Shower + Sink on One Electric Tankless? Here’s When
Can an instant wall-mounted electric water heater run a shower and a sink at the same time?
Sometimes—but only if the heater is sized for the combined flow rate and the incoming water temperature isn’t too cold. Instant (tankless) wall-mounted electric water heaters don’t store hot water; they heat on demand. That means performance is limited by how many gallons per minute (GPM) the unit can raise to your target temperature.
A typical shower can draw around 1.5–2.5 GPM, and a bathroom sink may add another 0.5–1.0 GPM. When both run together, the heater has to deliver roughly 2.0–3.5+ GPM at the desired temperature rise. If the unit can’t keep up, the most common result isn’t “no hot water”—it’s a noticeable drop in temperature or reduced flow as you open the second faucet.
What determines whether it will work?
1) Heater capacity (kW and rated GPM)
Electric tankless units are often rated by how much hot water they can produce at a specific temperature rise. Higher kW models generally support higher simultaneous demand, but only when installed on the correct electrical service and breakers.
2) Incoming water temperature
Cold inlet water requires more heating power to reach the same outlet temperature. In warmer climates, the same unit can support a shower plus a sink more easily than it can in colder regions.
3) Your target shower temperature
Setting the heater to a very hot outlet temperature increases the required temperature rise and reduces the maximum GPM the unit can maintain. A moderate setpoint can improve simultaneous performance.
How to make shower + sink more reliable
Look for a unit with enough rated flow for your worst-case conditions (winter inlet temperature plus both fixtures running). Low-flow showerheads and faucet aerators can also reduce total demand. For sizing tips and control features, see the full guide here: instant wall-mount electric water heater LCD control guide.
FAQ
What size electric tankless water heater do I need for my bathroom?
Add the flow rates of the fixtures you want to run at once (shower, sink, etc.), then choose a model rated to deliver that combined GPM at your expected temperature rise. If your inlet water is very cold, you’ll typically need a higher kW unit to maintain comfortable shower temperatures.
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