Electric tankless water heaters are compact, need no gas line or vent, and lose almost no heat in standby. They also draw a lot of current while they run, and their output drops sharply when incoming water is cold. Whether one is a good fit comes down to three things: the flow you need, the temperature rise in your area, and the electrical service in your home.
How electric tankless sizing works
An electric tankless heater is rated in kilowatts (kW). That power sets how many gallons per minute (GPM) it can heat by a given number of degrees. You can estimate flow with this formula:
GPM ≈ (kW × 3,412) ÷ (500 × temperature rise in °F)
Here 3,412 converts kW to BTU/h, and 500 is the weight of a gallon of water (about 8.33 lb) times 60 minutes. Electric units turn almost all of their input into heat, so no efficiency factor is needed for a planning estimate.
Estimated flow by size
| Heater size | 40°F rise | 55°F rise | 70°F rise |
|---|---|---|---|
| 8 kW | 1.4 GPM | 1.0 GPM | 0.8 GPM |
| 13 kW | 2.2 GPM | 1.6 GPM | 1.3 GPM |
| 18 kW | 3.1 GPM | 2.2 GPM | 1.8 GPM |
| 24 kW | 4.1 GPM | 3.0 GPM | 2.3 GPM |
| 27 kW | 4.6 GPM | 3.3 GPM | 2.6 GPM |
| 36 kW | 6.1 GPM | 4.5 GPM | 3.5 GPM |
These are calculated estimates. Always use the manufacturer's flow chart for the final choice.
Step 1: Find your temperature rise
Subtract your coldest winter incoming water temperature from the outlet temperature you want (usually about 105°F at a shower or 120°F at the heater). Southern states may see incoming water in the 60s or 70s °F in winter. Northern states can see the 30s or 40s °F. Our tankless sizing guide has a regional table.
Step 2: Add up the fixtures you run at once
A typical shower uses 1.5–2.5 GPM, and a bathroom faucet uses about 1.0–1.5 GPM. Two showers at once can need 4–5 GPM. Compare that total with the table at your temperature rise.
Wiring and electrical requirements
This is where most electric tankless projects succeed or fail. Current draw is power divided by voltage:
Amps = watts ÷ volts
| Heater size (240V) | Total current |
|---|---|
| 8 kW | About 33 A |
| 13 kW | About 54 A |
| 18 kW | About 75 A |
| 24 kW | About 100 A |
| 27 kW | About 113 A |
| 36 kW | 150 A |
Larger whole-house units split this load across two, three or four heating chambers. Each chamber runs on its own double-pole breaker and wire run. The manufacturer's installation manual lists the exact breaker size, wire gauge and number of circuits for each model. Follow it and the National Electrical Code.
Questions to answer before you buy
- What is your main service size? A 27 or 36 kW unit is a large load. Many homes need a 200A service, and some need an upgrade. An electrician can run a load calculation.
- Is there panel space? Whole-house units can need up to four double-pole breaker slots.
- How far is the panel? Long runs of heavy-gauge copper add real cost.
- What else runs at the same time? Electric heat, an EV charger, a range and a dryer all compete for the same service.
Best uses for electric tankless
Whole-house in warm climates
Where winter incoming water stays above about 55–60°F, a 27–36 kW unit can often supply two fixtures at once, such as a shower and a faucet, or two low-flow showers. Examples are the Rheem RTEX-36 and the Stiebel Eltron Tempra 24 Plus.
Point of use
A small unit under a sink or near a remote bathroom gives fast hot water without a long pipe run. Smaller models such as the Rheem RTEX-13 suit a single fixture or a small bathroom. Also see our point of use heaters.
Booster for a solar or heat pump system
An electric tankless unit downstream of a storage tank can top up water that arrives already warm. In this role it only has to deliver a small temperature rise.
When gas or a tank is the better choice
In cold-water regions with two or more showers running at once, even a 36 kW unit may not keep up. A gas tankless, a heat pump water heater or a storage tank is often a better fit there.
Installation and maintenance tips
- Install isolation valves so the heat exchanger can be flushed. Hard water causes scale on elements.
- Check the minimum activation flow. Very low-flow faucets may not switch the unit on.
- Expect a short delay and a small temperature change when other fixtures open or close. Models with flow control and thermostatic control reduce this.
- Mount the unit where it can be serviced, and keep the clearances the manual requires.
Browse all models in our electric tankless water heaters collection, from 8 kW to 36 kW.
Safety note: Electric tankless heaters need dedicated 240V circuits sized for very high current. Wiring must be done by a licensed electrician to the manufacturer's instructions and local code. Undersized wire or breakers can overheat and cause a fire.
Frequently asked questions
What size electric tankless water heater do I need for a whole house?
In warm climates, a 27–36 kW unit can often support two fixtures at once. In cold climates, even 36 kW may only support one shower plus a faucet at the same time. Use the formula above with your own temperature rise.
Do I need a 200 amp panel for an electric tankless water heater?
Large whole-house models often require one. Small point-of-use units do not. An electrician's load calculation is the only reliable answer for your home.
Is electric tankless cheaper to run than an electric tank?
It avoids standby heat loss, so it usually uses somewhat less energy for the same hot water. Savings depend on your usage and electricity rates.
Can I install an electric tankless water heater myself?
The plumbing is simple, but the electrical work is not. Have a licensed electrician size and install the circuits, and check whether your area requires a permit.