If you look at your electric bill, you will see that you are billed by the kilowatt-hour (kWh)—the amount of energy your household consumes. One kWh is equal to 1,000 Wh—like running a 1,000 W toaster oven for an hour. “Power” (W) is instantaneous usage, while “energy” (Wh or kWh) is power used over time. If you look at the label on an appliance or light bulb, you will see its rated watts—this is the power that it draws at any given moment.
To figure out how much energy an appliance uses, you need to factor in the amount of time it is running. To compute daily kWh, take the wattage and divide by 1,000 to convert it into kW, and then multiply by the number of hours you’re using the item during one day.
If the device label shows amps rather than watts, you can use Ohm’s law to calculate the power:
Volts × Amps = Watts
An electric heater rated for 10 A at 120 V uses 1,200 W, or 1.2 kW. If left on for 2 hours, it would consume 2.4 kWh (1.2 kW × 2 hrs.).
Many electric bills will also show the monthly kWh used for the last year, and some will show average daily kWh. Pull out your bill and look closely at these numbers, as this is the simplest metric to evaluate energy consumption. For comparison, in 2008, the average electricity consumption for a U.S. residential utility customer was 920 kWh per month. How does your household measure up?