Instant water heaters do pretty much what the name implies; they heat water nearly instantly. Most homes come with a large water heater that sucks up a lot of energy. They have to in order to heat all of the water in the tank. But an instant water heater doesn't even have a tank. Instead, it works by heating water only when you turn on something like a faucet.
Some estimates say that instant water heaters can save as much as 50% when compared to traditional water heaters, but they do require a bit of money upfront to buy.