Instant hot-water dispensers are simply miniature electric water heaters that serve only one faucet. A small, under-sink tank heats and holds nearly boiling water, ready for steamy delivery through a sink-top spout that's separate from the main tap.

Unlike a conventional water heater, however, the tank never becomes pressurized. The system hooks up directly to a cold-water pipe under the sink; incoming water travels first through the body of the spout and then into the tank, where it's heated by an electric coil.

The heated water expands, filling an expansion chamber in the upper part of the tank. When you turn on the faucet, more cold water is released into the bottom of the storage tank, displacing hot water in the tank and expansion chamber and forcing the heated water up through the faucet.

The spout delivers water more slowly than a typical faucet does, and the water is much hotter than a standard hot-water tap's: about 190 degrees Fahrenheit instead of 120 degrees. The heated water arrives immediately; you don't have to wait for it to warm up.

An adjustable thermostat—factory set at 190 degrees—controls water temperature with most models. Adjustment is needed only if the water is too cool or if it's so hot that it boils away, causing the tank to overheat. (To prevent damage, the tank should be protected from overheating by a replaceable thermal fuse.)

Why we like it: • Tall, rounded gooseneck shape • Single lever control • Satin nickel finish

Tanks vary in size and by the heating elements' wattage. Most are 1/3 or ½ gallon and range from 500 to 1,300 watts. A 750-watt, ½-gallon tank will produce up to about 60 cups of hot water per hour. Higher wattage tanks can deliver up to 100 cups per hour.

How much does it cost to keep the hot water ready and waiting? The usage is a little more than ½ kilowatt hour per 24 hours, which, depending on your local electric rate, equates to about 6 or 7 cents a day.

As with any water heater, hot-water dispensers may accumulate scale in hard-water areas. Some units have drain plugs at the bottom to allow you to drain the tank once or twice a year.