Most electric water heaters use 240-volt power, because the higher voltage provides faster heating than the 120-volt electricity that powers lights and small appliances. You won't find a switch on your water heater -- it's hardwired to a breaker in your home's service panel (breaker box), and you can use the breaker as a switch. If your heater isn't working, even though the power is on, the high limit reset button on one or both of the tank's heating elements may have tripped; consult your owners manual for instructions.
Starting a Tank-Style Electric Heater
If you're starting your water heater for the first time, or you drained it to make repairs, it's critical to allow the tank to fill completely before turning on the power, or the heating elements will quickly burn out. The process is roughly similar for Rheem, GE and other models.
Open the cold-water supply valve and turn on a nearby faucet so it can discharge hot water.
Allow the heater to fill until water emerges from the faucet at full stream for three minutes.
Close the faucet.
Locate the breaker in the service panel that controls the heater. The breaker should be labeled, but if not, look for a double-gang breaker with the same amperage rating as the heater, which is displayed on a label on the tank. Flip the breaker on.
Wait for about an hour, then turn on the hot water at a nearby faucet to check the water temperature and confirm the heater is working. You should notice the water turning warm, but it might take a few hours to fully heat the tank.
About Tankless Heaters
Like tank-style heaters, electric tankless heaters are usually powered by 240-volt electricity, but most units have a switch. When the unit is connected to power, turn on the switch, which is located on the front of the cover; a green LED should light to signal that the unit is on. Tankless heaters have a thermistor that senses incoming water temperature and compares it to the temperature selected by the main temperature control. If your heater doesn't come on, try turning the thermostat control clockwise to raise the temperature setting.