Voltage Current and Resistance
As mentioned earlier, the number of electrons in motion in a circuit is called the current, and its measured in amps. The pressure pushing the electrons along is called the voltage and is measured in volts. If you live in the United States, the power outlets in the wall of your house or apartment deliver 120 volts each.
If you know the amps and volts involved, you can determine the amount of electricity consumed, which we typically measure in watt hours or kilowatt hours. Imagine that you plug a space heater into a wall outlet. You measure the amount of current flowing from the wall outlet to the heater, and it comes out to 10 amps. That means that it is a 1,200 watt heater. If you multiply the volts by the amps, you get the wattage. In this case, 120 volts multiplied by 10 amps equals 1,200 watts. This holds true for any electrical appliance. If you plug in a light and it draws half an amp, its a 60 watt light bulb.
Lets say that you turn on the space heater and then look at the power meter outside. The meters purpose is to measure the amount of electricity flowing into your house so that the power company can bill you for it. Lets assume we know its unlikely that nothing else in the house is on, so the meter is measuring only the electricity used by the space heater.
Your space heater is using 1.2 kilowatts 1,200 watts. If you leave the space heater on for one hour, you will use 1.2 kilowatt hours of power. If your power company charges you 10 cents per kilowatt hour, then the power company will charge you 12 cents for every hour that you leave your space heater on.
Now lets add one more factor to current and voltage resistance, which is measured in ohms. We can extend the water analogy to understand resistance, too. The voltage is equivalent to the water pressure, the current is equivalent to the flow rate and the resistance is like the pipe size.