Electric tankless water heaters run on 240 volts in the US; “220 volts” is legacy shorthand for the same service.
The label on the box says 240V, the old forum post says 220, and the electrician keeps saying “it’s all the same thing.” That mix-up is exactly why so many buyers stall before they order — one wrong assumption here means a unit that underperforms or a breaker that trips on day one. The short version: whether a tankless water heater uses 220 or 240 volts isn’t a choice between two products, and once you see why, picking the right model takes about five minutes.
220V Vs 240V: Same Circuit, Different Labels
The 220 vs 240 volt question is a labeling issue, not two separate categories of appliance. Standard US residential service delivers a nominal 240 volts, and the “220” you see in older writing is informal shorthand for that same line. Some manufacturers rate their units at 208/240V, and a few print 220–240V because the appliance tolerates a range. What matters far more than the number on the sticker is that your specific model’s manual governs how it must be wired.
Rheem’s RTEX installation manual illustrates the stakes. Its electric specifications cover 8 kW, 11 kW, 13 kW, 18 kW, 24 kW, 27 kW, and 36 kW heater inputs, all listed at 240 volts. The same manual notes that the heating elements are built for 240V and produce less heating power when supplied with lower voltage — so a unit fed 208V won’t fail outright, it just won’t heat water as fast as the rating promises.
Stiebel Eltron’s Tempra manual takes a different tack, listing models in 220–240V and 208V AC configurations, and it adds a wiring instruction most people never hear: size the conductors so voltage drop stays under 3% under load. That’s a quiet admission that real-world voltage sags when a big resistive load kicks on, and the wire gauge is part of the fix.
| Model Family | Input Voltage Listed | Notable Wiring Guidance |
|---|---|---|
| EcoSmart ECO series | 240V | Dedicated circuit; household service of 100A to 300A depending on model |
| EcoSmart POU 6 / 6T | 240V | Point-of-use units with model-specific breaker and wire sizing |
| Rheem RTEX | 240V | Each wire pair connects to its own breaker; verify breaker and wire gauge before operation |
| Stiebel Eltron Tempra | 220–240V / 208V AC | Size conductors to hold voltage drop under 3% under load |
| Units rated 208/240V | Dual-rated | Lower output at 208V; check the manual’s derated performance |
Will A 220V Unit Actually Work On Your Home’s Service?
The physical circuit in a US home is the same either way, but the load a tankless heater draws is the part that decides whether it works. Tankless electric units pull enormous amperage, which is why every major manufacturer calls for a dedicated circuit sized to that model, and why EcoSmart’s electrical requirements page lists recommended household electric service ranging from 100A to 300A depending on which unit you choose.
That service panel number is the real gate. A small point-of-use heater can run on modest service, while a whole-home 36 kW unit may demand 300A and a panel upgrade before it ever gets wired. Before buying, confirm three things: your panel’s total capacity, the breaker and wire gauge the manual specifies for your exact model, and whether your utility delivers a full 240V at the meter. If the voltage runs low in your area, a unit designed for 240V-only will underperform rather than fail, which is a slower and more frustrating problem to diagnose.
If you already know you’re working with a 220V-nominal setup and want models rated for it, a tested roundup of the best 220 volt tankless water heater options can narrow the field fast. Otherwise, match the manual, not the listing title.
The Mistakes That Cost People A Rewire
Two errors show up again and again, and both come from treating voltage labels as interchangeable when they aren’t.
- Treating “220V” and “240V” as universal swaps. Official documentation shows some units are designed for 240V only, while others accept 208/240V or 220–240V. The exact model manual is the only authority — a product page headline is not.
- Undersupplying the voltage and expecting full output. Rheem’s manual is explicit that elements designed for 240V produce less heating power at lower voltage. You don’t get a fault code; you get lukewarm water and a mystery.
- Skipping the panel math. Buying a 36 kW heater before checking service capacity can turn into a panel upgrade, which typically dwarfs the cost difference between models.
- Ignoring the wire-pair rule. Rheem instructs connecting each wire pair to one breaker and confirming breaker size and wire gauge are correct before operation.
Frequently Asked Questions
Is 220V the same as 240V for a water heater?
Functionally, yes — 220V is legacy shorthand for the nominal 240V US residential service. The distinction matters only in the documentation: some models are rated 240V only, others 208/240V or 220–240V. Always check the specific unit’s manual rather than assuming the labels are interchangeable.
Can I run a tankless water heater on 208 volts?
Some models are dual-rated for 208/240V and will run, but with reduced heating output. Rheem notes its elements are designed for 240V and produce less heating power at lower voltage. If your building supplies 208V, confirm the model accepts it before ordering.
What size circuit does an electric tankless heater need?
It varies by model and wattage. EcoSmart’s electrical requirements page lists recommended household service from 100A to 300A depending on the unit, and Rheem directs each wire pair to its own breaker. The manual for your exact model has the final breaker and wire gauge spec.
References & Sources
- EcoSmart. “Electrical Requirements” Lists 240V specs, breaker/wire recommendations, and household service ranges by model.
- Rheem. “RTEX Installation Manual” Documents 240V electrical specs and the wire-pair-to-breaker wiring rule.
- Stiebel Eltron. “Tempra Installation Manual” Shows 220–240V / 208V configurations and the 3% voltage-drop conductor rule.
