Tankless Water Heater Size for Two Bathrooms
Part of our Ventilation & Hot Water guides.
Short answer: 27-36 kW. Two showers running together roughly doubles the demand. Expect 27 kW in warm climates and 36 kW in cold ones, which may exceed a 100 A electrical service.
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.
Simultaneous use is the key question
If two showers rarely run at the same time, size for one shower plus a sink. If mornings mean two showers at once, size for both: about 4-5 GPM. At a 55°F rise, 4.5 GPM needs about 36 kW.
Electrical reality
A 36 kW unit draws about 150 A, often split across four 40 A breakers. That usually requires 200 A service with spare capacity. Get an electrician's load calculation before ordering.
Alternatives for two bathrooms
- Two smaller point-of-use units, one per bathroom.
- A propane or natural gas tankless unit if available.
- A tank heater for the main bathroom plus a point-of-use unit for the second.
Staggering showers
Households that stagger showers by 10 minutes can often get by with a smaller unit. Smart scheduling is free.
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 one tankless heater serve two bathrooms far apart?
Yes, but hot water still travels through pipes; a recirculation system helps.
Is gas better for two bathrooms?
Gas or propane units often deliver more flow without electrical upgrades.
What service size do I need for 36 kW?
Large electric tankless units often require 200 A service with spare capacity.
Can I add a second heater later?
Yes, some homes add a point-of-use heater for the second bathroom.