Whenever you plug a given device into a wall outlet, it completes a circuit with the electrical box attached to that outlet and through the device itself, allowing electricity to flow. The appliance draws the amount of current that it needs to function. Electrical current is measured in amperes, or amps for short. Higher-powered devices, such as refrigerators or laundry machines, draw more current than do televisions or computers.
Electrical power consumption and requirement is the product of two quantities: voltage and current. You may not know the current of your device, but if you have a multimeter and can check resistance you can derive the current by dividing the voltage by the resistance. Once you know the current used, you multiply it by the voltage, or electrical pressure, to get wattage, or power usage. Watts divided by 1,000 equals kilowatts. You can purchase a handheld multimeter for under $25 at hardware stores, electronics shops or online.
Kilowatts are most often used in the context of kilowatt-hours, the measurement your utility company uses to determine how much power you consume. Utility companies charge by the kilowatt-hour. Some electrical components, such as laptop power supplies, have the amperage written on the label, and you can use amperage to calculate wattage. For example, a 3-amp power supply plugged into a 120-volt socket uses 360 watts. If this is left plugged in for two hours, it uses 720 watts, which converts to 0.72 kilowatt-hours. Some devices such as light bulbs have the wattage listed, making the calculation of kilowatt-hours simpler: multiply by the number of hours used, then divide by 1,000. A 60-watt bulb left on for three hours uses 0.18 kilowatt-hours.
Not all power supplies are created equal. The standard household voltage in the United States varies between 110 and 120 volts. Larger appliances require more voltage. Washers, dryers and some furnaces may be plugged into a 220-volt or 240-volt socket. If you are abroad, the standard voltage may be different. European countries, for example, use 240 volts as the standard. Failure to take into account different voltages could cost you a lot of money.