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.