One of the first considerations in getting involved with solar power is looking at power usage in your house. Therefore, an overview of power and energy as it relates to your electric bill is useful. Power is the rate of energy consumption (energy per time) and the most familiar unit is Watts, such as with a 60 W light bulb. Energy is power multiplied by the time the power is on and the most common household measure is the kilowatt-hour (kWh), where the kilowatt is 1000 watts and the time is measured in hours. Utility companies use the kWh measure because it gives a convenient number (at least several hundred) over the course of the typical 30 day billing cycle. Most household appliances are rated in watts. To obtain energy used for any appliance in kWh, divide the watts rating by 1000 and multiply by the number of hours that it is used.
Energy in kWh = (Power rating in watts x hours used)/1000
Examples:
A 60 W lightbulb that runs for 2 hours consumes 0.12 kWh of energy.
A 1200 Watt blow dryer used for 10 minutes consumes 0.2 kWh of energy.