Tankless Water Heater Size for One Bathroom (Shower + Sink)
Part of our Ventilation & Hot Water guides.
Short answer: 18-27 kW. One shower at about 2 GPM plus a sink is the common target. In southern states with warm groundwater, around 18 kW can work; in northern states plan on 24-27 kW.
How the math works
Required power depends on flow (gallons per minute) and temperature rise (desired hot temperature minus incoming water temperature). A shower at 105°F with 50°F incoming water needs a 55°F rise; with 70°F water, only 35°F. That difference changes the size you need by a third or more.
Electrical reality check
Every kW at 240 V draws about 4.2 A. An 18 kW unit draws about 75 A; a 36 kW unit about 150 A spread over multiple breakers. Have an electrician confirm your panel can support it.
Try your own numbers
Use the tankless sizing calculator with your local water temperature.
Worked examples
| Scenario | Flow | Rise | Approx. kW |
|---|---|---|---|
| Florida, 1.8 GPM shower | 1.8 GPM | 75°F → 105°F = 30°F | about 8 |
| Texas, shower + sink | 2.3 GPM | 65°F → 105°F = 40°F | about 13.5 |
| Ohio winter, shower + sink | 2.3 GPM | 45°F → 105°F = 60°F | about 20 |
| Minnesota winter, 2.5 GPM shower | 2.5 GPM | 38°F → 105°F = 67°F | about 24.5 |
Calculated with kW ≈ GPM × rise ÷ 6.83.
Flow rates to use
Many US shower heads are rated at 1.8-2.5 GPM; bathroom faucets about 0.5-1.2 GPM. Check your fixtures' ratings, usually printed on the head.
If it's not quite enough
A low-flow shower head reduces the kW needed. Alternatively, keep an existing tank and use tankless only for a specific bathroom.
Frequently asked questions
Will a tankless heater work in winter?
Yes, but output drops as incoming water gets colder, so size for winter conditions.
Is propane better for whole-house?
Propane units don't need big electrical service; see electric vs propane tankless.
Is 18 kW enough for one bathroom?
In warm climates often yes; in cold climates you may need 24-27 kW.
What flow rate should I plan for?
About 2-2.5 GPM for a shower plus a sink.
Can I use a low-flow head to reduce size?
Yes; lower flow lowers the required kW.
Where should the unit be installed?
Close to the bathroom to reduce wait time for hot water.