Choosing a tankless water heater is a smart upgrade—but choosing the right size is what makes the difference between endless comfort and daily frustration. Proper tankless water heater sizing ensures your system delivers consistent hot water, maximizes efficiency, and performs exactly as your household expects.
In this guide, we’ll break down how tankless water heater sizing works, what factors matter most, common mistakes to avoid, and how to confidently select the ideal system for your home.
Why Tankless Water Heater Sizing Matters
Tankless water heaters don’t store hot water. Instead, they heat water instantly as it flows through the unit. That means performance depends entirely on whether the system can keep up with real-time demand.
If the unit is undersized:
Hot water temperatures fluctuate
Multiple fixtures can’t run simultaneously
Showers may turn lukewarm during peak use
If the unit is oversized:
Upfront costs increase unnecessarily
Electrical infrastructure may need upgrading
You won’t gain meaningful performance benefits
Correct sizing balances comfort, efficiency, and long-term value.
Understanding Flow Rate (GPM)
The foundation of tankless water heater sizing is flow rate, measured in gallons per minute (GPM).
Every hot-water fixture in your home uses a certain amount of water:
Shower: 2.0–2.5 GPM
Bathroom sink: 0.5–1.0 GPM
Kitchen sink: 1.0–1.5 GPM
Dishwasher: 1.0–2.0 GPM
Washing machine: 1.5–3.0 GPM
To size a tankless water heater properly, you must account for how many fixtures may run at the same time.
Example:
If two showers and a sink could be running simultaneously:
Shower 1: 2.5 GPM
Shower 2: 2.5 GPM
Sink: 1.0 GPM
Total demand: 6.0 GPM
Your tankless water heater must support at least this flow rate at your desired temperature rise.
Temperature Rise: The Other Critical Factor
Flow rate alone isn’t enough. You also need to consider temperature rise—the difference between incoming groundwater temperature and your desired output temperature.
Typical target hot water temperature:
120°F (recommended for safety and efficiency)
Groundwater temperatures vary by region:
Southern U.S.: ~60–70°F
Northern U.S.: ~40–50°F
Example:
If incoming water is 55°F and you want 120°F water:
Temperature rise required = 65°F
Your tankless water heater must be rated to deliver your total GPM at that temperature rise.
This is where professional-grade systems truly stand apart.
Why Electric Tankless Water Heaters Require Precise Sizing
Electric tankless water heaters are known for:
Compact design
Exceptional efficiency
Precise temperature control
However, they must be sized carefully to match both electrical capacity and water demand.
Unlike gas systems that may struggle at low flow, electric tankless heaters deliver consistent performance—when properly sized.
Point-of-Use vs. Whole-House Systems
Tankless water heater sizing also depends on how the system is used.
Point-of-Use Systems
Serve a single fixture or room
Require lower GPM
Ideal for bathrooms, kitchens, or additions
Deliver instant hot water with minimal energy loss
Whole-House Systems
Supply hot water to the entire home
Require higher GPM capacity
Must handle simultaneous demand
Replace traditional tank systems completely
Choosing the right configuration can reduce system size, improve performance, and optimize energy use.
Household Size Isn’t the Whole Story
A common mistake is sizing based solely on the number of people in the home. While occupancy matters, usage patterns matter more.
Consider:
Do multiple showers run at the same time?
Is hot water used heavily in the morning?
Are luxury showers or rain heads installed?
Does laundry often run during peak hours?
Tankless water heater sizing should reflect how your home actually operates, not just how many people live there.
Electrical Capacity Considerations
Electric tankless water heaters require sufficient electrical service to perform at full capacity.
Proper sizing includes evaluating:
Available amperage
Panel capacity
Dedicated circuit requirements
Professional installation ensures your system is matched not only to your water demand—but also to your home’s infrastructure.
The Efficiency Advantage of Proper Sizing
When sized correctly, tankless water heaters deliver:
Endless hot water
Lower energy consumption
Reduced standby losses
Longer system lifespan
Oversizing or undersizing compromises these benefits. Precision is the key to performance.
Why Homeowners Trust Stiebel Eltron
Stiebel Eltron is a global leader in tankless water heating, known for:
German-engineered precision
Industry-leading efficiency
Compact, space-saving designs
Exceptional reliability
Consistent temperature control—even during peak demand
Stiebel Eltron tankless systems are designed with real-world usage in mind, making proper sizing straightforward and dependable when installed professionally.
Common Tankless Water Heater Sizing Mistakes to Avoid
Ignoring simultaneous usage
Underestimating temperature rise
Assuming one-size-fits-all
Choosing based on price alone
Skipping professional assessment
Avoiding these mistakes ensures your system delivers on its promise—day after day.
When to Upgrade Your Water Heating System
The best time to size and install a tankless water heater is before your old system fails.
Proactive upgrades offer:
Better system selection
Planned installation timelines
Avoidance of emergency replacements
Immediate efficiency gains
Tankless water heating is a long-term investment—and sizing it right from the start ensures maximum return.
Choose Stiebel Eltron for Superior Tankless Water Heating Solutions
Tankless water heater sizing isn’t guesswork—it’s precision. When done correctly, it delivers endless comfort, lower energy costs, and reliable performance for years to come.
With Stiebel Eltron tankless water heaters, you get expertly engineered systems designed to match your lifestyle, your home, and your expectations.
Choose smarter sizing. Choose lasting comfort. Choose Stiebel Eltron.