Electric Tankless Water Heater

An Electric Tankless Water Heater passes water over bare resistance coils inside a box about the size of a briefcase, switching them on in stages as flow increases. There is no tank, no flue, no gas line and no condensate. What it has instead is an electrical appetite that surprises almost everyone: a whole house model rated 27 kW draws 112 amps at 240 volts, arriving as three separate 40 amp double pole breakers and three cable runs, and a 100 amp service usually cannot host it.

Do the flow math before anything else. 27 kW yields about 3.7 gallons per minute at a 50 degree rise and only 2.6 at a 70 degree rise. A northern winter leaves you with roughly one shower. A Gas Tankless Water Heater at 199,000 BTU is doing about three times the work of the largest common electric unit. This is the whole reason whole house electric tankless is a warm climate product. Set against an Electric Storage Tank Water Heater, it trades slow recovery for a hard ceiling on simultaneous flow; set against a Heat Pump Water Heater it loses badly on running cost, since resistance heating returns one unit of heat per unit of power while a heat pump returns three or more.

Where it makes clear sense is small and local. A 3.5 to 8 kW unit under a bathroom sink delivers roughly 0.5 to 1.4 gallons per minute at a 40 degree rise, enough for handwashing and nothing else, and it removes the 40 second wait on a long pipe run. Cabins, garages, salon basins and additions all fit that pattern. Some warm climate homes do run one as a whole house heater successfully. They are the same homes where incoming water rarely drops below 65 F.

Judge it on the electrical drawing, not the brochure. Have an electrician confirm panel space, service capacity and wire size before you buy, since a service upgrade can cost more than the heater. Look for modulating power control rather than simple on and off staging, because staged units produce visible temperature swings and can flicker lights on a weak supply. Maintenance is light: no anode, no tank to flush. The coils still scale, though, so hard water needs the same annual vinegar flush a gas tankless unit gets, through isolation valves you should insist on at installation.