Size by required temperature rise and simultaneous fixture flow first. Only then compare brands, price and controls.
Best value range: ThermoMate
ThermoMate currently sells models from small point-of-use units through 36 kW whole-home systems. The 18 kW ET180 is listed around $330 and advertises up to 4.58 GPM under favorable conditions.
The key constraint is electrical demand: the 18 kW model requires 240 V, up to 76 amps and two 40-amp double-pole breakers.
Strong alternative: Camplux
Camplux sells electric tankless models in 14, 18, 24 and higher-kW sizes plus gas and RV heaters. Current U.S. pricing lists the 18 kW electric model around $420 and 24 kW around $480.
Camplux is especially useful if the buyer also wants gas, RV or outdoor heater options from the same brand.
Why advertised GPM can mislead
Heating capacity is fundamentally about energy. Colder incoming water requires more power to reach the same shower temperature, so a heater that performs well in Florida can deliver much less flow in a cold northern winter.
Use the manufacturer sizing table for the expected inlet temperature and desired outlet temperature.
Electrical service is often the real limiter
Whole-home electric tankless units can require multiple large breakers and substantial panel capacity. In some homes, the electrical upgrade can cost more than the heater itself.
Confirm service size, breaker space and wire requirements before buying.