There are 3 different showers available on the market today and choosing which one you want or need is a bit confusing. Below is a guide to the 3 types available.
An electric shower is a style of the shower that only has a cold water feed into it and it heats up the water in a heat exchanger, a bit like a high-performance kettle, and mixes it with the cold water feed to exit the shower at the desired temperature. The flow is determined by the flow output knob and the temperature needed as the shower can only heat up the water at a certain rate so it cannot give you the maximum flow at the maximum temperature. There are many outputs available for electric showers ranging from 7.0kw (the lowest) to 10.8kw (the highest). You should check with your electrician as the higher output electric showers need quite a big cable run to them.
A mixer shower is like an elaborate tap in that you feed it with hot and cold water through the inlet valves and it mixes the water to the desired temperature and pressure. The main disadvantage with this style of the shower is that the water can only come out as quick as it goes in. A high-pressure water system is required for a good flow of water.
A power shower is where the water comes out of the shower head very powerfully. There are 3 ways of achieving a power shower. The first is to have a high-pressure water system such as a combination boiler and fit a full flow shower onto this plumbing system which will give you about 1.5 bar pressure as there will be no flow restrictions. The second is to fit a shower pump onto your existing system and simply increase the flow of water too, and consequently out of the shower. The third method is to fit a power shower such as the image right which has an inbuilt pump and sucks the hot water out of the tank and increases the flow.
The disadvantages of the second and third method are that it may empty your hot water cylinder very quickly.