Rheem 18kW 240V Tankless Electric Water Heater: A Space-Saving Alternative to a 50 Gallon Water Heater
For many homeowners, the 50 gallon water heater is simply what was already in the closet. It works, until a long shower, a dishwasher cycle, and a laundry load all land in the same hour. Then the tank runs low and the water turns cold.
The Rheem 18kW 240V Tankless Electric Water Heater in Gray (model RTEX-18, part of the Classic Professional series) was built to avoid that problem. It heats water only when you open a tap, so there is no tank to empty. This guide covers how it compares with a 50 gallon water heater, how much hot water it can deliver, what the installation requires, and who it suits best.
Why Look Beyond the 50 Gallon Water Heater?
A 50 gallon water heater is popular for good reasons: it is familiar, usually cheaper to buy, and simple to replace. But a storage tank has built-in limits:
- A fixed supply. Once the 50 gallons are used, you wait for the tank to reheat.
- Standby heat loss. The tank keeps water hot all day, even when nobody is home.
- A big footprint. A typical 50 gallon water heater stands roughly four to five feet tall and takes up real floor space.
- Tank aging. Over the years, sediment and corrosion can shorten a tank’s life and raise the risk of leaks.
Tankless models solve these issues by changing how hot water is made.
Meet the Rheem RTEX-18
The RTEX-18 is a wall-mounted electric tankless water heater rated at 18kW and 240 volts. Water flows through copper tubing past heating elements, and the unit warms it to your chosen temperature on the way to the tap. When you close the tap, the heating stops.
What stands out about this model:
- Digital temperature display. A clear screen shows your temperature setting, such as 105°F, and a dial lets you adjust it.
- Compact size. It measures about 18¼ inches tall, 14½ inches wide, and 3½ inches deep. That is far smaller than a 50 gallon water heater.
- Copper heat exchange design. Copper tubing is known for good heat transfer.
- No gas line, no venting. It runs on electricity alone, which can simplify installation.
- Clear hot and cold connections. Red and blue fittings at the bottom mark the hot outlet and cold inlet.
- Gray finish. The neutral, brushed-metal look fits easily in a garage, utility room, or basement.
Size Comparison: Tankless vs. a 50 Gallon Water Heater
Here is where the RTEX-18 makes the strongest first impression. A 50 gallon water heater sits on the floor and needs clearance around it. The Rheem unit hangs on the wall, with a depth of only 3½ inches. That frees the floor beneath it for shelving, storage, or even bikes and tools.
If you are converting a closet, finishing a basement, or working in a tight garage corner, that space gain can matter as much as the hot water itself.
How Much Hot Water Can It Deliver?
Tankless heaters are rated by flow (gallons per minute, or GPM), not by tank size. Flow depends heavily on how cold the incoming water is. Rheem’s selection guide for the RTEX-18, based on a 105°F temperature setting, lists these maximum flow rates:
| Incoming water temperature | Approx. max flow |
|---|---|
| 37–47°F (cold climates) | 2.1 GPM |
| 47–57°F | 2.6 GPM |
| 57–62°F | 2.9 GPM |
| 62–77°F (warm climates) | 4.4 GPM |
For context, Rheem’s guide counts a typical shower at about 1.5 GPM and a faucet at about 0.5 GPM. That means:
- In very cold regions, the RTEX-18 can comfortably handle about one shower plus a faucet at the same time.
- In warmer regions, it can serve several fixtures at once, such as multiple showers and a faucet.
Higher temperature settings reduce flow, so a 105°F setting stretches capacity further than a hotter one. Because of this, the RTEX-18 can feel like an upgrade from a 50 gallon water heater for a small or medium household, but a busy family in a cold climate should review the chart carefully.
Electrical Requirements: Plan This First
This is the part many buyers underestimate. Swapping a 50 gallon water heater for a tankless unit is not just a plumbing change. Rheem’s published electrical requirements for the RTEX-18 include:
- Power: 18kW at 240 volts
- Maximum draw: 75 amps
- Breakers: two 40-amp double-pole breakers, dedicated solely to the unit
- Wiring: two runs of 8 AWG wire
- Household service: 150-amp service recommended
- Panel space: enough open slots to add both breakers
A traditional 50 gallon water heater typically uses a single circuit, so most homes will need electrical work to support a tankless unit. Rheem recommends professional installation and suggests consulting an electrician before you buy. Always confirm details in the official manual, and follow local codes and permit rules.
Benefits of the Rheem RTEX-18
Hot Water On Demand
The marketing line on Rheem’s own product imagery says it plainly: endless hot water, no tank required. Within its flow rating, the heater keeps producing hot water for as long as you need it, so long showers stop being a gamble.
Less Standby Energy Loss
A 50 gallon water heater reheats its stored water throughout the day. A tankless unit heats only when you ask for it, which can reduce wasted energy, especially in households that are away often.
Easy Temperature Control
The dial and digital readout make it simple to set and check your temperature. Many households choose a moderate setting, which is also safer around children and kind to your energy bill.
Flexible Placement
Since it needs no flue or gas connection, you have more freedom in where to mount it. A garage wall, utility room, or basement can all work, subject to code and the manufacturer’s clearance guidance.
No Large Tank to Fail
Without a big tank holding dozens of gallons, the risk of a major tank rupture and flooding is lower.
Who Should Choose This Heater?
The RTEX-18 is a strong match if you:
- Want to replace a 50 gallon water heater and gain back floor space
- Live in a mild or warm climate, where flow rates are higher
- Have a small or medium household, an apartment, a cabin, or a granny flat
- Prefer an all-electric setup with no gas or venting
- Have, or are willing to add, the required electrical capacity
It may be less ideal if you live in a very cold region with a large household, or if your panel cannot support two dedicated 40-amp breakers. In those cases, a larger unit from the same family, or a different setup, may fit better.
Tankless vs. 50 Gallon Water Heater: Quick Comparison
| Factor | 50 gallon water heater | Rheem RTEX-18 |
|---|---|---|
| Hot water supply | Limited to tank volume | On demand, limited by flow rate |
| Size | Large, floor-standing | Compact, wall-mounted |
| Standby loss | Yes | No stored water to keep hot |
| Upfront cost | Usually lower | Often higher |
| Electrical needs | Typically one circuit | Two dedicated 40-amp breakers |
| Venting | Not for electric tanks | None needed |
Practical Tips for Getting the Most From It
- Check your incoming water temperature. Use the selection guide to see what flow to expect in your region and season.
- Hire licensed pros. A qualified electrician and plumber help ensure safe, code-compliant installation.
- Choose efficient fixtures. Low-flow showerheads and faucets let the heater serve more outlets at once.
- Mind your water quality. If you have hard water, descaling or a softener can protect performance over time.
- Set a safe temperature. Start around 105°F for general use and adjust as needed within the manufacturer’s guidance.
Is It Worth the Switch?
Think in terms of total cost, not just the sticker price. A 50 gallon water heater is cheaper up front, but a tankless unit can offer savings in energy use, a longer expected service life, and the value of reclaimed space. Your actual results depend on local electricity rates, installation costs, and how you use hot water. If panel upgrades are needed, factor those in too.
Frequently Asked Questions
Can the RTEX-18 replace my 50 gallon water heater?
Often yes, for small to medium households, but check your incoming water temperature and the number of fixtures you run at once.
How many showers can it run at once?
Based on Rheem’s guide at 105°F, about one shower plus a faucet in cold regions, and several fixtures in warm regions.
What size is it?
About 18¼” tall, 14½” wide, and 3½” deep.
What electrical setup does it need?
Two dedicated 40-amp double-pole breakers, 8 AWG wire, and a 75-amp maximum draw. A 150-amp household service is recommended.
Does it need a gas line or vent?
No. It is fully electric.
Is professional installation necessary?
Rheem recommends it, and it is the safest route given the electrical load.
Final Thoughts
The 50 gallon water heater has earned its place in millions of homes, but it is not the only smart option anymore. The Rheem 18kW 240V Tankless Electric Water Heater offers on-demand hot water, a slim wall-mounted body, and easy temperature control in a package far smaller than a storage tank.
Before buying, check three things: your climate’s incoming water temperature, how many fixtures you run at once, and whether your electrical panel can support two dedicated 40-amp breakers. If all three line up, moving from a 50 gallon water heater to the RTEX-18 could mean more comfort, more space, and fewer cold-shower surprises.
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