Selecting the right size tankless water heater for your home requires understanding how much power (measured in kW or BTUs) your appliance needs to heat water on demand. The power requirements depend on factors like your home’s water usage, desired flow rate, and incoming groundwater temperature.
Quick Answers
Here are some quick answers to common questions about power needs for tankless water heaters:
- For a single bathroom home, look for a unit with at least 18-27 kW power.
- For a 2-3 bathroom home, look for 27-36 kW.
- For a large 4+ bathroom home, look for 36 kW or more.
- Higher flow rates and colder groundwater require more power.
- Gas tankless heaters range from 150,000 – 199,000 BTUs for larger homes.
- Electric tankless units range from 12 – 27 kW.
How Much Hot Water Does Your Household Use?
The first step in determining the right tankless water heater size is estimating your peak hot water demand, or the maximum amount of hot water your household could use at one time. Here are some guidelines:
- Low demand household (1 bathroom): 3-5 gallons per minute (GPM)
- Medium demand household (2-3 bathrooms): 5-7 GPM
- High demand household (4+ bathrooms): 8-10 GPM or more
To get a more accurate measure, conduct an audit by running hot water appliances simultaneously and measuring flow rates. Target a tankless unit that meets but does not dramatically exceed your peak demand.
Groundwater Temperature in Your Area
The temperature of your incoming groundwater has a big impact on the power needed to heat it. Colder groundwater requires significantly more energy to raise to your target temperature. Here are some general groundwater temps for different climates:
- Northern U.S. cities: 50-54°F
- Southern U.S. cities: 55-59°F
- Western coastal cities: 60-65°F
- Hawaii: 70-75°F
For example, a tankless unit in Alaska may need to boost 40°F water up to 120°F, while the same unit in Hawaii only needs a 50°F temperature rise for the same output temperature.
Recommended Power by Household Size
Given your household’s hot water demand and groundwater temperature, you can determine the appropriate power range for your tankless water heater:
| Household Size | Demand | Power Range (kW) |
|---|---|---|
| 1 bathroom | Low | 18-27 |
| 2-3 bathrooms | Medium | 27-36 |
| 4+ bathrooms | High | 36+ |
For gas tankless heaters, the power range would be:
| Household Size | Demand | Power Range (BTU) |
|---|---|---|
| 1 bathroom | Low | 150,000 – 180,000 |
| 2-3 bathrooms | Medium | 180,000 – 199,000 |
| 4+ bathrooms | High | 180,000 – 199,000 |
These are general guidelines only. To choose the optimal size, always consider your unique water usage.
Power Boosters
In some situations, an extra power boost is needed:
- Low groundwater temperature: Upgrade to the next higher power range.
- High demand household: Choose a model rated for your maximal calculated demand.
- Remote bathrooms: May require localized electric boosters.
- Extreme cold climates: Select units rated down to -30°F.
With electric tankless heaters, you can also install multiple units in parallel to deliver a power boost. Gas tankless units require a more powerful individual unit.
Electric vs. Gas: How Power Is Measured
Electric and gas tankless water heaters use different units for their power rating:
- Electric: Power is measured in kilowatts (kW). Most models range from 12 kW to 27 kW.
- Gas: Power is measured in British Thermal Units (BTUs). Models range from 150,000 to 199,000 BTUs.
While the units are different, you can still follow the same capacity guidelines outlined above when choosing between electric or gas. The higher the rating, the more heating power the unit can deliver.
How Flow Rate Affects Power Needs
The flow rate of your tankless unit also influences power requirements. Flow rate tells you how many gallons per minute (GPM) the heater can raise to your target temperature. Some key points:
- Higher GPM = higher power demand. For example, an 8 GPM unit needs more power than a 5 GPM unit.
- Choose a GPM at least as high as your peak household demand. Otherwise, you may experience loss of hot water at times.
- Most consumer models range from 5-10 GPM. Commercial units can be 20 GPM or higher.
When shopping for a unit, don’t sacrifice flow rate to get a lower power rating. You want an appropriately-sized power rating and flow rate.
Typical Flow Rates by Power Range
| Power | Typical Flow Rate |
|---|---|
| 18-27 kW (electric) | 4-6 GPM |
| 27-36 kW (electric) | 6-7 GPM |
| 150,000 – 180,000 BTU (gas) | 6-8 GPM |
| 180,000 – 199,000 BTU (gas) | 8-10 GPM |
Voltage Considerations for Electric Models
With electric tankless water heaters, available voltage options can also impact your power capabilities:
- Standard voltage: 220-240V. Typical for residential installations.
- High voltage: 277V. Provides more power from the same amperage. Requires special wiring.
Some high-powered electric units mandate 277V. Check this when choosing a model, as wiring upgrades add cost if you only have 220-240V.
Use Multiple Units for Extra Power
One option to boost your electric tankless power is installing multiple units in parallel:
- A 27 kW and a 36 kW unit together provide 63 kW.
- You can also combine different flow rates.
- Use manifold plumbing to link the units.
- Allows you to tailor capacity to your needs.
With gas tankless heaters, you are limited to the power of a single unit. Make sure to spec your model accordingly.
Common Power Sizing Mistakes
It’s important to choose an appropriately sized tankless heater for your home. Some common mistakes include:
- Too small – Leads to loss of hot water when demand exceeds capacity.
- Too large – More expensive and can cycle inefficiently.
- Ignoring groundwater temperature – Colder water needs more power.
- Ignoring peak demand – Size for maximum simultaneous use.
A professional sizing analysis is recommended if you are unsure about your household’s hot water requirements. They can assess your needs and recommend the optimal tankless unit.
Working With an Electrician
When installing a tankless water heater, an electrician will ensure your home electric service can handle the load. Key points:
- Check your main breaker amperage rating. This limits the power you can draw.
- Each heating unit needs a dedicated circuit with proper amperage.
- Electrical panel may need upgrades for a large unit. Expense varies based on work needed.
- Electrician can advise you on ideal power options for your home.
Improper electrical setup can lead to nuisance tripping or failure to run the unit. Consulting a qualified electrician is highly recommended.
Gas Line Sizing
For gas tankless heaters, your plumber will ensure the gas lines can supply sufficient fuel:
- Gas line sizing depends on length from meter, fittings, and unit BTU rating.
- Existing lines may need to be replaced with larger diameter pipes.
- Proper sizing depends on gas type, pressure, and other factors.
- Consult an experienced plumber for your fuel requirements.
Tankless heaters need a greater gas supply than traditional storage tanks. Make gas line upgrades part of your installation budget if needed.
Energy Efficiency Comparison
When comparing tankless water heater models, also look at their Energy Factor (EF):
- The higher the EF, the more energy efficient the unit.
- Gas models range from around 0.80 to 0.95 EF.
- Electric models range from around 0.99 to 1.00+ EF.
- A higher EF rating helps offset increased power demands.
While not directly related to power output, energy efficiency helps reduce operating costs. Prioritize the highest rated units your budget allows.
Installation Costs
Here are some typical costs for installing a tankless water heater:
- Unit cost: $600 – $1500
- Venting: $500 – $1000+
- Gas line upgrades: $200 – $800 (gas models)
- Electrical work: $200 – $800
- Plumbing labor: $500 – $2000+
- Total: $2000 – $6000+
Power venting, electrical upgrades, and manifold plumbing can significantly add to costs. Get multiple quotes before purchasing a unit.
Operating Costs
Operating costs depend on local energy rates and your hot water usage:
- Electricity: $200 – $400 per year
- Natural gas: $150 – $300 per year
- Propane: $250 – $500+ per year
Choosing an appropriately sized unit prevents energy waste from cycling. Look for the highest Energy Factor rating when comparing models.
Tax Credits and Rebates
Incentives can help offset the upfront costs of a tankless water heater:
- Federal tax credit: 26% of cost, up to $300 (through 2032)
- State/local rebates: Up to $500 in some areas
- Manufacturer rebates: $100 – $500 for select brands/models
Contact your state energy office and local utility company to learn about available incentives for high-efficiency tankless water heaters.
Conclusion
Determining hot water heater power requirements involves balancing considerations like household demand, groundwater temperature, flow rate, and installation factors. While tankless heaters need more power than conventional water heaters, they can be highly efficient and supply endless hot water when properly sized. Use the guidelines provided to choose a model with sufficient power for your home’s needs. If unsure, consult a professional sizing specialist for their recommendations.
