Tankless size/capacity is based on what temperature rise is needed to reach the desired target temperature from the lowest input temperature that occurs (during coldest part of winter season), at the needed flow rate. Tankless unit specs for any given model state how much rise it can produce at X GPM flow rate.

Showers typically run at 2.5 GPM (some water saver designs may be less) and the target temperature is generally accepted to be 105°F ... but that may not feel very warm at a sink faucet, and some people may want a higher shower temp. Running the shower simultaneously with one sink faucet (or both) entails a higher flow rate.

A rainfall shower head or a body shower installation with multiple spray heads requires higher flow rate ... in case you're planning anything like that.

Turning the shower and/or faucet down to a lower flow rate increases the temp rise capacity ... but you can't necessary expect guests to be accepting of that.

Find models that meet your requirements, and check what electric circuit size is needed ... it may be heftier than you expect, and your service drop may not be accommodating without an upgrade. You shouldn't need one as large for your limited application ... but as an example an electric tankless large enough to handle a whole-house load typically takes at least a 120 amp circuit, which is FOUR TIMES more power than a typical electric tank unit. It doesn't always need to pull that much power but the circuit must be able to deliver that much when needed.

We have a small guest unit with a tankless heater that works fine. It runs a washing machine and a full bathroom. You will need at least 80 amp breakers minimum if I recall correctly, but as stated by dadoes, this depends on your supply water temp.

Look at Stiebel Eltron units and always plumb it with a cleanout setup. There are online videos regarding this.

A vacation home is unlikely to have sufficient electrical service to add any electrical tankless. Most people don’t want to upgrade the entire home’s electrical service just to get hot water. What’s your panel’s capacity?

Do you not have gas? Gas tankless would be the better choice.

dadoes has the numbers right.

Just a clarification - we design for 105 water temp - but that's what it gets mixed down to, not what the heater is set to (usually 110-120 F). If someone wants a hotter shower or hotter water at the sink, they have that option. It doesn't become an issue unless they are at the limits of the heater output (coldest inlet temps and max amount of gpm). At least on the gas units, the heaters will throttle the water volume to maintain the set temp.

At the same time - The Cook's Kitchen is not wrong when she says that electrical service is an issue.

There is potentially a better answer: use an electric tank water heater and switch it off when there are no guests. There are "smart controls" for water heaters that would let you control it from your phone, etc. if that's an issue.

We have propane but I wanted to avoid trenching from the main house or adding an additional propane tank if possible. The house is In Virginia and it is doubtful it would be used much in the winter. Frankly it would not be heavily used much at all. As for electrical I don’t know too much about the capacity, but I do know I have lots of space on my panel. ...I actually have 2 panels.

And let me reiterate....it’s not a whole house situation....this is a guest suite with only a single shower plus two sinks. I can’t imagine anyone needing to wash their hands in hot water while someone is showering. I feel like their must be a tankless electric capable of heating a single shower to a reasonable temperature that will fit on my panel...which has lots of room.

The number of empty slots in your panel has nothing to do with the service size. Your contractor should be able to advise you on what is possible. Second guessing hiim is just gonna cost you money.

Okay Branson...what does determine it and how do I find out what it is? My contractor hasn’t seen my panel...we are just in the design stage.

Service size is the maximum amperage that can be fed from the utility service pole into your your main panel, which is usually where the meter is located. That panel typically has a master breaker that shuts off the entire house. The rating on that breaker is an indication of the service size. A secondary panel (sometimes more than one) may be located elsewhere with breakers for the individual branch circuits.

We put in an master bedroom n bathroom a few years ago and put in a tankless. Never had a problem with it and we like long hot showers. Also have radiant floor heat.

Yeah, number of slots in your panel is like number of checks in your checkbook... it' has nothing to do with the balance in the account.

Assuming you don't want to take my brilliant advise and use a standard tank heater...

First determine what your needs are: you will need a shower (2.5 gpm and possibly a sink 1 gpm) = 3.5 gpm. And let's just say that you will need to go from 55 degree F water to 105 F (temp rise of 50 degrees). A heater to do that would be either an 18kw unit (2.5 gpm@ 49 degree rise - almost but not quite) or a better fit would be a 24 kw unit (3 gpm@55 rise).

The 24kw unit would require 100 amps and would need 3 - 40 amp two pole breakers.

Standard electrical service is 100 - 200 amps. If you had a 200 amp service, it's not likely you would have enough capacity for half of that to be allocated to heat water in the guest bath.

You would need to know the size of service you have (check the main breaker) and you would need to know how much of that you are using already (an electrician can help you with that).

Note that you may have "peak usage" billing also - if you do, a tankless electric water heater can send your electric bill through the roof.

Again, I would suggest a standard tank heater and just turn it off when not needed.

I may take your advice about a standard heater, I’m just trying to get the best fit. No peak usage billing.
If a 100-200 amp service is normal and you need half of that or all of that to heat for one shower, it’s a wonder anybody has a electric tankless system at all. Do all these other people with electric tankless water heaters have 1000 amp service to their homes? How are these companies marketing their products if nobody normally has the capacity to use them? I must be missing something.

They have to install 300 or 400 amp service - and that's just crazy stuff!

Older homes with mostly gas appliances will often have 100 amp service. Newer homes were getting 200 amp service standard. Very large homes with lots of mechanicals might get 300 amp service.

The service required for whole house tankless doesn't make a lot of sense for sure.

BTW: I said "half of a 200 amp service" you aren't allowed to size your usage at 100% of breaker rating - so that's not really accurate, but you get the idea. Normally you design for 80% or less of the circuit ampacity,

My house (built in 2004) has 200 amp service, a 5-ton heat pump with accordingly-sized auxiliary heat strips, water well, aerobic septic system, and electric tankless on a 120a circuit. I haven't any exhibited electrical issues.

The factors in this discussion are oriented toward worst-case scenario .. which of course must be accommodated ... but electric tankless does moderate its current draw to fit the load of the hot-water task at hand. Operational characteristics will of course vary depending on
environmental location and incoming water supply temperature range, as
well as usage habits.

I may reach maximum capacity for example if I raise the setpoint to 140°F for zapping a load of whites laundry during winter weather (it doesn't get quite that high during summer weather). Showering at 103°F, I've seen the tankless read low as 8% of capacity during summer weather.

I was not involved in construction of the house, the builder's decision to install electric tankless. There is no natural gas service in this little subdivision outside town so the alternative would be propane. Both bathrooms have whirlpool tubs, which a "normal"-sized tank water heater couldn't handle. I've not had occasion to fill them simultaneously but I've filled one with 112°F to 115°F water in winter weather (with the heat pump running).

Contact your local power company and ask them if they have a program to review your home's electrical use efficiency and ask about the potential use of an electric in line tankless water heater for your guest suite as a part of that review. Ask about any cost involved for the review.

Thanks for input dadoes... I feel like my situation is probably the best case situation possible...a auxiliary guest place at a mostly summer used vacation home in a mid Atlantic location...yet I’m being told no...this is not possible. Yet comments from other users seems like it is certainly possible. And I feel like if it’s not feasible for this situation what situation would it possibly work for...only a cup heater in the tropics? I will take a no if it’s the true answer, but goodness if my situation means no, what possible situation would work for a shower in a mostly mild climate one shower situation?

When I researched, no one seemed to be a fan of electric tankless water heaters (vs. gas). Even the solar-panel salesman said it’s better to use gas. (FWIW, since it’s mostly summer use, maybe a solar heated water tank—I think the tubes sit on the roof). Sounds like you have a lot of site-specific issues to consider. Maybe time to bring in a local contractor and ask their opinion? Best of luck.

You will never save enough in costs over a tanked to justify the added infrastructure increase needed. While anything is possible with enough money and stubbornness, the point of some things isn’t money and stubbornness. Then again, sometimes, for some people, that’s IS the point and they’ll spend anything to prove their idea to be right. The 10K flyswatter.

Reread the original post. Since you’re building new, have you considered adding a very small kitchenette, so guests could make their own coffee in the morning or have snacks at night. maybe a mini-fridge, mini-sink, small counter space, one cupboard?

I think this is being asked in the planning stages and so hopefully the OP will make sure about all of this. Where I live electric is cheaper than gas so most of us have electric hot water tanks. Tankless hot water for a tiny spce with a shower should not break the bank or out a huge starin on 200AMP service IMO.They are used all over Europe because they are energy efficient and take up very little space.I have no idea why someones electric bill would sky rocket with the install of a tankless. BTW They are not efficient if they have to serve long runs between spots where the water is used. I think it is a great idea for a guest cottage. A quick check by a good electrician will tell you if your service can handle what you want.

With tankless, because water is heated quickly and only when it's needed, it's more energy-efficient. In fact, our condensing models start at an energy factor of 0.94, which means 94% of the fuel is being used to heat the water and only 6% is wasted.

Looking at my panel, there is a breaker at the bottom with a 200 on it so I am assuming that means I have 200 amp service. Girl wonder - yes, when I posted about a double vanity, I actually will have one sink in the bathroom and one on the other side of the wall in a coffee bar.
I’ve been skeptical of some of the local contractors because I’ve worked with many before...this is in the boondocks. I’ve had valves put on the wrong side of filters, I had to explain the product Trex to one who hadn’t heard of it.
I’m not an idiot— live wire oak—if I have to change my infrastructure significantly with significant costs I’m not going to do it. So this is why I’m asking the questions. “We’ve always done it that way” is just as stupid and stubborn as change for change’s sake.
It may be that I’ll have to go old school because it makes more sense. Or a self contained tankless electric may be the best solution.

Okay, so I supplied you with the numbers and the assumptions that got to those numbers. It's possible that some of the assumptions (like the cold water temp) is wrong... but what we designed for was 3 gpm (slightly more than a shower) and 50 degree temp rise.

The heater to do that has a max draw of 100 amps. That's all math.

You may only load a circuit to 80% of its rated capacity. That's code.

Now, if you are able to run this heater on a 200 amp service depends on the rest of your power usage. You have 60 amps left to work with. It's borderline.

Patricia Colwell Consulting is not correct. An electric tank heater and an electric tankless heater are both almost 100% efficient at heating water. An electric tank heater has standby losses of about 3% in normal usage.

The issue is that any tankless heater must heat water very quickly. That requires a lot of energy over a short amount of time. For that reason, both gas and electric tankless require very large inputs. For example, my gas tankless heater is 200k btu/h which is twice as much gas input as my furnace.

There is no magic here at all. You can do the math yourself - 1 BTU = the energy to raise 1 pound of water 1 degree F. Water is 8.34 pounds per gallon - so it takes 8.34 BTU to raise a gallon 1 degree F or 1251 BTU per minute to raise 3 gal of water 50 degrees in one minute or 75,060 BTU/hour.

There's 3412 kw per BTU/hr so that's about 22 kw (the heater was a 24 kw model- so slightly higher).

dadoes is correct - this heater isn't going to be running at 100 amps all the time. However, you do have to design it for the times that it will run at maximum capacity (when someone is taking a shower and the inlet water temp is 50 degrees)