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Written from the listing and 2,187 buyer reviews, not from our own use · How we rate
We like the Rheem 18 kW Electric (240V) for the mix: up to 4.4 GPM, 18 kW of heating power and self-modulating power. It's our #1 pick for whole-house electric tankless water heaters, and a natural fit for a smaller home or one main bathroom on electric.
These are sold as variants of one listing family. Each variant has its own page.
| Brand | Rheem |
|---|---|
| Category | Electric tankless water heaters |
| Fuel | Electric |
| Stated maximum flow | 4.4 GPM |
| Stated heating power | 18 kW |
| Stated voltage | 240 V |
| Buyer reviews | 2,187 · 4.2/5 |
| ASIN | B01MR7Z39V |
Full listing title: Rheem 18kW 240V Tankless Electric Water Heater, Gray
| Fuel | 18kWFull listing title |
|---|---|
| Stated maximum flow | up to 4.4 GPMListing text |
| Stated heating power | 18kWFull listing title |
| Stated voltage | 240VFull listing title |
#1 of 5 in our Whole-House Electric Tankless Heaters ranking
What it's made for: a smaller home or one main bathroom on electric.
The two picks next to it in our whole-house electric tankless water heaters ranking, with their rating and a dated price.
The heating power the listing states. At 240 V it draws about 75 A. Before you think about showers, your electrician has to say yes to that number at the panel in the garage.
Why it made our list
4.4 GPM max. That is the headline figure, usually at a modest temperature rise. Our calculator counts a shower at 2 GPM, so think one shower at a time.
Self-modulating. It draws only the power the flow and the setpoint need, instead of slamming every element on. Steadier temperature, and no wasted amps on a summer afternoon.
Warm tap water: where it comes in around 62 °F (much of the South), 18 kW gives about 2.9 GPM of 105 °F water on our calculator's maths. That's one 2 GPM shower at a time.
A cold January: where it comes in at 42 °F, it's about 1.9 GPM, less than one 2 GPM shower. Up north, that's the figure to size against, and it's why the honest answer is sometimes a second unit or gas.
The seller states, word for word, in the Rheem 18 kW Electric (240V) listing:
External digital thermostatic control with LED display (+/1 degree accuracy).
Durable Copper immersion two heating elements, field Serviceable.
That's the seller talking, not us. No listing tells you how it copes with 42 °F water at 6:45 in the morning.
What the Rheem 18 kW Electric (240V) listing names:
For the Rheem 18 kW Electric (240V), the installation manual settles the breakers and the wire.
At the price on Sep 30, 2026, each kW of stated heating power works out at about $23.50 (price ÷ stated kW, our arithmetic). That puts it in the pricier third of the 27 whole-house electric tankless heaters we have a price and a stated power for.
The electrician comes on top: new double-pole breakers, heavy wire and, in some houses, a panel upgrade.
The button opens the exact listing (ASIN B01MR7Z39V), where today's price, stock and seller apply. Sizes and variants of the same family often sit at different prices.
Colder tap water where you live? The WINTEMP WN-36kW Electric, 240V (8.1/10) is rated 36 kW.
Only one sink far from the heater? The ThermoMate 14 kW Electric, 240V (7.9/10) is a 14 kW point-of-use unit.
Want to spend less? The SIVUATEK 18 kW Electric, 240V (7.7/10) was listed at $245.99 on Sep 30, 2026.
Our take
If a friend asked us for one whole-house electric tankless water heater, we'd point them to the Rheem 18 kW Electric (240V). 2,187 buyers give it 4.2/5, good but not spotless. Where it falls short: no word on the breakers it needs. If it covers your morning rush in January, we'd start here. If not, the WINTEMP WN-36kW Electric (240V) is plan B.
New breakers, the unit on the wall next to the old tank's spot, water in and out. An electrician does the panel side.
Hot in a few seconds once the water in the pipes has passed, and the power steps up and down with the flow (self-modulating, per the listing). Set it, then leave it.
With 42 °F water coming in, 18 kW gives about 1.9 GPM at 105 °F on our calculator's maths. Low-flow shower heads stretch that further.
Clean the inlet screen and flush out scale on hard water. Our flushing and descaling guide has the routine.
Buyers give it 4.2/5, a solid record, slightly below this ranking's 4.3 average. And with 2,187 reviews behind it, that number is hard to argue with.
Before the Rheem 18 kW Electric (240V) goes in your cart, check your panel: 18 kW at 240 V is roughly 75 A, so an electrician should look at the panel first.
Take a photo of the panel label (main breaker size and free slots) and send it to the electrician with the listing. Our electrical requirements guide explains what they will look for.
On our calculator's maths, 18 kW gives about 1.9 GPM with 42 °F winter water and 2.9 GPM with 62 °F water. Our flow-rate calculator does it for your region.
Hardwired 240 V work needs a licensed electrician and often a permit. Our install cost guide shows what usually ends up on the quote.
Hard water still builds up on the elements and the inlet screen. Our flushing and descaling guide covers the routine.
How the Rheem 18 kW Electric (240V) stacks up against the closest alternative in our ranking, the WINTEMP WN-36kW Electric (240V).
| Criterion | Rheem 18 kW Electric, 240V | WINTEMP WN-36kW Electric, 240V |
|---|---|---|
| Our rating | 9.3/10 | 8.1/10 |
| Heating power | 18 kW | 36 kW |
| Max flow | 4.4 GPM | 8.7 GPM |
| Breakers | Not stated | 1 × 50 A |
| Warranty | Not stated | 2 years |
| Buyer reviews | 2,187 · 4.2/5 | 203 · 4.4/5 |
Pick the WINTEMP WN-36kW Electric (240V) if more power (36 kW) and more stated flow (8.7 GPM) matter more to you; on our rating, the Rheem 18 kW Electric (240V) comes out ahead.
We rate it 9.3/10 (excellent), #1 of 5 in our whole-house electric tankless water heater ranking. It is based on 2,187 buyer reviews averaging 4.2/5 and the specifications stated in the listing.
The listing states a maximum flow of 4.4 GPM and a heating power of 18 kW on 240 V. The headline flow is a mild-weather number: with cold winter water coming in, the same unit gives you less. Read the maker's flow chart at your winter rise, or let our flow-rate calculator do the sums.
The listing states 240 V and 18 kW. At 240 V, that is roughly 75 A. A licensed electrician checks the panel, the breakers and the wiring before it goes on the wall.
Its listing states up to 4.4 GPM, 18 kW of heating power and self-modulating power. These are the seller's figures, not our measurements: how steady the temperature is in your shower and how it copes with your winter water can't be read from a listing.
Your panel: take a photo of the panel label (main breaker size and free slots) and send it to the electrician with the listing. Our electrical requirements guide explains what they will look for. Your winter flow: on our calculator's maths, 18 kW gives about 1.9 GPM with 42 °F winter water and 2.9 GPM with 62 °F water. Our flow-rate calculator does it for your region. The install: hardwired 240 V work needs a licensed electrician and often a permit. Our install cost guide shows what usually ends up on the quote. Scale: hard water still builds up on the elements and the inlet screen. Our flushing and descaling guide covers the routine.
We link to the exact listing on Amazon, where today's price, stock and seller apply. It is an affiliate link, and it has no effect on the rating.
No, we haven't used it ourselves. Everything on this page comes from its listing and 2,187 buyer reviews, read line by line and weighed with our published formula.
If the Rheem 18 kW Electric, 240V hasn't sold you yet, see where it ranks, read the guides that feature it or browse all electric tankless water heaters.