Prices, ratings and stock were checked on Amazon on October 22, 2026. Amp figures are simple math from kilowatts and volts. Have a licensed electrician check your panel.
A tankless water heater can be great or a wiring headache. Here are six in stock, the amp math and how cold water changes the flow.
Table of Contents
Tank, Tankless or Mini-Tank?
A tank heater stores hot water and loses a little heat all day. A tankless heater heats water on demand and can run out if you ask for too much. A mini-tank serves one sink. The right one depends on your household size and your power or gas supply.
What to Look for in a Water Heater
- Fuel: electric, gas or propane.
- Flow rate: gallons per minute at your winter water temperature.
- Power: kilowatts and amps for electric units.
- Size: number of showers or taps at once.
- Venting: gas units need proper venting.
Check Your Panel First
An 18 kW tankless heater draws about 75 amps at 240 volts (18,000 watts divided by 240 volts). A 24 kW unit draws about 100 amps. Your service panel and wiring have to handle that on top of everything else. A licensed electrician can check. A multimeter from HVAC multimeters helps with basic checks, but installation is pro work.
Here is where water heaters are heading: heat pump models and electric tankless units with Wi-Fi. A Stiebel Eltron Tempra 24 lists at $514.23 and Wi-Fi tankless units from Westinghouse and Electrolux sit near $450 to $480. Gas still leads for big homes, but electric and heat pump models are taking share because of rebates and clean-energy rules.
6 Water Heaters to Consider
Rechecked on Amazon on October 22, 2026. Prices, ratings and buttons are below.
Price on Amazon: $432.20 | 4.2 stars from 2,188 ratings (checked October 22, 2026)
The Rheem 18kW lists a flow of up to 4.4 gallons per minute, an external digital thermostat and self-modulating power. It costs $432.20, rated 4.2 (2,188).
Price on Amazon: $542.99 | 4.3 stars from 896 ratings (checked October 22, 2026)
The Rheem RTEX-24 is the 24 kW sibling for larger homes. It costs $542.99, rated 4.3 (896).
Price on Amazon: $514.23 | 4.6 stars from 96 ratings (checked October 22, 2026)
The Stiebel Eltron Tempra 24 Trend is a 24 kW, 240V electric tankless heater. It costs $514.23, rated 4.6 (96).
Price on Amazon: $265.98 | 4.3 stars from 142 ratings (checked October 22, 2026)
The CAMPLUX 18kW is an electric on-demand unit with a digital display. It costs $265.98, rated 4.3 (142).
Price on Amazon: $199.99 | 4.4 stars from 1.4K ratings (checked October 22, 2026)
The CAMPLUX ME25 is a 2.5 gallon electric mini-tank for a single sink. It costs $199.99, rated 4.4 (1.4K).
Price on Amazon: $159.99 | 4.2 stars from 3.3K ratings (checked October 22, 2026)
The CAMPLUX 5L portable is a propane unit rated at 1.32 gallons per minute, made for camping, RVs and outdoor use. It costs $159.99, rated 4.2 (3.3K).
The downside is the electrical bill and the wiring. An 18 kW electric tankless heater draws about 75 amps at 240 volts, and a 24 kW unit about 100 amps. Many older homes do not have the spare capacity, and a panel upgrade can cost more than the heater. Cold winter groundwater also cuts the flow, so a unit that gives 4.4 gallons a minute in July may give far less in January. Expect surprise installation bills and cold showers. Check your panel before you buy.
Flow Rate and Cold Water
Heater makers list a top flow, but the real number depends on how cold your water is. In winter, incoming water can be 40 degrees or colder. A unit that heats water by 70 degrees at 2 gallons a minute may deliver less if the water is colder. Pick a unit with room to spare for your coldest month.
Hard Water and Scale
Hard water leaves scale on heating elements. That cuts life and efficiency. Consider a softener or whole-house filter, see whole-house water filters for hard water and whole-house reverse osmosis systems. Flush tank heaters yearly, see how to flush a water heater.
Running Cost
Electric tankless heaters avoid standby loss but draw huge power while they run. Gas and heat pump tank heaters can cost less to run per gallon. Compare per-gallon costs with your local rates before you decide. Learn how heating systems fit together in how HVAC systems work.
How to Pick
Whole house, electric, known brand? Rheem 18kW or 24kW. Highest rated big unit? Stiebel Eltron. Tight budget? CAMPLUX 18kW. One sink? CAMPLUX ME25. Camping or outdoor shower? The CAMPLUX 5L propane unit.
FAQ
How many amps does an 18kW tankless heater need?
About 75 amps at 240 volts (18,000 watts divided by 240 volts).
Do tankless heaters run out of hot water?
They can if you ask for more flow than the unit delivers at your water temperature.
Is a mini-tank worth it?
For a single sink or a distant tap, yes.
Can you install a tankless heater yourself?
Electric and gas units need proper wiring or venting. Hire a licensed pro.
Final Word
Size by winter flow, check the panel and hire a pro for the install.
What comes next: more heat pump tank heaters, more smart controls and more limits on gas hookups in new homes. For a small bathroom or a cabin, a tiny electric unit still works. For a whole house, size by winter flow, not summer.
Related Guides
Keep going with how to flush a water heater, whole-house water filters for hard water, whole-house reverse osmosis systems, HVAC multimeters and how HVAC systems work.
