A 120V tankless heater serves one low-demand point of use, while 240V models cover whole-home demand and higher flow rates.
One bathroom sink on a standard household circuit points to 120V. A house running a shower, a dishwasher, and two faucets at once points to 240V. Most homes land somewhere between, so the 120V vs 240V tankless water heater call follows the load first, and then the wiring has to agree.
Voltage leads every spec sheet, but breaker size, wire gauge, and household service capacity decide whether a unit can be installed at all.
How Do You Know Which Voltage You Need?
Match the unit to the job before the price. A single low-demand point of use — a bathroom sink, a small kitchen tap — is what 120V models are built for, while whole-home supply and multi-fixture demand belong to 240V.
Four checks settle the purchase, and every number comes from the manufacturer’s electrical table:
- Draw at the point of use. 120V point-of-use models run 3.5–4.0 kW. Anything beyond a sink points to 240V, where output starts near 5.5 kW and reaches 24 kW on the largest whole-home units.
- Breaker and wire. An EcoSmart POU 3.5 calls for a 1 x 30 amp breaker on #10 AWG wire, while the 240V EcoSmart ECOS 27 calls for three 40 amp double-pole breakers on #8 AWG.
- Household service. 120V point-of-use models list a 100 amp service. High-load 240V units such as the EcoSmart ECOS 27 list 200 amps.
- Physical fit. Confirm the connection size and the cabinet space before ordering: the Rheem RTEX-04 measures 5-7/8 x 10-7/8 x 2-3/4 inches with a 1/2 inch NPT connection, while the RTEX-24 wants 3/4 inch NPT.
Notice what the 120V column shares: lower wattage, a single breaker, and a short wire run. That is why these units sit close to the fixture they serve, under the sink or in a nearby cabinet, while whole-home 240V models mount on a utility wall with room for multiple breakers.
If the load points to 120V, a roundup of tested 120V tankless water heater picks narrows the model list fast — then bring that model’s spec sheet back to these four checks.
Choosing Tankless Water Heater Voltage: What The Spec Sheets Show
The two categories split cleanly: 120V point-of-use models stay in the 3.5–4.0 kW range, while 240V units push from 5.5 kW to 24 kW with heavier breakers to match.
EcoSmart’s electrical requirements show what those ratings demand at the panel — the 120V POU 3.5 and POU 4T each pull 29 amps and need a 1 x 30 amp breaker with #10 AWG wire.
| Model | Voltage And Draw | Breaker And Household Service |
|---|---|---|
| EcoSmart POU 3.5 | 120V, 29A | 1 x 30A breaker, 100A service |
| EcoSmart POU 4T | 120V, 29A | 1 x 30A breaker, 100A service |
| EcoSmart POU 6 | 240V, 29A | 1 x 30A breaker, 100A service |
| EcoSmart ECOS 27 | 240V, 112.5A | 3 x 40A double-pole, 200A service |
| Rheem RTEX-04 | 120V, 3.5 kW, 29A | 1 x 30A breaker |
| Rheem RTEX-08 | 240V, 8.0 kW, 33A | 1 x 40A breaker |
| Rheem RTEX-24 | 240V, 24.0 kW, 100A | 3 x 40A breakers |
Rheem’s RTEX spec sheet covers the 240V side, topped by the RTEX-24 at 100 amps across three 40 amp breakers.
What To Verify Before Ordering
Three details catch buyers off guard: these heaters hardwire to a dedicated circuit instead of plugging in, 240V units lose output on 208V service, and a slow trickle may not switch the heater on.
- Hardwiring, not a plug. At least one 120V point-of-use model must be hardwired to a dedicated 30 amp circuit. A 29 amp draw is far past anything a household outlet carries.
- 208V service. A unit sized for 240V will feel weaker there.
- Minimum flow. The RTEX-04, RTEX-06
