The electric demand of a tankless water heater varies widely based on the unit’s heating capacity, the flow rate you need, and the water temperature rise in your home. Understanding watts helps homeowners plan electrical service, breaker sizes, and installation costs. This guide explains typical wattage ranges for electric and gas-powered tankless models, how to calculate your electrical needs, and practical considerations for homeowners in the United States.

How Tankless Water Heaters Use Power

Tankless water heaters heat water on demand rather than storing it in a tank. Electric models use electric resistance elements to raise water temperature, so their power consumption is measured in watts (or kilowatts). The higher the flow rate and the greater the temperature rise required from incoming cold water, the more watts are demanded. Gas-powered tankless units use gas combustion to heat water; they still require electricity for ignitions, sensors, and control boards, but their continuous heating power is driven primarily by gas rather than electrical load.

Electric Versus Gas Tankless: Where Watts Matter

Electric tankless water heaters have the most direct relationship between watts and performance. They typically require a large electrical service and dedicated circuits. Common configurations include multiple 40-amp or 50-amp circuits, depending on the model’s kilowatt rating. Gas-fired tankless heaters use electrical power mainly for ignition and control components, so their wattage demand is much lower and more dependent on the unit’s electronics rather than heat output. In short, the visible wattage concern is far more significant with electric models.

Typical Wattage Ranges for Electric Tankless Models

Electric tankless heaters come in a spectrum of heating capacities, usually described in kilowatts (kW). The most common ranges are:

Below 12 kW models: Suitable for point-of-use or single fixtures with mild climate zones; often require smaller electrical upgrades but limit simultaneous hot water usage.
12–24 kW models: Popular for a single bathroom or small apartments; typically use two to three 40A circuits at 240V.
27–36 kW models: Common in homes needing multiple taps or higher flow; commonly require three 40–50A circuits at 240V and may demand a higher service rating (100–150A total).
High-end 36–45 kW models: Designed for larger homes or busy households; can require four 40–50A circuits or more and a substantial electrical service upgrade.

As a rough rule of thumb, a 24 kW unit at 240V draws about 100A (24,000 W ÷ 240 V = 100 A). A 36 kW unit draws about 150A (36,000 W ÷ 240 V = 150 A). These numbers illustrate why many homes need a dedicated, up-sized electrical service and multiple circuit breakers to support electric tankless heaters.

How to Calculate Your Electrical Requirements

To determine the electrical needs for an electric tankless heater, consider three factors: maximum kilowatts, voltage, and the number of heating elements activated at once. Start with the unit’s nameplate rating in kilowatts. Divide by your supply voltage (commonly 240V in the U.S.) to get the current draw in amperes. For example, a 27 kW unit: 27,000 W ÷ 240 V = 112.5 A. Then plan for continuous load, upgrade allowances, and a safety margin when sizing the service panel and feeders. Consult a licensed electrician and, if possible, verify local electrical codes and the National Electrical Code (NEC) requirements for service upgrades and dedicated circuits.

Practical steps to ensure proper sizing include:

Audit your peak hot water demand based on showerheads, faucets, and simultaneous usage.
Check water temperature rise at your location; colder climates increase wattage needs to achieve a comfortable output.
Factor in future needs or renovations that might raise hot water demand.
Plan for a dedicated electrical feed with appropriate disconnects and surge protection.

Gas Tankless Water Heaters: Where Watts Enter Less

Gas-powered tankless heaters consume relatively little electricity, primarily for ignition and electronics. Typical electrical loads for gas models range from a few hundred watts to a couple of kilowatts, depending on the model. The primary considerations for gas units are gas supply pressure, venting requirements, and venting efficiency. While electricity is still essential for controls, the overall electrical impact is modest compared with electric tankless units.

Practical Considerations and Tips

When evaluating tankless options, consider these practical tips to manage wattage effectively:

Match heater size to demand: Oversizing a unit increases electrical costs and can complicate installation, while undersizing reduces hot water reliability.
Plan for a dedicated service: Electric tankless heaters often require a separate 100–150A service with multiple 40–50A circuits, depending on the model and climate.
Temperature rise awareness: A greater temperature rise (e.g., from 50°F to 120°F) increases wattage demand significantly; model selection should account for this.
Install peripheral components: Adequate cold-water supply, proper venting for gas models, and correct piping reduce inefficiencies and protect performance.
Seek professional guidance: A licensed electrician or plumber can assess your home’s electrical capacity, gas line provisions, and local code requirements.

Cost and Efficiency Considerations

Electric tankless heaters typically have higher up-front electrical installation costs due to the service upgrade and multiple circuits. However, they can be highly efficient because they heat water on demand with minimal standby energy loss. Gas tankless models may offer lower operating costs in some regions with cheap natural gas, but installation complexity and venting requirements add to the total cost. Understanding wattage helps forecast not only installation expenses but also ongoing electricity bills or gas consumption, influencing the total cost of ownership.

Summary: What To Know About Watts For Tankless Heaters

Electric tankless heaters demand significant electrical power, with common models ranging from 12 kW to 36 kW and corresponding current draws of roughly 50 A to 150 A at 240V. Gas-powered units rely less on electrical load, focusing on gas supply and venting. Accurate wattage estimation is essential for electrical service upgrades, proper circuit sizing, and adherence to code requirements. For homeowners, this means planning ahead, consulting professionals, and selecting a unit that aligns with both hot water needs and available electrical or gas infrastructure.