How the math works
Tanklessly mathAmps = kW × 1,000 ÷ volts. A 240-volt element on 208-volt service delivers (208 ÷ 240)², about 75%, of its rated power, so it heats less and draws less; Stiebel Eltron's sizing guide says to go one model size up at 208 volts.
Tanklessly mathNatural gas: cubic feet an hour = BTU/h ÷ 1,036 (EIA heat content of natural gas). Propane: gallons an hour = BTU/h ÷ 91,452 (EIA heat content of propane).
Licensed workThese figures tell a licensed electrician or gas fitter what the unit needs. They don't say your panel, service, wire or gas line can carry it: that depends on every other load in the house, the pipe length and the codes where you live.
Questions
How many amps does a 27 kW tankless water heater draw?
About 112.5 amps at 240 volts (27,000 ÷ 240). Every 27 to 29 kW listing in our catalog that states its breakers splits that across three 40-amp breakers; check the maker's manual for the exact breakers and wire.
Will a 199,000 BTU tankless heater work on my gas line?
At full fire it burns about 192 cubic feet of natural gas an hour. Whether your line can deliver that with the furnace and range running depends on the pipe size and length; a gas fitter checks it against the code tables.
Can I run a 240 V tankless heater on 208 V?
Many are rated for both, but the output falls to about 75% at 208 V, so the flow at your rise drops by the same share. Size one model up.