Household electricity and its current flow can be easier to understand if you compare it to the plumbing in your home. Both water and electricity enter your home from the utility service lines and exit it after being distributed throughout the house. Water flows through pipes and is used at faucets and other fixtures. Electricity flows through a network of wiring and is used by lights, appliances and other electrical devices.
Pressure Makes It Flow
Water flows throughout a home's supply piping because it is pressurized by the water company or by a well system. Once it is used at a fixture it no longer has pressure and must rely on gravity to flow through the drain pipes. Therefore, all water exiting the house through drains can be considered to have no pressure. Likewise, electricity has specific pressure that is regulated by the power company. This pressure allows the electrical current to flow through the wiring leading into the house and throughout the home's electrical system. Electricity is used by each electrical device or appliance. Anything unused returns to the electrical service entry point (and ultimately back onto the power grid) via neutral circuit wires. This return is considered to be without pressure, much like water going down the drains of your home.
Pressure = Voltage
Increasing pressure in a pipe means the water flows through with greater force. The same is true with electrical wiring; more pressure means greater force. Water pressure is measured in pounds per square inch, or psi. Electrical pressure is measured in volts, or voltage. All electrical devices are rated for specific voltages. Most devices and small appliances in a home are rated for 120 volts, while high-voltage appliances, such as electric dryers, ranges, and many baseboard heaters, are rated for 240 volts.
Flow = Current (Amperage)
Just as larger water pipes carry more flow, or volume, of water, larger wires carry more electrical current. Current is measured in amps, or amperage. Along with voltage, all electrical components are rated for safe amperage levels. The circuit breakers in your home's breaker box have specific amp ratings. Most circuits are rated for 15 or 20 amps, while large-appliance circuits are rated for 30, 40, 50 or more amps. When you plug too many things into one circuit, you're effectively increasing the flow to the point of an overload. This causes the breaker to trip and shut off the power to the circuit.
Voltage x Amperage = Watts
A watt is a measure of how much electricity is used by an electrical device. Watts are a function of both voltage (pressure) and amperage (flow, or current). Multiplying voltage and amperage gives you wattage. For example, your microwave oven might be rated for 10 amps, and you plug it into a 120-volt outlet. Therefore, 10 (amps) x 120 (volts) = 1,200 watts. That's how much electricity the microwave is using whenever it's running on high (other settings, like defrost, probably use fewer watts). Returning to our plumbing comparison, an energy-efficient light bulb is like a low-flow showerhead. Where we used to have 60-watt incandescent bulbs we now use 12- or 14-watt LED bulbs. Old showerheads had flows of 5 gallons per minute, but today's low-flow models use no more than 2.5 gallons per minute. Depending on how you look at it, that means more power going back to the grid or less water going down the drain.