Sizing the Tankless Water Heater
OK, so you now know the required temperature rise (desired hot water temperature - incoming ground water temperature = temperature rise) and you have added up the required flow rates for all the faucets and shower heads to be heated by the unit that may be on at one time. So let's say you require 11 GPM to accommodate 2 shower heads (1 pre 1992 at 4 GPM, 1 at 2.2 GPM), 1 lavatory faucet at 2.2 GPM and the kitchen faucet at 2.2 GPM. Based on our previous section "Calculating Temperature Rise" you need a 38°F temperature rise if you live in south Florida. So you need a whole house unit capable of handling 11 GPM (4+2.2+2.2+2.2=11) at a 38°F rise and you'd like an electric model.
Well that's pretty big for an electric tankless water heater but a 240 volt Tempra 29 model by Stiebel Eltron can produce about 5.5 GPM at a 38°F rise, so you need two units this size to meet the 11 GPM requirement. Now if you live in the northern half of the USA, say in Michigan or Minneapolis or Maine or Washington, then you're incoming ground water will be much colder than our Florida example. Probably at least 30°F colder which means you'll be looking for a unit that can handle a 68°F temperature rise. Once you require a bigger unit like that, you'll have to move up to a gas or propane fueled tankless water heater or heaters as the photo above shows.