If you are asking, “What size electric tankless water heater do I need?”, the answer depends on two things: how much hot water you want at one time and how much the unit needs to raise the water temperature in your home. Choosing the right size matters because an undersized unit will struggle to keep up, while an oversized unit can cost more than necessary.
How Electric Tankless Water Heaters Are Sized
Electric tankless water heaters are usually sized by flow rate and temperature rise.
- Flow rate is measured in gallons per minute (GPM).
- Temperature rise is the difference between your incoming groundwater temperature and your desired hot water temperature.
To choose the correct size, you need to know how many fixtures may run at the same time and how much hot water each one uses.
Step 1: Add Up Your Peak Hot Water Demand
Start by estimating the maximum number of fixtures or appliances that may use hot water at the same time. Here are common flow rates:
| Fixture or Appliance | Typical Flow Rate |
|---|---|
| Bathroom sink | 0.5 to 1.0 GPM |
| Kitchen faucet | 1.0 to 2.0 GPM |
| Shower | 1.5 to 2.5 GPM |
| Dishwasher | 1.0 to 2.0 GPM |
| Washing machine | 1.5 to 2.5 GPM |
| Bathtub faucet | 3.0 to 4.0 GPM |
For example, if your household may run one shower and one kitchen faucet at the same time, your peak demand might be around 3.5 to 4.5 GPM.
Step 2: Calculate the Temperature Rise
Most homeowners want water heated to around 120°F. The temperature rise is the difference between the incoming cold water temperature and that target temperature.
Example:
- Desired hot water temperature: 120°F
- Incoming water temperature: 55°F
- Temperature rise needed: 65°F
The colder your incoming water, the harder the unit has to work. That directly affects how large the electric tankless water heater needs to be.
General Sizing Guide for Electric Tankless Water Heaters
While every home is different, this chart gives a helpful starting point:
| Household Use | Suggested Flow Rate | Approximate Unit Size |
|---|---|---|
| Single sink or point-of-use | 0.5 to 1.5 GPM | 8 to 13 kW |
| One shower or small apartment | 1.5 to 2.5 GPM | 13 to 18 kW |
| One shower + one sink | 2.5 to 3.5 GPM | 18 to 24 kW |
| Two showers or whole small home | 3.5 to 5.0 GPM | 24 to 27 kW |
| Larger household with multiple fixtures | 5.0+ GPM | 27 to 36 kW |
In many homes, a whole-house electric tankless system will need to be in the 24 kW to 36 kW range, depending on the number of bathrooms and simultaneous demand.
Quick Examples
Small Apartment or Guest Suite
If you only need to serve one shower and one sink, a unit around 13 kW to 18 kW may work, depending on your local water temperature.
Two-Bedroom Home
If you expect one shower, one sink, and occasional kitchen use at the same time, you may need something in the 18 kW to 24 kW range.
Family Home with Two Bathrooms
If two showers could run at once, plus a faucet or appliance, you may need a 24 kW to 36 kW electric tankless water heater.
Important Limitation: Electrical Capacity
One of the biggest factors with electric tankless water heaters is not just hot water demand, but whether your home’s electrical system can support the unit.
Larger electric tankless units often require:
- High amperage draw
- Multiple dedicated breakers
- Heavy-gauge wiring
- A service panel with enough available capacity
That means the “right size” is not only about hot water needs. It is also about what your home can safely handle. In some cases, homeowners discover that upgrading the electrical panel is necessary before installing a whole-house electric tankless system.
Signs You Are Choosing the Wrong Size
Too Small
- Hot water turns lukewarm when another fixture turns on
- Showers lose temperature during peak use
- Unit cannot keep up in colder months
Too Large
- Higher upfront equipment cost
- Possible electrical upgrades you did not need
- Paying for more capacity than your home uses
Electric Tankless vs. Traditional Tank Sizing
Traditional water heaters are sized by tank capacity, such as 40-gallon or 50-gallon models. Electric tankless water heaters are different because they heat water on demand. Instead of asking how many gallons you need stored, you need to ask how much hot water you need at one time.
That is why two households with the same number of people may need different tankless sizes. Their simultaneous usage habits matter more than household size alone.
What Size Electric Tankless Water Heater Do Most Homes Need?
For many homes, the answer falls into one of these categories:
- Point-of-use applications: 8 to 13 kW
- Small apartment or light-use home: 13 to 18 kW
- Average small-to-mid-size home: 18 to 24 kW
- Larger whole-house applications: 24 to 36 kW
However, the only reliable way to size the system correctly is to evaluate your peak GPM demand, temperature rise, and electrical capacity together.
Professional Sizing Helps Avoid Expensive Mistakes
If you are investing in an electric tankless water heater, it is worth having a professional assess your plumbing fixtures, water usage patterns, and electrical setup before installation. That helps ensure you choose a unit that can deliver enough hot water without overloading your home’s electrical system.
A proper recommendation can also help you compare whether an electric tankless system is the best fit, or whether another water heating option may be more practical for your home.
Need Help Choosing the Right Electric Tankless Water Heater?
If you are unsure what size electric tankless water heater you need, our team can help you evaluate your household’s hot water demand and recommend the right solution. We can also inspect your electrical and plumbing setup to make sure the system is sized correctly for safe, reliable performance.
Contact us today to schedule an assessment and get expert guidance on electric tankless water heater installation.
