Tankless water heaters, also known as on‑demand or instant water heaters, heat water only when a hot tap is turned on. This targeted operation can lead to significant energy savings compared to traditional tank models, but power consumption varies widely based on the type (electric vs gas), the required hot water flow, and the temperature rise from incoming supply to the desired output. Understanding how power is used helps homeowners choose the right unit, estimate operating costs, and size equipment for peak demand.

How Tankless Water Heaters Use Power

Electric tankless models draw electricity to power heating elements that raise water to the selected temperature. The key metric is kilowatts (kW), which reflects the rate of electrical power consumption. Gas-fired units, by contrast, burn natural gas or propane and use a heat exchanger to raise water temperature; their power consumption is typically described in BTU per hour (BTU/hr) or hundreds of thousands of BTU. Real-world energy use depends on two factors: the flow rate at which hot water is drawn and the temperature rise required to reach the outlet temperature. A higher flow rate or a larger temperature rise increases energy use, even with high-efficiency equipment.

Measuring Power Consumption for Electric Models

Electric tankless heaters list a maximum electrical load, often 15 kW to 36 kW for whole-house sizing. This rating determines the electrical circuit requirements (voltage, amperage, and dedicated circuits). The actual energy drawn varies with demand: at a 1–2 GPM (gallons per minute) draw with a modest 60–70°F rise, an electric unit may operate at a fraction of its maximum kW. When sizing for a household, consider the peak simultaneous hot-water needs (showers, laundry, dishwasher) to ensure the unit can deliver the necessary flow without overheating or triggering a limiter.

Gas vs Electric: Power Consumption and Sizing

Gas tankless heaters use a gas input rating, typically 140,000 to 199,000 BTU/hr (or higher for large households). Software and sensors modulate the flame to meet the required temperature rise while maintaining efficiency. Because gas units rely on combustion rather than electricity as the primary energy source, their electrical power use is limited to the blower, control electronics, and ignition. In practice, the ongoing energy cost for gas models is tied to gas consumption for heating rather than electrical usage, making them attractive in regions with low electricity costs or high hot-water demand. Electric models, while potentially more expensive to run per BTU, avoid venting and can be easier to install in some homes.

Estimating Your Load and Energy Use

Accurate estimates begin with the desired hot-water temperature, incoming water temperature, and peak flow rate. A common approach is to determine the temperature rise required for each fixture—for example, raising 50°F for a shower plus 20–30°F for other uses—and select a unit or units that can deliver the combined flow at that rise. The following simplified example illustrates the concept:

Incoming water temperature: 60°F
Desired hot-water temperature: 120°F
Temperature rise: 60°F
Peak simultaneous flow: 4–6 GPM (shower plus faucet)
Electric unit size: 27 kW for whole-house capability (typical range 15–36 kW)
Gas unit size: 150,000–199,000 BTU/hr (typical range)

With electric units, 1 kW can heat roughly 0.5–1 GPM at a 40–60°F rise, depending on efficiency. This is a rough rule of thumb; manufacturers provide exact performance curves for different inlet temperatures and outlet setpoints. For gas units, manufacturers publish hot-water output at various flow rates and temperatures, helping to size the unit to meet peak demand.

Efficiency and Operating Costs

Tankless heaters boast high energy efficiency; many electric and gas models exceed 0.8–0.95 Energy Factor (EF) or similar metrics. A higher EF indicates lower energy use for the same hot-water output. Since tankless systems heat water only when needed, they often have lower standby losses than traditional storage tanks, which reduces annual energy consumption. However, if a unit isn’t sized correctly for the home, it may cycle on and off frequently or fail to meet hot-water demand, leading to higher electricity or gas use per gallon heated.

Operating costs hinge on local utility rates and the unit’s efficiency. In regions with high electricity prices, properly sized gas or hybrid systems may offer savings, while in areas with cheap electricity, electric tankless models can be economical. For a rough annual cost estimate, multiply the unit’s energy usage by the local rate and consider the expected number of hot-water draws per day. A correctly sized system minimizes both energy waste and user inconvenience.

Practical Tips To Optimize Power Consumption

Size correctly. A unit that’s too small will run constantly or fail to meet demand; one that’s oversized wastes upfront costs but may also operate efficiently at partial loads.
Consider staged or dual units. In larger homes, installing two smaller units or a staged electric system can reduce peak power requirements and improve reliability.
Install a tempering valve. A tempering valve helps maintain safe outlet temperatures and reduces the heat duty required to maintain a comfortable shower temperature, saving energy.
Delay or stagger hot-water use. Spreading high-demand activities (laundry, multiple showers) can prevent simultaneous peaks and improve overall efficiency.
Regular maintenance. For gas models, annual venting and burner adjustments ensure efficient combustion. For electric models, check heating elements and replace them when worn to maintain efficiency.
Insulate hot-water pipes. While tankless heaters heat on demand, protecting hot-water lines reduces heat loss as water travels from the heater to fixtures.

Common Pitfalls And How To Avoid Them

Homeowners often overestimate a tankless unit’s ability to handle multiple simultaneous uses or underestimate incoming water temperature limitations, leading to undersized equipment or unsatisfactory performance. Always perform a load calculation that accounts for all major fixtures and typical usage patterns. If the incoming water supply is cold (e.g., in winter in colder climates), the required temperature rise increases, which can elevate electrical or gas demand. Consulting with a licensed installer to conduct a detailed sizing analysis and recommend model options tailored to the home ensures optimal power consumption and reliable hot-water delivery.

Summary: Making Informed Choices On Power Use

Choosing between electric and gas tankless water heaters hinges on local energy costs, available electrical capacity, and the home’s hot-water demand profile. Electric models emphasize high efficiency and simpler venting, with power consumption rising with required flow and temperature rise. Gas models emphasize robust heat output and potentially lower operating costs in areas with high electricity prices, though they require venting and gas supply considerations. By accurately sizing for peak demand, employing energy-saving practices, and maintaining the system, homeowners can optimize power consumption while enjoying on-demand hot water.