Add the two shower flow rates, calculate the coldest design temperature rise, then use the manufacturer chart to find the kW class that can deliver that flow. In colder climates, two simultaneous showers can push electric tankless sizing well above entry-level whole-home units.
Calculate combined shower demand first
Two 1.8 GPM showerheads create about 3.6 GPM of simultaneous hot-water demand before adding a sink, dishwasher or other fixture. Higher-flow showerheads increase the requirement quickly.
Use actual fixture flow where possible instead of assuming every shower uses the same amount of water.
Cold inlet water is the multiplier
The heater must raise incoming water to the desired outlet temperature. The colder the inlet water, the more energy each gallon requires and the less flow a given kW rating can support.
Size around the coldest meaningful inlet-water condition if you want the system to maintain performance through winter.
Electrical service can become the bottleneck
Higher-power electric tankless heaters may require multiple large breakers and significant service capacity. Before choosing a model, verify panel space, conductor requirements and the home service rating.
A technically adequate heater can still be the wrong economic choice if the electrical upgrade is unusually expensive.
Compare brands only after the load is defined
Once the required flow and temperature rise are known, compare ThermoMate, Camplux and other brands within the kW range that actually meets the load.
This is more reliable than selecting a brand first and trying to make its most popular model fit the house.
How to size and install What Size Electric Tankless Water Heater for Two Showers?
Equipment performance depends on matching capacity to the load rather than choosing the largest model. For pumps and heaters, that means required flow, pressure or temperature rise. For dehumidifiers and pool equipment, it means the environmental or hydraulic load. For UV treatment, it means validated flow at the required dose with appropriate upstream water quality.
Check electrical service, drainage, plumbing connections, clearance and ambient operating limits before purchase. A system can look ideal on a specification table but become expensive when the installation requires a new circuit, condensate pump, larger plumbing, structural changes or professional labor that was not included in the hardware price.
Operating cost, maintenance and failure points
Compare energy use and consumables over several years. Pumps, heaters, dehumidifiers and UV systems add electricity cost; pool and spa equipment may add filter or chemical maintenance; leak-detection devices may require batteries, valves or hub hardware. A lower purchase price can lose its advantage when operating cost or replacement parts are materially higher.
Look for serviceable components, warranty terms, replacement-part availability and the maintenance tasks the owner will actually perform. Equipment that depends on inaccessible proprietary parts or frequent neglected maintenance carries more long-term risk than a slightly more expensive system with clear service support.