Most repairs to an electric water heater involve the two elements and their thermostats. If water leaks onto the floor, the tank is rusted through, and the unit must be replaced.
-- If water is not hot enough, try turning up the thermostat settings for both elements.
-- If the unit suddenly stops heating water, press the reset button, usually on the high-limit cutoff. If you hear a click, the unit is reset and the problem may be solved. Also check that power is reaching the unit; call in an electrician if you are not sure.
-- If water gets warm but not hot, replace the upper element and/or thermostat.
-- If the hot water runs out quickly, replace the lower element and/or thermostat.
-- If the unit is noisy, drain the tank and refill it. If that does not solve the problem, remove and clean the elements.
-- If water is too hot even with the thermostats turned down, replace either the thermostat or the high-limit cutoff.
An hour or two for most repairs
Screwdriver, voltage tester, groove-joint pliers
Testing for electrical power, unscrewing and replacing devices
Prepare the household for doing without hot water for a few hours; arrange for a work light while the power is off.
New element, thermostat, or high-limit cutoff for your make and model of water heater
The amount of power an electric water heater uses is dangerous. Check with an electrician if you are unsure about shutting off power. At the service panel, shut off the breaker or unscrew the fuse controlling the water heater. Be sure no one will turn the power on while you work.