Electric water heaters heat and retain a quantity of water in an insulated cylinder, ready for use. They provide hot water at mains pressure, and as they can deliver the whole stored quantity immediately it’s needed, can service many outlets at the one time. And it will deliver to them all at mains pressure. When emptied, it takes time to reheat to have you back in hot water, but they heat water quickly, especially with more powerful or twin elements, and can be run on lower cost off-peak electricity.