Match the unit’s published GPM at your local temperature rise to your peak simultaneous hot-water demand, then confirm your home’s electrical service can carry it.
A 36 kW unit that looks powerful on paper can leave a shower cold in Minnesota and run perfectly in Florida — the difference is inlet water temperature, not the wattage badge. Picking a whole-house electric tankless water heater comes down to three numbers: peak flow demand, temperature rise, and available amperage.
Two 2.5 GPM showerheads running at once demand 5 GPM. That alone rules out most 17.3 kW units in a cold climate, because the same model that delivers 2.6 GPM at a 45°F rise only manages 1.8 GPM at 65°F.
How Much Hot Water Does Your Peak Demand Require?
Add up every hot-water fixture likely to run during your busiest 60 minutes of the day. Sizing starts with that peak number, not with square footage.
Count simultaneous uses honestly:
- Each standard showerhead: roughly 2.0–2.5 GPM
- Bathroom faucet: about 1.5 GPM
- Kitchen faucet: about 1.5 GPM
- Dishwasher: about 1.5 GPM during fill cycles
- Clothes washer: about 2.0 GPM during fill
If two showers and the dishwasher can plausibly overlap, you need a unit rated near 6 GPM at your worst-case temperature rise. Stiebel Eltron publishes sizing guides and videos for exactly this step. GE Appliances’ tankless guide takes the same approach: confirm your climate zone, find the matching GPM and usage level, and size up if you land near a limit.
Why Inlet Water Temperature Decides Your Real Flow Rate
Ground water in northern states can enter at 40–45°F in winter, while southern states see 65–75°F. A tankless heater must raise that water to roughly 110–120°F, so the colder the inlet, the more of the unit’s capacity goes to heating rather than flowing.
That’s why kW ratings mislead. A 26.9 kW unit at 208–240 V delivers 3.9 GPM at a 45°F rise, 3.3 GPM at 55°F, and 2.8 GPM at 65°F. The 17.3 kW unit drops from 2.6 GPM to 1.8 GPM across the same range. Same box, same wiring — very different output depending on where you live.
Lower supply voltage cuts output further. A unit rated at 240 V running on 208 V produces noticeably less hot water, so confirm your actual service voltage before trusting the spec sheet.
| Model Rating | Peak Flow (45°F / 65°F Rise) | Circuit & Service Needs |
|---|---|---|
| 36.0 kW | Highest output of the three | 180 A through three 60 A breakers; 300 A minimum home service |
| 26.9 kW | 3.9 GPM / 2.8 GPM | 120 A through three 40 A breakers; 200 A minimum home service |
| 17.3 kW | 2.6 GPM / 1.8 GPM | 80 A through two 40 A breakers; 150 A minimum home service |
| 12 kW | 1.8 GPM / 1.2 GPM | Best for single-fixture or point-of-use duty |
| Activation flow | 0.6–0.8 GPM to fire | A trickle won’t trigger the heating element |
| Voltage effect | Lower output at 208 V | Verify nameplate against actual service |
| Climate factor | Cold inlet = less GPM | Size up in northern regions |
Can Your Home’s Electrical Service Handle It?
Often the real bottleneck is the breaker panel, not the water heater. Whole-house electric tankless units draw enormous amperage the moment they fire.
A 36.0 kW unit needs 180 A of circuit protection split across three 60 A breakers and at least a 300 A home service. The 26.9 kW unit needs 120 A across three 40 A breakers on a 200 A service minimum, and the 17.3 kW unit needs 80 A across two 40 A breakers with at least 150 A available. Many older US homes run 100 A service, which means a service upgrade and panel work before any whole-house electric unit is even possible. Factor that cost in before you buy.
Stiebel Eltron’s technical specifications list the exact electrical requirements and flow rates for each model, worth checking against your panel before ordering. If the numbers work out, our breakdown of the best whole-house electric tankless models compares units against these same service limits.
Where Whole-House Electric Tankless Makes Sense
Warmer-climate homes with 200 A service and moderate inlet water temperatures get the smoothest results. Colder regions often need either a very large service upgrade or a different water-heating strategy altogether.
Run the three checks in order — peak GPM, temperature rise for your climate, then breaker and service capacity. If a model clears all three, it will keep up with your household. If it fails the electrical check, no amount of kW shopping fixes it.
FAQs
Is a higher kW rating always better?
No. A higher kW unit delivers more hot water, but only if your home’s electrical service can support its amperage and breaker configuration. A 36.0 kW model requires 180 A of circuit protection and at least a 300 A service. Without that capacity, a smaller unit matched to your temperature rise and peak demand performs more reliably.
Why does my flow rate drop in winter?
Colder inlet water forces the heater to spend more of its capacity on raising temperature rather than pushing volume. A unit delivering 3.9 GPM at a 45°F rise may only manage 2.8 GPM at 65°F. That’s why northern homes often need a larger model than the same-size home in a warmer state.
Can a whole-house electric tankless unit run on 100 A service?
Generally no. Even the smallest whole-house model in this class requires 80 A of circuit protection across two 40 A breakers and a minimum 150 A home service. Standard 100 A panels typically need an upgrade before installation, which adds significant cost to the project.
References & Sources
- Stiebel Eltron USA. “Tankless Sizing Guides” Official model-selection guidance for matching unit size to demand.
- Stiebel Eltron USA. “Technical Specifications” Source for GPM flow rates, activation flow, and electrical requirements.
- GE Appliances Air & Water. “Tankless Water Heater Sizing Guide” Climate-zone, GPM, and usage-based sizing method.