Electric Tankless Water Heater Caveats

We've posted on tankless water heaters before, but an inquiry from a client prompts us to revisit some of our reservations about tankless units. Wonderful idea, of course, good for energy, wish I'd thought of it myself, and all; but do your homework and keep your eyes open. Claims made for tankless heaters are larger than they seem in real life.
First, flow rate. You need at least three gallons per minute of hot water at 125 degrees fahrenheit to operate a laundry machine, dishwasher, shower, kitchen sink or any combination of two faucets or appliances in the house. if your teenager is in the shower and you go downstairs to start the dishwasher, you will be cited by Family Services in this litigious society, for cruelty to a teenager. Sharing the output of a tankless electric unit is dicey. And families living in multi-bathroom houses will, sooner or later, need to share that output.
Second, power needs. The only electric tankless that begins to fill the bill for a family is something like the Bosch AE 125 . The power requirement of this water heater is app. 125 amps at full load. Do you have a 100 amp service feeding your entire house, as I do? Fuhgeddabouddit. You can't install electric tankless in your house. Do you have a 200 amp service? Expect to give away 60% of that capacity while using hot water, which means that you can't operate your electric range, air conditioning, and clothes dryer all in tandem with this water heater. You have to do what we call "load management," in which you stop to think, ok, toaster is 110 watts, dryer 4500, range is 8000 unless I only use one burner, turn up the air conditioning thermostat, and,,,, ok, now we can do hot water. And if you have electric heat, you'll have to shut some of it off to avoid an overload, even with a 200 amp service. No, you can't have a 300 amp service on a house, not without paying lots of money. Perhaps in the "home of the future."
If it's just two of you in the house, or if the kids only come home for Christmas, this all may work out well. You can save up to 25% over electric tank hot water by virtue of lowering your standby costs (the expense of keeping the tank hot and losing heat to the surrounding air). If your house is large, full of kids, or if you have a big kitchen and you're always in it, beware.
Electric tankless water heaters are growing in popularity, and they should. But i'm always concerned when a past or potential client buys one off the internet and asks for a quote to install it. My bill for installation will commonly exceed the cost of the water heater, if indeed I can even shoehorn it into the house's electrical system. Then I'm delivering the bad news, the phone goes "click," and the unhappy client is off down the road to a plumbing company which knows not-so-much about electrical loading and is willing to take the client's money for installing an inadequately sized unit. Happens several times a year.
Other technologies are more practical. Oil, natural gas, LP gas, almost any fuel other than electric power makes for a better performance in water heating, due to the ability of those fuels to deliver larger amounts of energy instantaneously to the water, exceeding electricity by far in the critical category of "recovery rate." Watch your loading, watch your pricing, beware of claims made by salesmen bearing gifts, and consider all your options. Sometimes a heavy insulation blanket and a simple timer can turn an old electric tank into a lean, mean green machine, for a lot less money.