Electric showers only take water from the mains cold water supply.
They heat the water when you turn the shower on, by passing it over a heating element inside the shower in a similar way to how a kettle works.
When using the shower, you do not use up stored hot water. Therefore, this make electric showers ideal for households where there is a limited supply of hot water.
Electric showers are always ready to use, any time of the day or night.