Tankless Water Heater Size for A Single Sink Or Powder Room
Part of our Ventilation & Hot Water guides.
Short answer: 8 kW or a mini-tank. A low-flow bathroom faucet uses about 0.5 GPM. An 8 kW unit or a 2.5 gallon mini-tank is enough, and both have small electrical needs compared with whole-house units.
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.
Why small units work here
A bathroom faucet uses about 0.5-1.2 GPM, and hand washing needs only warm water (around 95-105°F). An 8 kW tankless unit or a 2.5-gallon mini-tank is enough for most sinks.
Mini-tank vs mini-tankless
- Mini-tank (2.5-8 gal): often plugs into a standard outlet, instant warm water, limited volume.
- Small tankless (8 kW): unlimited at low flow, needs a 240 V, 40 A circuit.
Common uses
Remote half baths, garage or shop sinks, ADUs, and bathrooms far from the main water heater where you'd otherwise wait a minute for hot water.
Installation notes
Install under the sink with access for service, a T&P valve discharge where required, and an isolation valve. Check that the cabinet has room for the unit plus pipes.
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.
Can a small tankless heater feed a shower?
Small units like 8 kW aren't designed for showers.
Do mini-tanks plug into a regular outlet?
Many 120 V mini-tanks do; check the plug and circuit requirements.
How much space do I need under the sink?
Measure cabinet height and depth; 2.5-gallon tanks are compact.
Is there a wait for hot water?
Mini-tanks deliver hot water almost instantly because they're close to the tap.