How Many Amps Does A Tankless Water Heater Use? 2026: Guide

Planning to install a tankless water heater? The electrical requirements might surprise you.

An electric tankless water heater uses 100-167 amps while a gas tankless water heater uses only 1-4 amps for controls and ignition. This massive difference in electrical demand is crucial for determining whether your home’s electrical system can support the unit.

I’ve helped hundreds of homeowners navigate tankless water heater installations, and electrical requirements are consistently the biggest hurdle. After reviewing dozens of installations and consulting with electricians, I’ll break down exactly what you need to know about amperage requirements.

By the end of this guide, you’ll understand amp requirements for both electric and gas units, how to calculate breaker sizes, and whether your electrical panel needs an upgrade.

Electric Tankless Water Heater Amp Requirements

Electric tankless water heaters draw substantial power to heat water on demand. Unlike traditional tank heaters that use 30-40 amps, electric tankless units require 100-167 amps depending on the model and your hot water needs.

The amp draw directly correlates with the unit’s power rating (in kilowatts) and voltage. Most residential electric tankless water heaters operate on 240V and range from 13kW to 36kW. Using the formula Amps = Watts/Volts, a 24kW unit draws 100 amps (24,000W ÷ 240V = 100A).

Power Rating	Amperage (240V)	Typical Application	Flow Rate (GPM at 70°F rise)
13-18 kW	54-75 amps	Point-of-use, small apartment	1.5-2.0
18-24 kW	75-100 amps	2-bathroom home	2.0-2.7
24-27 kW	100-113 amps	3-bathroom home	2.7-3.0
27-36 kW	113-150 amps	Large home, high demand	3.0-4.0

What factors affect amp draw? The primary factors are temperature rise (incoming water temperature vs. desired output) and flow rate. Colder climates require higher temperature rise, increasing power demands. Simultaneous hot water usage also affects the required capacity.

From my experience, homeowners often underestimate their actual hot water needs. A 24kW unit might seem adequate, but when someone showers while the dishwasher runs, you’ll need more capacity. I recommend sizing for your peak demand scenarios, not average usage.

Most electric tankless water heaters require dedicated circuit breakers. Some smaller units (under 24kW) might use two 40-50 amp breakers, while larger units require multiple 60-80 amp breakers. Always check the manufacturer’s specifications for exact breaker requirements.

For those considering tankless water heater sizing, remember that bigger isn’t always better. An oversized unit will draw more power than necessary, increasing your electrical costs. Proper sizing ensures efficiency while meeting your hot water demands.

Gas Tankless Water Heater Electrical Requirements

Gas tankless water heaters are surprisingly energy-efficient from an electrical standpoint. Unlike their electric counterparts that draw 100+ amps, gas units use minimal electricity—typically 1-4 amps during normal operation.

What do these 1-4 amps power? Gas tankless units use electricity for: electronic control boards, ignition systems (usually intermittent or hot surface ignition), exhaust fans (if power vented), temperature sensors and safety circuits, and display panels and thermostats.

The control board is the main electrical component, managing gas flow, temperature regulation, and safety monitoring. This typically draws 1-2 amps continuously. Ignition systems consume additional power only when lighting the burner, usually for just a few seconds.

Most gas tankless water heaters can plug into a standard 120V outlet, requiring just a 15-amp circuit. Some units might need a dedicated 20-amp circuit, but this is rare. The electrical requirements are so minimal that your existing household wiring typically suffices.

However, don’t confuse electrical requirements with gas line requirements. Gas units need properly sized gas lines (often 3/4″ to 1″) and adequate gas pressure. The electrical component is straightforward, but gas line sizing requires separate calculations.

From an installation perspective, gas units are often more feasible for older homes with limited electrical service. You avoid the massive electrical upgrades required for electric units while still enjoying the benefits of on-demand hot water.

The trade-off? Gas units require venting and have more complex installation requirements. You’ll need proper exhaust venting (usually stainless steel), gas line installation, and potentially outdoor mounting in some jurisdictions.

How to Size Circuit Breakers for Tankless Water Heaters?

Proper breaker sizing is critical for safety and code compliance. The National Electrical Code (NEC) requires that continuous loads (like water heaters) use only 80% of a breaker’s capacity. This is the 125% rule: multiply the appliance’s amp draw by 125% to determine the minimum breaker size.

For example, a tankless water heater drawing 80 amps requires a minimum 100-amp breaker (80A × 1.25 = 100A). Never round down—always round up to the next standard breaker size.

125% Continuous Load Rule: NEC requirement that continuous loads (operating 3+ hours) use only 80% of breaker capacity, calculated by multiplying appliance amps by 125%.

Breaker Sizing Examples:

18kW Electric Unit: Draws 75 amps (18,000W ÷ 240V). Required breaker: 94A minimum → 100A breaker
24kW Electric Unit: Draws 100 amps. Required breaker: 125A minimum → 125A or 150A breaker
27kW Electric Unit: Draws 113 amps. Required breaker: 141A minimum → 150A breaker
Gas Tankless Unit: Draws 2 amps. Required breaker: 2.5A minimum → 15A standard breaker

Electric tankless water heaters often require multiple breakers. A 36kW unit might need three 60-amp double-pole breakers. Always follow the manufacturer’s specific wiring diagram—don’t guess!

Wire sizing is equally important. For 100-amp breakers, you’ll typically need 4 AWG copper wire. Larger applications might require 3 AWG or even 2 AWG wire. Always consult NEC tables or a licensed electrician for proper wire sizing.

I’ve seen installations fail because of incorrect breaker sizing. Undersized breakers will trip frequently, while oversized breakers create fire hazards. The 125% rule isn’t optional—it’s a critical safety requirement.

⏰ Time Saver: Some electric tankless water heaters come with multiple heating modules that can be staged to reduce peak amp draw, potentially allowing smaller breakers and wire.

Electrical Panel Requirements and Service Upgrades

Your electrical service size determines whether you can install a tankless water heater. Most older homes have 100-amp service, while newer homes typically have 200-amp service. Here’s how to determine if your service is adequate.

Calculate your total electrical load by adding up all major appliances: air conditioner (30-60A), electric stove (40-50A), electric dryer (30A), plus lighting and outlets (typically 30-50A). If you’re already near your service limit, adding a 100+ amp tankless water heater will definitely require an upgrade.

For homes with 100-amp service, installing an electric tankless water heater is typically impractical. Even if your calculated load shows available capacity, most electricians won’t install without a service upgrade due to safety concerns.

Service upgrades typically cost $2,000-5,000 depending on your location, utility company requirements, and the complexity of the installation. This includes: new 200-amp main breaker panel, possible utility service line upgrade, new weather head and meter socket, and updated grounding system.

If you’re considering upgrading to support an electric tankless water heater, look into energy efficient heating options that might reduce your overall electrical needs.

Alternative solutions for limited electrical service include: point-of-use electric units at specific fixtures (require 20-30 amps each), gas tankless units (minimal electrical requirements), hybrid systems combining smaller electric units with gas backup, or solar pre-heating systems that reduce the tankless unit’s load.

Before committing to a service upgrade, get multiple quotes from licensed electricians. Some utility companies offer rebates for service upgrades when installing energy-efficient appliances like tankless water heaters.

Safety Considerations and Code Requirements

Working with high-amperage circuits requires strict adherence to safety protocols and electrical codes. Improper installation can lead to fire hazards, electrical shock, or damage to your water heater and home.

The National Electrical Code (NEC) has specific requirements for tankless water heaters: Article 422 covers water heaters, dedicated circuits are required for most units, proper grounding is mandatory, disconnect switches must be accessible, and GFCI protection may be required depending on installation location.

Always use licensed electricians for tankless water heater installations. DIY electrical work on these high-amperage circuits is extremely dangerous and often violates local codes. Additionally, improper installation can void your water heater warranty.

Common installation mistakes I’ve seen include: undersized wiring creating fire hazards, improper breaker sizing causing frequent tripping, missing disconnect switches preventing emergency shutdown, inadequate grounding creating shock hazards, and incorrect voltage connections damaging the unit.

Permit requirements vary by jurisdiction, but most areas require permits for both electrical and plumbing work on tankless water heaters. Don’t skip this step—permits ensure code compliance and protect your investment.

Regular maintenance is crucial for safety and longevity. Electric units require annual inspection of wiring and connections, while gas units need professional servicing of gas lines and venting systems.

✅ Pro Tip: Install a dedicated disconnect switch within sight of the water heater. This allows emergency shutdown without accessing the main panel.

When researching installation options, check tankless water heater reviews for units with safety features like automatic shutoff, leak detection, and temperature limiters.

Frequently Asked Questions

Is 200 amp service enough for a tankless water heater?

Yes, 200-amp service is typically sufficient for most electric tankless water heaters, provided you calculate your total electrical load carefully. A standard electric tankless unit uses 100-150 amps, leaving 50-100 amps for other appliances. However, if you have multiple high-draw appliances like central air conditioning, electric heat, or an electric vehicle charger, you might need load management or a larger service. Always perform a complete load calculation before installation.

What size breaker do I need for a tankless hot water heater?

Breaker size depends on the water heater’s amp draw and follows the NEC 125% rule. For example: 18kW unit drawing 75A needs a 94A minimum breaker → use 100A breaker, 24kW unit drawing 100A needs a 125A minimum breaker → use 125A or 150A breaker, 27kW unit drawing 113A needs a 141A minimum breaker → use 150A breaker. Always check the manufacturer’s specifications and consult a licensed electrician for proper installation.

Do I need to upgrade my electrical panel for an electric tankless water heater?

You likely need an upgrade if your home has 100-amp service or less. Electric tankless water heaters typically require 100-167 amps, which would exceed your entire service capacity on a 100-amp system. Homes with 150-amp service might accommodate smaller electric units, but most installations require 200-amp service for safety and reliability. Get a professional load calculation to determine your specific needs before making a decision.

Can a tankless water heater be installed on a 100 amp service?

Gas tankless water heaters can easily install on 100-amp service since they only use 1-4 amps for controls and ignition. Electric tankless water heaters generally cannot install on 100-amp service because they require 100-167 amps themselves. The only exception might be very small point-of-use electric units drawing 20-30 amps, but these won’t serve a whole home. Most electric tankless installations require at least 150-amp service, preferably 200-amp.

How many circuits does an electric tankless water heater need?

Most electric tankless water heaters need 1-3 dedicated circuits depending on their power rating. Smaller units (18-24kW) typically require two 40-60 amp double-pole breakers. Medium units (27-30kW) might need three 50-60 amp breakers. Large units (36kW+) often require four 60-80 amp breakers. Always follow the manufacturer’s wiring diagram exactly—each circuit serves specific heating elements within the unit.

What voltage do electric tankless water heaters use?

Most residential electric tankless water heaters use 240V single-phase power. Some smaller point-of-use units might operate on 120V, but these have limited capacity and flow rates. Commercial units might use 208V or 277V three-phase power. Always verify the voltage requirements of your specific model and ensure your electrical service can provide the required voltage and amperage.

Final Recommendations

After analyzing hundreds of installations and consulting with electricians nationwide, here are my final recommendations:

For homes with existing 200-amp service and newer electrical systems, electric tankless water heaters offer excellent efficiency and unlimited hot water. The electrical investment pays off over time through energy savings and convenience.

For older homes with 100-amp service or limited electrical capacity, gas tankless water heaters provide the best balance of efficiency and practicality. The minimal electrical requirements make them suitable for most homes without costly upgrades.

Always get a professional electrical assessment before purchasing any tankless water heater. A licensed electrician can perform load calculations, verify your service capacity, and identify any necessary upgrades. This $200-500 assessment can save you thousands in unexpected upgrade costs.

Consider your long-term plans. If you’re planning to add electric vehicle charging, solar panels, or other high-draw appliances, now might be the time to upgrade to 200-amp service even if you choose a gas tankless unit.

Tankless water heaters represent excellent investments in home efficiency and comfort. Understanding the electrical requirements upfront ensures a smooth installation process and years of reliable hot water on demand.