Electric tankless · kW and amps
Electric tankless water heater size
Electric tankless sizing is pure arithmetic: kW = 500 × gpm × rise ÷ efficiency ÷ 3,412. Two standard showers at a 70°F rise need about 40.7 kW, roughly 170 amps at 240 volts.
Calculate kW and amps
Start with the required flow from the tankless calculator (it counts only the hot share of each blended fixture), or enter a flow you already know.
Electric input needed
40.4kW
- Temperature rise
- 70°F
- Heat into the water
- 136,500 BTU/hr
- Current at 240 V (total)
- 168 A
| Unit size | Flow at 70°F rise |
|---|---|
| 13 kW | 1.3 gpm — not enough |
| 18 kW | 1.7 gpm — not enough |
| 24 kW | 2.3 gpm — not enough |
| 27 kW | 2.6 gpm — not enough |
| 36 kW | 3.5 gpm — not enough |
At 99% efficiency. Current is total power ÷ voltage; units split it across several circuits. Breaker and wire sizes come from the manufacturer and the electrical code, not this number.
kW needed by flow and rise
Electric input at 99% efficiency. Find your required flow on the left and your temperature rise along the top.
| Flow | 40°F rise | 50°F rise | 60°F rise | 70°F rise | 80°F rise |
|---|---|---|---|---|---|
| 1.5 gpm | 8.9 | 11.1 | 13.3 | 15.5 | 17.8 |
| 2.0 gpm | 11.8 | 14.8 | 17.8 | 20.7 | 23.7 |
| 3.0 gpm | 17.8 | 22.2 | 26.6 | 31.1 | 35.5 |
| 4.0 gpm | 23.7 | 29.6 | 35.5 | 41.4 | 47.4 |
| 5.0 gpm | 29.6 | 37.0 | 44.4 | 51.8 | 59.2 |
| 6.0 gpm | 35.5 | 44.4 | 53.3 | 62.2 | 71.0 |
What common unit sizes deliver
The same arithmetic run backwards: the flow each size can supply at each rise, at full power.
| Unit | Current at 240 V | 40°F rise | 50°F rise | 60°F rise | 70°F rise | 80°F rise |
|---|---|---|---|---|---|---|
| 13 kW | 54 A | 2.2 | 1.8 | 1.5 | 1.3 | 1.1 |
| 18 kW | 75 A | 3.0 | 2.4 | 2.0 | 1.7 | 1.5 |
| 24 kW | 100 A | 4.1 | 3.2 | 2.7 | 2.3 | 2.0 |
| 27 kW | 113 A | 4.6 | 3.6 | 3.0 | 2.6 | 2.3 |
| 36 kW | 150 A | 6.1 | 4.9 | 4.1 | 3.5 | 3.0 |
Real units also have a maximum flow and may limit output at low flow. Use the manufacturer’s table for the final choice.
Climate decides most of it
Power scales with the rise. The same two showers need 25.9 kW with 70°F water, 40.7 kW with 50°F water and 48.1 kW with 40°F water. That is why a unit that handles two showers in Florida may manage only one in Minnesota in January. Use your coldest-month incoming water temperature.
Amps, breakers and service capacity
Current is total watts ÷ volts. Whole-house electric tankless units split that load across two to four double-pole circuits, and each needs its own breaker and conductors sized to the manufacturer’s instructions and the electrical code. That is outside what any sizing calculator can decide. The number that matters before you buy is whether your service can carry the extra load: a 36 kW unit alone is about 150 A at 240 V.
Note on classification: the federal definition of a consumer electric instantaneous water heater is a nameplate input of 12 kW or less (10 CFR 430.2). Most whole-house electric tankless models are larger than that.
Electric tankless questions
What size electric tankless water heater do I need?
Multiply the flow you need by your temperature rise and by 500 to get BTU/hr, divide by efficiency, then by 3,412 to get kW. Two 2.5 gpm showers with 50°F water and a 120°F outlet need about 40.7 kW. With 40°F water, 48.1 kW; with 70°F water, 25.9 kW.
How many amps does an electric tankless water heater draw?
Current is power divided by voltage. A 27 kW unit at 240 volts draws about 113 amps in total, usually split across several double-pole circuits. The manufacturer's installation manual gives the required breakers and wire sizes; an electrician confirms them against the electrical code and your service capacity.
Why do electric tankless heaters work better in warm climates?
The power a unit needs rises in direct proportion to the temperature rise. Heating 40°F winter water to 120°F takes twice the power of heating 80°F water to the same temperature, so the same unit delivers half the flow in a cold climate.
Is my electrical service big enough for a whole-house electric tankless?
Possibly not. A 36 kW unit draws about 150 amps at 240 volts, most of a 200-amp service. An electrician should run a load calculation before you buy.
Water Heater Sizing Calculator for iPhone
Take the calculator to the job site
The app runs the same sizing engine as this site, offline. Basic tank and tankless sizing is free. Lifetime Pro, a one-time $4.99 purchase, adds tank vs. tankless comparison, custom fixtures, saved jobs with notes and PDF reports you can send to a customer.
