Electric Tankless Water Heater Sizing Calculator
Find the ideal electric tankless water heater size for your needs based on voltage, amperage, flow rate, and temperature rise.
List of the Best Electric Tankless Water Heater:
Electric Tankless Water Heater Sizing Calculator Guide
Our electric tankless water heater sizing calculator helps homeowners, contractors, and DIYers accurately determine the right unit size for their hot water needs. Whether you're replacing an old unit or installing a tankless system for the first time, proper sizing is essential for performance, efficiency, and comfort. This guide will walk you through everything you need to know to size your electric tankless water heater correctly.
Why Use an Electric Tankless Water Heater?
Electric tankless water heaters are compact, energy-efficient, and provide endless hot water on demand. Unlike traditional tank-style heaters, they only heat water when it is needed, reducing standby energy losses. They're ideal for homes with limited space, remote bathrooms, or off-grid setups with sufficient electrical capacity.
How Sizing Works: The Basics
Electric tankless water heater sizing depends on two primary factors: flow rate and temperature rise.
- Flow Rate (GPM): The total amount of hot water needed at one time. For example, if you're running a shower and a washing machine, you may need 5 GPM or more.
- Temperature Rise: The difference between the incoming cold water temperature and your desired hot water temperature. In colder climates, this rise can be as much as 70°F or more.
Step-by-Step: How This Calculator Works
This electric tankless water heater sizing calculator estimates the required power (in watts or kilowatts) based on the flow rate and desired temperature rise. It then compares this value to the electrical capacity you enter (voltage × amperage) to determine whether your electrical system can support the desired load.
Common Fixture Flow Rates (GPM)
| Fixture | Flow Rate (GPM) |
|---|---|
| Shower | 2.0–2.5 |
| Bathroom Sink | 0.5–1.0 |
| Kitchen Sink | 1.0–1.5 |
| Dishwasher | 1.5 |
| Washing Machine | 2.0–2.5 |
Required Power Calculation
The general formula used in this electric tankless water heater sizing calculator is:
Power (watts) = (Flow rate in GPM × Temperature Rise × 500) / 3.41The number 500 is a BTU conversion factor, and 3.41 is used to convert BTUs to watts. This formula gives a good estimate of the power needed to meet your hot water demands.
Typical Electric Tankless Heater Sizes
| Unit Size (kW) | Approx. GPM @ 60°F Rise | Breaker Requirement |
|---|---|---|
| 11–13 kW | 1.5–2.0 | 60A |
| 15–18 kW | 2.5–3.0 | 2×40A |
| 24–27 kW | 3.5–4.0 | 2×50A |
| 36 kW | 5.0+ | 3×50A or more |
Regional Temperature Rise Map (U.S.)
Knowing your region's average incoming water temperature helps estimate temperature rise:
- Northern U.S.: 40–50°F incoming temp → 70–80°F rise
- Central U.S.: 50–60°F incoming → 60–70°F rise
- Southern U.S.: 60–70°F incoming → 50–60°F rise
Advantages of Proper Sizing
- Ensures consistent hot water supply
- Maximizes energy efficiency
- Prevents system overload or underperformance
- Protects your home’s electrical infrastructure
Can My Electrical System Support a Tankless Heater?
Before installing an electric tankless water heater, it's important to verify that your electrical panel can support the load. Many whole-home electric units require two or more 40–50A double-pole breakers and heavy-gauge wiring. If your panel is underpowered, consider an upgrade or a point-of-use model.
FAQs About Electric Tankless Water Heater Sizing
Can I install an electric tankless water heater with 100 amps?
Yes, but your available amperage must be shared with other household appliances. Most whole-home units require 120A or more. If your panel only provides 100A, a point-of-use unit may be more suitable.
What’s the best size for a family of 4?
Most families of 4 need a unit with 18–27 kW output depending on climate. Use the electric tankless water heater sizing calculator to refine your estimate.
Can I use two smaller units instead of one large one?
Yes. Many homeowners install multiple smaller units for zone-based hot water (e.g., one for kitchen, one for bathroom). This can also reduce lag time and energy use.
What happens if the unit is undersized?
An undersized electric tankless water heater will struggle to keep up during high demand, causing cold water surges or reduced flow. Proper sizing avoids this problem entirely.
Are electric tankless units better than gas?
They are more energy efficient and easier to install, but less powerful. Electric units are best for smaller homes or areas without gas service.
Installation and Safety Tips
- Hire a licensed electrician to install dedicated breakers
- Use the correct wire gauge (typically 6 or 8 AWG depending on amperage)
- Check that your unit is UL-listed and meets local code
- Use temperature-safe plumbing (PEX, CPVC, or copper)
Conclusion: Use the Electric Tankless Water Heater Sizing Calculator with Confidence
This calculator removes the guesswork from choosing the right unit. By entering a few key values—voltage, amperage, desired flow rate, and temperature rise—you'll get a reliable estimate of what electric tankless heater will work for your space. Bookmark this tool and refer to it anytime you or a client needs sizing guidance.