Electric showers heat water on demand, taking the supply from the mains cold water supply and passing it over a heating element inside the shower unit. An electric shower will produce an adequate shower (water pressure dependant) but will never give a 'Power Shower' performance. The higher the kilowatt rating you choose, the better the flow will be.
On the plus side, you will not run out of hot water or have a wait for the cylinder to heat up and electric showers are a consideration as secondary shower for this reason as they heat water as needed. They are also economical on water usage