Picture this: You’ve just unboxed a shiny new electric tankless water heater. You follow the installation guide, flip the breaker, and the whole house goes dark. The panel pops, and you’re standing in the basement with a dead unit and a headache.
This happens way more often than it should. Nobody told you that a tankless water heater doesn’t sip power like a standard tank. It gulps. Understanding the amperage draw before you buy is what separates a smooth install from a call to the electrician at 9 PM on a Saturday.
By the time you finish this article, you’ll know exactly how many amps your model will pull, how to check if your panel can handle it, and what numbers to hand your electrician. I’ll walk through the math, the trade-offs, and the gotchas that install guides often skip.
Why Amps Matter for Tankless Heaters
Every electrical device in your home has three numbers that matter: voltage, amps, and watts. Think of watts as the total power needed to do the job. A typical tank-style water heater uses about 4,500 watts. A tankless unit that heats water on demand? That same unit can pull 18,000 to 36,000 watts.
Watts = volts × amps. So if you know the wattage and the voltage, you can solve for amps.
Most residential tankless water heaters in North America run on 240 volts. A 24,000-watt electric tankless at 240 volts draws exactly 100 amps (24,000 ÷ 240 = 100). That’s more than the entire main breaker on many older houses, which is often 100 or 150 amps total.
That single number is why a tankless install is rarely a simple swap. It’s not just the heater itself — it’s the wire, the breaker, and often a service upgrade. Ignore the amp draw, and you’ll trip breakers in winter when the water’s coldest and the demand is highest.
Electric vs Gas Tankless: The Real Amp Difference
Here’s where people get confused. A gas tankless water heater still needs electricity. It has a fan, a control board, and sometimes a recirculation pump. But the electrical load is tiny compared to an all-electric unit.
A typical gas tankless pulls 1 to 3 amps at 120 volts. That’s around 120 to 360 watts — less than a hair dryer on low. Most gas units plug into a standard wall outlet. You don’t need new wiring or a bigger panel.
Electric tankless heaters are a different animal. The smallest point-of-use models (for a single sink) might draw 20 to 30 amps at 240 volts. Whole-house electric tankless units commonly pull:
- 50 to 60 amps for low-flow models rated for warm climates (1-2 bathrooms)
- 80 to 100 amps for mid-range units (2-3 bathrooms in colder regions)
- 120 to 150 amps for high-output models that can handle 3+ simultaneous showers in a northern winter
Those numbers are for the heater alone. Your house still needs power for lights, appliances, and everything else. That’s why electric tankless heaters demand a dedicated circuit and often a 200-amp main service panel.
I’ve seen homeowners install a 36 kW unit on a 150-amp panel and wonder why the main breaker trips every time the dryer runs. The answer is math — not a defect. The panel just cannot supply that much current.
How to Calculate Your Actual Draw
The nameplate on the heater lists its maximum wattage. That’s the number to use for breaker sizing. But real-world draw can vary, and there’s a smart way to estimate yours.
First, find your incoming water temperature. In the southern US, groundwater might be 70°F. Up north, it can drop to 40°F in February. Colder water needs more heat, which means more wattage and higher amp draw. A tankless heater’s electronics modulate power to match demand — it won’t always run at full blast.
Still, you must size the circuit for the worst case. Here’s the formula:
Amps = Watts ÷ Volts
For a 27 kW unit on 240 volts: 27,000 ÷ 240 = 112.5 amps. The National Electrical Code says you need a breaker rated 125% of the continuous load. So 112.5 × 1.25 = 140.6 amps. You’d use a 150-amp breaker and wire rated for at least 150 amps. That’s a serious cable — usually 1/0 or 2/0 AWG copper, depending on distance.
Typical Tankless Amperage Comparison
| Type / Size | Power (kW) | Voltage | Amps (approx) | Wire Size (copper) | Breaker |
|---|---|---|---|---|---|
| Point-of-use electric (single sink) | 3.5 | 240 | 15 | 14 AWG | 20A |
| Small whole-house electric (warm climate) | 13 | 240 | 54 | 6 AWG | 60A |
| Mid-range whole-house electric (cold climate) | 24 | 240 | 100 | 3 AWG | 110A |
| High-output whole-house electric (large family, north) | 36 | 240 | 150 | 2/0 AWG | 175A |
| Gas tankless (all models) | 0.15–0.36 | 120 | 1–3 | 14 AWG or plug-in | 15A |
Note that wire size also depends on run length. A 100-foot run needs a thicker gauge than a 10-foot run because of voltage drop. Always have a licensed electrician do the final calculation.
What Your Electrician Needs to Know
You don’t have to become an electrician. But you should know enough to have a productive conversation with one. Here’s the list of info they’ll need:
- Heater’s nameplate wattage at your voltage — Some units can be wired for 208V (common in commercial buildings) or 240V. Make sure you’re using the right number.
- Distance from panel to heater — This determines wire gauge and whether you need a sub-panel.
- Your home’s main breaker size — If it’s 100 amps and you’re adding a 100-amp heater, you’ll need a service upgrade to 200 amps. That alone can cost $1,500–$3,000.
- Available space in the panel — Most electric tankless units require a double-pole breaker. If your panel is full, you’ll need a sub-panel or a tandem breaker.
- Local permit requirements — Many jurisdictions require a permit for a new circuit over 50 amps. Some even demand an energy audit. Skip the permit at your own risk — insurance may not cover a fire from unpermitted work.
One thing I’ll be blunt about: don’t take shortcuts with wire gauge. I’ve seen a 100-amp heater wired with 8 AWG because the homeowner wanted to save $50. The wire got hot enough to melt the insulation in under an hour. Wire is cheap insurance compared to a house fire.
Frequently Asked Questions
Can a tankless water heater run on a 30-amp breaker?
Only if it’s a very small point-of-use unit rated under 5.5 kW at 240 volts. Most whole-house electric tankless units need at least a 50-amp breaker, and many need 100 amps or more. A 30-amp breaker supplying a full-size tankless will trip instantly when you open the hot water tap. Stick to the manufacturer’s spec — breaker size is not a suggestion.
Do gas tankless water heaters need electricity?
Yes, almost all do. The control board, ignition system, and exhaust fan all run on 120V power. The draw is small — usually around 1 to 3 amps. If your power goes out, the heater shuts off. A few models offer a battery backup for the control board, but the fan still needs AC power. In an extended outage, you’re without hot water.
What size wire for a 50-amp tankless water heater?
For a 50-amp continuous load, you need a 6 AWG copper wire (or 4 AWG aluminum) rated for 55 or 60 amps, connected to a 60-amp breaker. The wire gauge must handle the full load plus a safety margin. If the run is more than 100 feet, step up to 4 AWG copper to prevent voltage drop. Always check local codes — some jurisdictions require an even larger wire for continuous loads.
How many amps does a small point-of-use tankless use?
Typical under-sink tankless heaters rated at 3.5 kW pull about 15 amps at 240 volts. That fits on a 20-amp double-pole breaker with 14 AWG wire. Some models are 120 volts and draw around 10–12 amps. These small units work well for a single sink or as a booster for a distant bathroom, but don’t expect them to run a shower unless you like a lukewarm trickle.
Will a tankless water heater reduce my electric bill?
Maybe, but not for the reason most people think. Electric tankless heaters can reduce standby losses — you’re not keeping 40 gallons hot all day. But they also pull huge power for short bursts. If your utility charges high demand fees (common in commercial settings), an electric tankless could raise your bill. Gas tankless units typically lower energy costs because gas is cheaper than electricity per BTU. Your actual savings depend on your local rates, usage patterns, and the efficiency of your current tank. For most homes, the payback on electric tankless is slow unless you also go solar or have a high standby loss from your old tank.
Quick Guide Before You Buy
- Check your main breaker size first. Anything under 200 amps likely needs an upgrade for a whole-house electric tankless.
- Gas tankless draws 1–3 amps and plugs into a standard outlet. Electric tankless draws 50–150 amps and needs a dedicated heavy circuit.
- Use the nameplate wattage divided by voltage to get minimum amps. Then multiply by 1.25 for breaker sizing.
- Wire gauge depends on amp load and distance. Avoid undersizing — it causes voltage drop and fire risk.
- Cold incoming water increases actual amp draw because the heater works harder. Size for winter, not summer.
- Point-of-use tankless units (3–5 kW) work on a 20-amp circuit. Whole-house units (18–36 kW) need 60- to 150-amp breakers.
- Get a permit and hire a licensed electrician. The cost of fixing a mistake is higher than doing it right the first time.
