More KW. For example the Rheem RTE7 uses 240V and is 7KW. It has max flow rate of 2.5 gallons with 20³ temp rise. NOT enough for a shower.
The RTE 18 is 18KW, operates on 240v, requires (2) 40 amp breakers and can support 50³ temp rise at 2.46 gallons per minute.
WDRP MCMHU is only 4 mags totaling 6K watts. If this unit is only for under a bathroom sink it might work fine for washing your hands but for showering the unit would be need to be a MCMCMCMCMCMHU.