How do water heaters work, and why not make them like electric tea kettles?

Electric hot water heaters have a heating element (usually coiled) that is powered on whenever the thermostat detects that the temp in the tank is below what has been set.

Your electric kettle has, at most, a quart or so of water to heat. An average size water heater is 40 gallons, and many are as large as 50–60 in a modern home. Heating 50 gallons takes time.

Your kettle warms to boiling within 2–3 minutes. A water heater, from a cold state, takes a half hour or more. A home water heater ‘s thermostat is set to whatever temp that owner wants—usually 120–130 degrees. And the thermostat keeps it at that level.

In parts of Europe, and in a very small scale in the US, there are ‘tankless’ water heaters, which heat the water immediately before use. (Tankless is a misnomer, there is a small tank, a couple of gallons maximum). These have not been popular in the US because people like hot showers and the tankless heaters can’t keep up with the demand.

It’s one thing to heat up a liter of water at a time. Quite another to heat 40–50 gallons. That’s going to require a quite a large magnetron. And considering the relative power draw of your average microwave oven your house’s electrical wiring probably isn’t going to handle such a load.

A traditional heating element, on the other hand, doesn’t pull much electricity when heating. If, indeed, it even is electric to begin with (the water heater in my house is gas).

When a tea kettle gets up to temperature, it vents/whistles. A water heater is not vented so if it were to get up to boiling, it would explode. So a tea kettle doesn’t need a thermostat to control it’s maximum temperature, whereas a water heater does. And a water heater has a safety valve to prevent it from exploding, which could kill you and/or severely damage your house.