kW Rheem for a friend and that required four dedicated 40A breakers and four 8/2 cables. You could hear the breaker panel hum when that thing came on.

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Now that’s a lot of juice!

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Yeah, pretty much the big downside with them is how much power you have to run. Otherwise they are great for POU, installed a small unit in a laundry room cabinet just for the washer and sink. I've installed others as hot water boosters for tubs and showers because that's the other weakness for any tankless is the temperature delta they can achieve. If you feed them water that is already fairly warm then even a small electric unit can supply plenty of hot water.

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fyi, In a bind, I've hooked my electric tank up to 120v on my generator and it worked. I don't remember the amp draw but it wasn't a big generator. Seems like when I looked up info on it after the fact, a website said running 120 on a 240 water heater would use 1/4th of the power and thus take 4x as long to heat the water. Not sure why it wouldn't be 1/2 but I had hot water during the Great Freeze and DGAF about how I got it.

I had a tankless electric and you're SOL if needed hot water on anything but a large generator. iirc it was a small one and it drew around 50amps. Goes to show just how much energy is needed to heat water.

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Point of use electric water heaters are mostly 120 volt devices that will operate on a 20 amp circuit. They are only good for low flow rates. Whole home tankless water heaters can be as high as needed but most are around 120 amp 240 volt circuit. That will make your meter flow off the wall and run down the street....but only while in use,

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Point of use electric water heaters are mostly 120 volt devices that will operate on a 20 amp circuit. They are only good for low flow rates.

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even my 240v "whole home" tankless electric which, iirc, drew 40-50amps would not keep up with my shower when using it in the dead winter months. I'd have to not turn the water on all the way if I wanted a hot shower.

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even my 240v "whole home" tankless electric which, iirc, drew 40-50amps would not keep up with my shower when using it in the dead winter months. I'd have to not turn the water on all the way if I wanted a hot shower.

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We are building our retirement home from a design I did myself. It will have a tankless water heater (propane) in it BUT the plumbing manifold will be turned up inside a closet big enough to house a standard 40-50 gallon electric water heater so someone could easily change over without having to do a lot of plumbing. We decided to go tankless because 90% of the time it will be just the 2 of us and we will, if health allows, be living in the house about half the time for the first 6-10 years of retirement, and away from home for extended periods of time. IN that scenario it makes sense....I am not sure it does with a couple of teen daughters showering simultaneously and the washing machine running 24-7.

I mean I've hooked 120 to it from my generator before and it worked. My electric tank does have exceptionally low standby losses, (heat loss) due to the nature of an electric WH, an additional blanket, and the way I've plumbed it. Pretty confident even if I only put a 100w into the heater that it would heat to full temp within a day or so....... Now that I've said that I've realized that should be pretty simple math:

incoming water temp of 73* raised to 120* so a delta of 57* and a 40gal water tank would take about 19000/btu which is 5500w. That is of course efficiency at 100% with no heat loss....which is probably pretty close for my setup. But with only 100w of input that would deliver 2400w/hrs per day so just over two days to bring it to full temp with no one using any hot water and no losses. Probably not good enough and the delta will be a lot greater if outage takes place in dead of winter. Oh, and odds I f;up the math somewhere near 66%.


Since I've taken the day off for the election ( went to bed at 3am) I think I'll fully drain the tank, fill it and record (as best I can) the incoming water temp, and put 120v with a kilawatt meter on it to see short term and long term consumption. .....Also might dick around with it and do other more pressing things

in that scenario yes a gas on demand tankless heater would work just fine. I have a 6gpm propane tankless that runs a sink, shower and bathtub just fine that all get daily use

A generator will run it.

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My water heater works when the power goes out. Tankless won’t do that.

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Mine does. Tankless will run on small battery backup or a generator if it’s natural gas. Just need electricity for the igniter. Wife is still amazed she can take a bath when we out of power for days.