If you are looking to install a shower in your bathroom then among the options open to you will be an electric shower. Choosing a mixer shower or power shower will depend on your heating system but electric showers run independently of your central heating so can be used in any household - providing it has water and electricity.
Electric showers work by instantly heating water. Cold, mains pressure water is fed into the unit and passes over a heating element inside a chamber. The power rating of this element will determine the amount of water that can be heated. As well as a model name, many electric shower manufacturers will include the power rating of the shower in the description, which will be given in kilowatts. Showers with a higher kilowatt rating will be able to heat more water in a given time, so a higher figure gives a better flow rate. Very early models had ratings less than 6kw so the spray that they produced was very fine with a low water flow rate, but over the years kilowatt ratings have steadily increased with some current models rated over 10kw.
The elements in the heating chamber do not get hotter as the temperature dial is turned up - the dial turns a flow restrictor. When the indicated temperature on the dial is turned up what actually happens is that the water passes more slowly over heating elements and is heated up more. If the dial is turned down the water passes over the elements faster and does not heat up as much. The temperature of the incoming water will also have an affect of the performance as the electric shower will have to work harder to raise the temperature. To raise the temperature of incoming, cold, winter water to a reasonably comfortable showering temperature means the unit will have to work harder than it would to raise the temperature of water from a warmer, summer mains supply. To achieve a decent temperature with colder water, the unit will have to restrict the flow so consequently electric showers will produce better flow rates in the summer than they will in winter months. Many electric showers will have a hi/low or winter/summer button to help overcome the varying incoming water temperatures and unless the unit is thermostatic the temperature/flow control will need to be adjusted accordingly throughout the year.
The larger kilowatt rated electric showers can produce a reasonable rate of flow but even the most powerful will be some way off the performance of a power shower or a mixer shower run off a combination boiler.
Hard water areas can result in the build up of limescale in the unit which can affect the heating elements and also clog up the spray head. A spray head affected by limescale can lead to a build up of pressure resulting in the pressure relief valve operating (this device prevents excess pressure damaging components within the shower body). Many manufacturers supply their electric showers with a "rub-clean" handset where the sprayhead has flexible jets. Limescale is hard and brittle so when the jets are rubbed with the finger or thumb they flex causing the limescale to crack and break away. Although there are many makes of replacement headsets available on the market most electric shower manufacturers insist on you using their own handset to maintain the warranty and to ensure the correct rate of flow.