18 kW vs 36 kW Tankless Water Heaters: Which Should You Buy?
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Choosing between an 18 kW vs 36 kW tankless water heater depends on your hot water needs. Both units are electric and operate at 240 volts. They provide on-demand hot water without a storage tank. The main difference is power output. The 18 kW model suits smaller households. The 36 kW model handles higher demand. Read on to see how these specs affect your daily routine.
Key differences
| Aspect | Rheem 18kW 240V Electric Tankless Water Heater | Stiebel Eltron Tempra 36 Trend Tankless Water Heater |
|---|---|---|
| Wattage | 18 kW | 36 kW |
| Flow Rate | 4.4 GPM | not listed |
| Temperature Control | ±1°F accuracy | 68° to 140°F range |
| Dimensions | 14.5 x 18.5 x 3.5 inches | 16.6 x 14.5 x 4.63 inches |
| Weight | 14.8 pounds | 19 pounds |
Which one should you choose?
Choose Rheem 18kW 240V Electric Tankless Water Heater if
- You need a compact unit for a small bathroom or kitchen sink.
- You prefer precise temperature adjustments with ±1°F accuracy.
- You want a lighter unit that is easier to mount.
Choose Stiebel Eltron Tempra 36 Trend Tankless Water Heater if
- You require higher power for multiple simultaneous showers.
- You need a wider temperature range up to 140°F.
- You value silent operation and auto-modulation features.
Verdict: Choose the 18 kW model if you have low hot water demand and limited space. Pick the 36 kW unit if you need robust performance for a whole house. Both are electric, so check your electrical service capacity before buying.
18 kW vs 36 kW Comparison
Wattage output
The Rheem model provides 18 kW of power, while the Stiebel Eltron unit delivers 36 kW. This difference is significant for your hot water capacity. Higher wattage generally allows for greater flow rates during colder months. The Stiebel Eltron Tempra 36 Trend offers double the heating power of the Rheem RTEX-18. You need more power to heat water faster or to support multiple fixtures simultaneously. If you require high-volume hot water, the 36 kW unit is the stronger choice. The 18 kW model is suitable for lower demand scenarios or point-of-use applications where less heat is needed.
Flow rate
The Rheem RTEX-18 specifies a maximum flow rate of up to 4.4 GPM. The Stiebel Eltron Tempra 36 Trend does not list a specific GPM value in the provided data. However, higher wattage typically supports higher flow rates. Since the Stiebel Eltron has 36 kW compared to the Rheem's 18 kW, it likely supports a higher flow. You should check the manufacturer's charts for the Stiebel Eltron to confirm the exact GPM at your incoming water temperature. For now, the Rheem offers a confirmed flow specification, while the Stiebel Eltron relies on its higher power for performance.
Temperature control
The Rheem unit features an external digital thermostatic control with LED display and ±1°F accuracy. This allows you to set precise temperatures. The Stiebel Eltron Tempra 36 Trend offers an output water temperature range of 68° to 140°F. It also includes a digital temperature display. Both units provide digital control for your convenience. The Rheem highlights its specific accuracy, which is useful for maintaining consistent temperatures. The Stiebel Eltron defines its operational range. Both are electric and use electronic controls, so you get digital precision with either choice. The Rheem's ±1°F accuracy is a specific technical advantage.
Dimensions and size
The Rheem RTEX-18 measures 3.5 x 14.5 x 18.5 inches and weighs 14.8 pounds. It is described as small. The Stiebel Eltron Tempra 36 Trend measures 14.5 x 16.6 x 4.63 inches and weighs 19 pounds. Both are compact for tankless heaters. The Rheem is slightly narrower in height but longer in length. The Stiebel Eltron is wider but shorter in height. You need to measure your installation space carefully. The weight difference is minimal, with the Stiebel Eltron being slightly heavier. Both fit in typical utility spaces, but the Rheem is lighter and has a different footprint.
Warranty coverage
The Stiebel Eltron Tempra 36 Trend includes a 7-Year Leakage and 3-Year Parts Warranty. This is explicitly stated in the product features. The Rheem RTEX-18 data does not mention specific warranty terms. You should verify the Rheem warranty details separately. The Stiebel Eltron provides clear, long-term protection against leaks and parts failure. This is a significant advantage for peace of mind. You get defined coverage with the Stiebel Eltron. Without warranty data for the Rheem, you cannot make a direct comparison. The Stiebel Eltron wins this round based on the available information.
Electrical requirements
Both units operate at 240 volts. The Stiebel Eltron Tempra 36 Trend requires a minimum recommended electric service of 300 A. This is a very high requirement for a single unit. The Rheem RTEX-18 does not list its amperage or service requirements in the data. You must ensure your electrical panel can handle the load. The 300 A recommendation for the Stiebel Eltron is unusual for one heater and may refer to the total home service or a specific configuration. You need to consult an electrician for both units. The Stiebel Eltron's high requirement is a notable constraint.
Which tankless heater fits your home
Small household with limited electrical service
The Rheem 18kW model operates at 240 volts and draws less power than the 36kW unit. It fits well in homes with lower electrical capacity. Its compact 3.5-inch height allows for easy installation in tight spaces. You get precise temperature control with ±1°F accuracy and a clear LED display for monitoring water heat levels.
Large family needing high flow rate
The Stiebel Eltron Tempra 36 delivers 36 kW of power to support higher flow rates for multiple simultaneous uses. It requires a minimum 300-amp electrical service, which suits robust home systems. The unit provides continuous hot water without interruption. Its digital display allows you to set temperatures between 68°F and 140°F for consistent comfort.
User prioritizing precise temperature adjustment
This Rheem unit features an external digital thermostatic control with ±1°F accuracy. You can fine-tune the water temperature to your exact preference. The advanced self-modulation adjusts power to meet your specific hot water demand. This precision helps prevent sudden temperature fluctuations, ensuring a stable shower experience every time you turn on the tap.
Buyer seeking extended warranty coverage
The Stiebel Eltron Tempra 36 includes a 7-year leakage warranty and a 3-year parts warranty. This coverage provides long-term protection for your investment. The unit also features silent operation and an electronic switch for easy activation. Its solid copper heating system ensures reliable performance while saving energy through auto-modulation when hot water is not in use.
Quick picks
| # | Product | Best for | Price |
|---|---|---|---|
| 1 | Rheem 18kW 240V Electric Tankless Water Heater | small households or single bathroom use | Check price of Rheem 18kW 240V Electric Tankless Water Heater on Amazon |
| 2 | Stiebel Eltron Tempra 36 Trend Tankless Water Heater | High-demand whole-house hot water needs | Check price of Stiebel Eltron Tempra 36 Trend Tankless Water Heater on Amazon |
The products compared
Rheem 18kW 240V Electric Tankless Water Heater
Best for: small households or single bathroom use
- Wattage
- 18 kW (what this guide is about)
- Fuel Type
- Electric
- Flow Rate
- 4.4 GPM
- Voltage
- 240 V
- Temperature Control
- ±1°F
- Dimensions
- 3.5 x 14.5 x 18.5 inches
View on Amazon: Rheem 18kW 240V Electric Tankless Water Heater Affiliate link
This 18 kW electric unit delivers up to 4.4 GPM of hot water. It features external digital thermostatic control with ±1°F accuracy and self-modulation technology that adjusts power to meet demand. The compact design measures 14.5 x 18.5 x 3.5 inches and weighs 14.8 pounds. It is suitable for whole-house use in smaller homes or single-bathroom applications where lower simultaneous demand is typical.
Pros
- 18 kW heating capacity for efficient energy use
- Up to 4.4 GPM flow rate
- Digital display with ±1°F temperature accuracy
- Self-modulation adjusts power to demand
Cons
- Lower 18 kW wattage limits simultaneous use
- Requires 240 V electrical connection
- No remote control included
Good fit if
- You need a compact unit for a single bathroom
- You have a 240 V electrical supply available
- You want precise digital temperature control
Skip if
- You need to run multiple showers at once
- You require a whole-house solution for large families
- You only have a 120 V outlet
Verdict: Rheem 18 kW Electric Tankless Water Heater suits smaller households with modest hot water needs. It provides precise temperature control and efficient modulation for single-point or light whole-house use.
Stiebel Eltron Tempra 36 Trend Tankless Water Heater
Best for: High-demand whole-house hot water needs
- Wattage
- 36 kW (what this guide is about)
- Fuel Type
- Electric
- Voltage
- 240 V
- Temperature Control
- 68° to 140°F
- Dimensions
- 14.5 x 16.6 x 4.63 in
- Use
- Whole House
View on Amazon: Stiebel Eltron Tempra 36 Trend Tankless Water Heater Affiliate link
The Stiebel Eltron Tempra 36 Trend is a 36 kW electric tankless water heater designed for high-demand households. It operates on 240V power and delivers water temperatures between 68°F and 140°F. This unit features a white, compact design measuring 14.5 x 16.6 x 4.63 inches. It uses an electronic switch for silent, on-demand heating. You should consider this model if you need substantial hot water capacity for multiple fixtures simultaneously.
Pros
- 36 kW output provides high flow rates for whole-house use
- Digital temperature display allows precise control from 68°F to 140°F
- Compact dimensions save space without requiring venting
- Silent operation with no moving parts during heating
Cons
- Requires a minimum 300 A electric service
- Heavier at 19 pounds compared to smaller 18 kW units
- Limited to electric fuel type only
Good fit if
- You have a 300 A electrical service available
- You need continuous hot water for multiple bathrooms
- You prefer a compact, vent-free electric unit
Skip if
- You only have a standard 100 A or 200 A electrical panel
- You are looking for a lower wattage 18 kW option
- You require gas fuel type for your installation
Verdict: Stiebel Eltron Tempra 36 Trend suits homes with robust electrical infrastructure that need high-volume hot water. It delivers reliable performance for whole-house demands where smaller units fall short.
Specifications side by side
| Spec | Rheem 18kW 240V Electric Tankless Water Heater | Stiebel Eltron Tempra 36 Trend Tankless Water Heater |
|---|---|---|
| Fuel Type | Electric | Electric |
| Flow Rate | 4.4 GPM | not stated |
| Wattage | 18 kW | 36 kW |
| Voltage | 240 V | 240 V |
| Temperature Control | ±1°F | 68° to 140°F |
| Dimensions | 3.5 x 14.5 x 18.5 inches | 14.5 x 16.6 x 4.63 in |
| Use | Whole House | Whole House |
| Weight | 14.8 pounds | 19 pounds |
| Size | Small | 36 (kW) |
| Color | Gray | White |
| Model | RTEX-18 | 36 Trend |
Frequently asked questions
Can an 18 kW tankless water heater serve a whole house?
Yes, but it is best for small households or single bathroom use. The Rheem 18 kW model provides efficient modulation for light whole-house demands. It suits homes with modest hot water needs. If you have multiple bathrooms running simultaneously, this unit may struggle to maintain consistent temperatures. It is ideal for single-point applications or very small residences.
What is the size difference between 18 kW and 36 kW units?
The 36 kW Stiebel Eltron unit is wider and taller than the 18 kW Rheem model. The 36 kW heater measures 16.6 inches wide and 14.5 inches high. The 18 kW Rheem unit is much slimmer, with a height of only 3.5 inches. Both are compact, but the higher wattage model requires more wall space for installation.
Do both 18 kW and 36 kW heaters use 240 volts?
Yes, both listed electric models operate at 240 volts. The Rheem 18 kW and Stiebel Eltron 36 kW units share this voltage requirement. This means your home needs a compatible electrical panel to support either choice. The 36 kW unit will draw significantly more current than the 18 kW model to achieve its higher power output.
Which unit is heavier, the 18 kW or 36 kW heater?
The 36 kW Stiebel Eltron unit is heavier. It weighs 19 pounds. The 18 kW Rheem model weighs 14.8 pounds. The difference is about four pounds. This weight difference is minor for mounting purposes, but you should ensure your wall brackets can support the load. Both units are relatively light for tankless heaters.
Is the 36 kW model better for high-demand households?
Yes, the 36 kW Stiebel Eltron unit suits homes with robust electrical infrastructure needing high-volume hot water. It delivers reliable performance for whole-house demands where smaller units fall short. If you have multiple showers running at once, the higher wattage helps maintain flow rates. The 18 kW model is not recommended for such high-demand scenarios.
What colors are available for these tankless water heaters?
The Rheem 18 kW model comes in gray. The Stiebel Eltron 36 kW unit is white. Color is a cosmetic difference that does not affect performance. You should choose based on how the unit blends with your utility space. Both models have a sleek, modern design suitable for indoor installation.
Can I install the 18 kW unit in a tight space?
Yes, the Rheem 18 kW model is designed for small spaces. It has a slim profile with a height of only 3.5 inches. Its width is 14.5 inches and length is 18.5 inches. This compact size allows for easy installation in closets or utility rooms where space is limited. The 36 kW unit is wider and taller.
How does wattage affect hot water flow rates?
Higher wattage generally allows for greater flow rates during colder months. The 36 kW unit provides double the power of the 18 kW model. This means it can heat more water per minute. In winter, when inlet water temperatures drop, the higher wattage helps maintain your desired output temperature without reducing flow. The 18 kW unit may reduce flow in cold climates.