Most disappointment with tankless water heaters traces back to one mistake: buying on price or brand instead of on math. A tankless unit that is correctly sized feels better than a tank. One that is undersized produces the notorious lukewarm shower whenever the dishwasher kicks on. The good news is that the sizing calculation involves two numbers and takes about five minutes.
Those two numbers are flow rate, measured in gallons per minute, and temperature rise, measured in degrees Fahrenheit. Every tankless unit is rated as a curve between them. The same heater might deliver 6 GPM in Florida and 3 GPM in Minnesota, because it is doing twice the work per gallon in the colder climate. Manufacturer marketing tends to quote the friendly number. Your job is to find the number that applies to your house.
Step One: Add Up Your Peak Flow
Start by deciding what you need running at the same time, not what you own. Typical fixture demands run about 1.5 to 2.5 GPM for a shower, 1.0 to 1.5 GPM for a kitchen sink, 1.0 GPM for a bathroom faucet, and 1.5 to 2.0 GPM for a dishwasher or washing machine. Low flow shower heads sit at the bottom of those ranges and older fixtures at the top.
A realistic peak for a small household is one shower plus one sink, roughly 3.0 to 3.5 GPM. A family morning with two showers and a running kitchen tap lands closer to 5.5 GPM. Be honest rather than optimistic here, but also avoid designing for a scenario that happens twice a year. Sizing for every fixture at once inflates cost and, on gas models, can push you into a larger gas line than the house has.
Step Two: Calculate Your Temperature Rise
Temperature rise is your target output temperature minus your incoming groundwater temperature. Most households set output between 105 and 120 degrees. Incoming water varies enormously by region and season, from the mid seventies in the Gulf states to the high thirties across the northern tier in winter.
The arithmetic is straightforward. Target 120 degrees with 50 degree incoming water gives a 70 degree rise. That same target with 70 degree incoming water gives only a 50 degree rise. Always size for your coldest month, because that is when the unit will be asked to work hardest and when a shortfall is most noticeable.
Once you have both numbers, read the manufacturer chart rather than the headline. A unit like the Rheem RTEX-18 at 18kW handles moderate rises for one or two fixtures, while the higher output EcoSmart ECO 36 targets whole house duty in colder regions. The spec sheets show the same relationship in both cases: as required rise climbs, deliverable GPM falls.
Electric Sizing Has a Hard Ceiling
Electric tankless capacity is governed by a clean formula. Roughly, GPM equals kilowatts divided by temperature rise, multiplied by 6.83. A 24kW unit facing a 60 degree rise yields about 2.7 GPM. The same unit facing a 40 degree rise yields about 4.1 GPM.
The constraint is your electrical service. A 24kW model such as the Stiebel Eltron Tempra 24 Plus draws 100 amps at 240 volts, typically split across three double pole breakers. A 36kW unit approaches 150 amps. Many homes with a 100 or 150 amp service simply cannot supply a whole house electric tankless heater without a service upgrade, and that upgrade often costs more than the heater. Check your panel before you shop, not after.
Gas Sizing and Why the Numbers Look Better
Gas units sidestep the amperage ceiling and generally deliver higher flow at a given rise. A propane model rated at 5.3 GPM like the Rinnai V53DeP covers a two bathroom household in a temperate climate comfortably. Their constraints move elsewhere: gas line diameter, combustion air, and venting.
Input ratings of 150,000 to 199,000 BTU commonly require a three quarter inch gas line. If your existing half inch line feeds a 40,000 BTU tank, that line will need replacing. Condensing models also need a condensate drain. None of this is exotic work, but it belongs in the budget from the start. Comparing input ratings and vent requirements across the water heaters category is the fastest way to see which models fit an existing gas supply.
Key Takeaways
- Size on two numbers: realistic simultaneous flow in GPM and temperature rise in degrees Fahrenheit.
- Use your coldest month groundwater temperature, not an annual average.
- For electric units, GPM is approximately kW divided by rise, times 6.83.
- Electric whole house models often demand 100 to 150 amps, which many panels cannot supply.
- Gas models reach higher flow but may require a larger gas line, venting, and a condensate drain.
Frequently Asked Questions
What happens if I undersize a tankless water heater?
The unit keeps flowing water but cannot reach the set temperature at that flow rate, so output arrives lukewarm. Reducing simultaneous demand restores full temperature, which is why the problem shows up mainly during busy mornings.
Can I oversize a tankless unit to be safe?
Modestly, yes, and modulating burners handle low demand well. The practical limits are cost and infrastructure, since larger units need heavier gas lines or more breaker capacity. Many units also have a minimum activation flow, so a very large model paired with a trickling faucet may not fire at all.
Do I need to size for the bathtub filling?
Tub fillers move a lot of water, often 4 GPM or more. If filling a large tub while another fixture runs is a regular event, include it. If it happens rarely, accepting a slower fill is usually cheaper than sizing the whole system around it.
Does hard water affect sizing?
Not the calculation, but it affects sustained performance. Scale buildup on the heat exchanger reduces heat transfer over time, so an annual descaling flush keeps a correctly sized unit performing at its rated output.
