Electric Tankless Water Heater Requirements: Will Your Panel Handle It?
Here's the number that surprises most people shopping for an electric tankless water heater.
The EcoSmart ECO 36 is a whole-house unit that sells for around $644. That's less than many gas models. But according to EcoSmart's own electrical requirements table, it draws 150 amps and calls for a 300-amp household electrical service.
Most American homes have a 100-amp or 200-amp service. A 300-amp service is uncommon in residential construction. So the cheapest-looking route to whole-home electric hot water can require an electrical upgrade that costs several times more than the heater itself — assuming your utility can even deliver that much power to your address.
This is the part of electric tankless shopping that product listings tend to bury. The unit price is the small number. The electrical service is the real one.
This guide explains how to work out what a given unit actually needs, how to check what your home can support, and how to tell — before you spend anything — whether electric is realistic for you at all.
One important note up front: everything below is general guidance to help you ask the right questions. Electrical service sizing and circuit design must be confirmed by a licensed electrician against your local code and your specific panel. Nothing here replaces that.
Why electric tankless units need so much power
A tankless heater has no storage tank. It heats water the instant you open the tap, which means it has to deliver all the energy that a tank water heater would have spread over hours — in the few seconds the water takes to pass through the unit.
That is an enormous instantaneous demand. A conventional electric tank heater typically draws around 4.5 kW. A whole-house electric tankless unit can draw 36 kW. That's roughly eight times the power, on demand.
Gas units sidestep this because burning fuel delivers heat far more densely than an electrical circuit can. That single physical difference is why electric tankless is easy for a bathroom sink and difficult for a whole house.
How to read the numbers yourself
Three figures matter, and they're related in a way you can check on any spec sheet.
1. Kilowatts (kW) — the unit's power draw.
This is the headline rating. Be careful: the model number is often not the kW rating. The EcoSmart "ECO 11" is a 13 kW unit. The Eemax "EEM24027" is a 27 kW unit — the 240 refers to voltage. Always read the specification, not the name.
2. Amps — what the circuit has to carry.
Nearly all whole-house electric tankless heaters in the US run on 240 volts. To convert:
Amps = (kW × 1000) ÷ 240
So an 18 kW unit draws 18,000 ÷ 240 = 75 amps. A 24 kW unit draws 100 amps. A 36 kW unit draws 150 amps.
That formula matches the manufacturer figures for every unit in the table below, so you can apply it to any model you're considering and sanity-check the seller's claims.
3. Breakers — and why they're bigger than the draw.
Water heating counts as a continuous load, and electrical code requires the circuit to be rated above the actual draw — generally 125% of it. That's why a 75-amp unit isn't fed by a single 75-amp breaker.
It's also why these units use multiple circuits. A 36 kW heater isn't wired to one enormous breaker; it's split across four separate double-pole 40-amp breakers, each with its own run of heavy cable. That means four free double-pole slots in your panel, not one.
What real units actually require
These figures come from the manufacturers' own published specifications, not from retailer listings. Prices are approximate and change constantly — check current pricing before making any decision.
| Unit | Approx. price | Power | Max draw | Breakers required | Min. service recommended |
|---|---|---|---|---|---|
| EcoSmart ECO 11 | $208 | 13 kW | 57 A | 1 × 60 A double-pole | 125 A |
| Marey ECO180 | $399 | 18 kW | 100 A | 2 × 50 A double-pole | 150 A |
| EcoSmart ECO 18 | $423 | 18 kW | 75 A | 2 × 40 A double-pole | 150 A |
| Eemax EEM24027 | $419 | 27 kW | 112.5 A | 3 × 40 A double-pole | — |
| Rheem RTEX-24 | $533 | 24 kW | 100 A | 3 × 40 A double-pole | 200 A |
| EcoSmart ECO 36 | $644 | 36 kW | 150 A | 4 × 40 A double-pole | 300 A |
| Stiebel Eltron Tempra 36 Plus | $849 | 36 kW | 150 A | 3 × 50 A | 300 A |
| Stiebel Eltron Tempra 29 Plus | $1,199 | 28.8 kW | 120 A | 3 × 40 A double-pole | 200 A |
A few things stand out.
Price tells you almost nothing about electrical demand. The $644 ECO 36 needs a 300-amp service. The $1,199 Tempra 29 Plus needs 200 amps. The more expensive unit is the easier one to install in a typical home.
The jump from 24 kW to 36 kW is where most projects break. Up to about 24 kW you're looking at a 200-amp service, which many modern homes already have. At 36 kW the manufacturers themselves recommend 300 amps — a different conversation entirely, involving your utility.
Several budget units couldn't be included here. A few electric models sold widely online don't publish accessible electrical specifications from the manufacturer. We've deliberately left them out rather than estimate. If a seller can't tell you the amperage and breaker requirement, treat that as a warning sign, not a detail to sort out later.
What your home can actually support
Knowing the unit's requirement is only half the problem. The other half is what's left in your panel.
Find your service size. Open your main electrical panel and look at the large main breaker at the top. The number stamped on it — 100, 150, 200 — is your service size in amps. That is the total your home can draw at once.
Remember that the heater doesn't get all of it. Your service is shared with everything else: air conditioning, electric range, clothes dryer, oven, pool pump, and increasingly an EV charger. Those loads compete for the same capacity. A licensed electrician performs a load calculation that accounts for existing demand before deciding what's available for a new circuit.
This is why "I have a 200-amp service, so a 100-amp heater fits" is not a safe conclusion. It depends entirely on what else is already drawing from that service, and on how those loads are counted under code.
Check for physical space too. A 36 kW unit needs four double-pole breaker slots. Even a panel with adequate capacity may simply not have the room, which can mean a subpanel or a panel replacement independent of any service upgrade.
The three realistic outcomes
Once you have both numbers, almost everyone lands in one of three places.
1. Point-of-use electric — usually straightforward.
If you want hot water at a single sink, a garage, a workshop, or an addition far from the main heater, a small electric unit is often genuinely easy. A 13 kW unit on a 60-amp circuit is a normal, manageable job in most homes with a 125-amp service or better. This is where electric tankless is at its best, and it's frequently overlooked because buyers jump straight to whole-house models.
2. Whole-house electric — possible, but price the electrical work first.
If your home already has a 200-amp service with genuine headroom, a 24 kW unit may work well. Get a load calculation before you buy. The order matters: confirm the electrical path, then choose the heater. Doing it the other way around is how people end up with an expensive box they can't energise.
3. Gas or propane — often the better answer.
If you have a 100-amp service, or a full 200-amp service, or you need high flow rates in a cold climate, gas or propane will usually get you there with far less electrical disruption. Gas units have their own requirements — venting, combustion air, and adequate gas line sizing — but they don't ask you to renegotiate your home's electrical capacity.
For a cold-climate whole-house replacement, this is frequently the honest recommendation, even though it's not the answer people searching for electric units expect.
What an electrical upgrade actually involves
If you need more capacity, it helps to know the two things people conflate.
A panel upgrade replaces the breaker box. This helps when you're short on breaker slots or the panel is old or unsafe, but your incoming service is adequate.
A service upgrade increases the power the utility delivers to your home. This is the bigger job: it involves the utility company, the service entrance conductors, the meter, and usually permits and inspection. Timelines depend on the utility, not just your electrician.
We're not going to publish a dollar figure, because a genuine range would be so wide it would be useless. Costs vary enormously by region, by whether your service is overhead or underground, by the distance from the transformer, by local permit requirements, and by what the utility charges. What we can say is that a service upgrade is typically a four-figure project, and it can exceed the cost of the water heater by a wide margin.
Get at least two quotes from licensed electricians before treating any of it as settled.
A checklist before you buy
Work through these in order. It takes an afternoon and can save a great deal of money.
- Read the actual specification for the unit you're considering — kW, maximum amperage, and the number and size of breakers. Not the model name.
- Check your main breaker to find your service size.
- List your major electrical loads — AC, range, dryer, oven, EV charger, pool equipment.
- Count your free double-pole breaker slots.
- Get a load calculation from a licensed electrician. This is the step that gives you a real answer.
- Price the electrical work before buying the heater, and compare that total against a gas or propane installation.
- Confirm requirements against local code, which can be stricter than the manufacturer's minimum.
If steps 5 and 6 reveal that whole-house electric needs a service upgrade, that's not a dead end — it's exactly the information you needed before spending money, and it's the point at which gas, propane, or a smaller point-of-use unit deserves a serious look.
Where to go next
- Compare the best electric tankless water heaters if your panel has the capacity.
- Look at gas and propane options if it doesn't.
- Work out the size you actually need with our sizing guide — flow rate and temperature rise determine the kW figure, which in turn drives everything above.
- Run the numbers on operating costs with our savings calculator, or answer a few questions in our quiz to narrow the field.
Specifications cited above are from manufacturer documentation and were accurate at the time of writing. Manufacturers revise specifications; always confirm against the current documentation for the exact model you intend to buy, and have a licensed electrician verify that your home can support it safely and to code.