How To Calculate Watt Hour


Watts Usage in Appliances

Watt usage ratings are vastly different for different appliances. Even similar appliances will have different watt usage ratings. Although generally a smaller appliance will use less watts than a larger appliance, this varies by function, environmental factors and how long the device runs.



The watt usage rating refers to the number of watt-hours or kilowatt-hours that an appliance uses. Watt-hours are calculated by multiplying the number of watts that device consumes by the number of hours it is on.

The watt-usage of a refrigerator increases as the outside temperature increases. The motor of the refrigerator must be on for longer periods to keep the refrigerator colder, because more energy is required to keep the refrigerator colder in warmer temperatures.


Surge power

The watt-usage rating is higher immediately after an appliance with a motor is turned on than when the motor has been operating for a few seconds. The amount of power required to make a motor turn initially, called the surge power, is higher than the power to keep it turning. This is due to the inertia of the motor.


Clothes dryer

The watt-usage rating for a clothes dryer is one of the highest of all appliances. A clothes dryer can require up to 5000 watts to operate. This is much higher than a refrigerator or microwave. One hour of dryer time can consume as much energy as 10 hours of refrigerator time.


Electric hair dryer

The watt rating of an electric hair dryer is very high considering its size. Electric hair dryers have watt ratings in the order of 1,000 watts, higher than most refrigerators. This means your hair dryer consumes more energy when it’s on than your refrigerator does.


How to Calculate Watt Hours

Your electric bill is probably calculated in kilowatt hours, which are 1,000 watts–for example, 10 light bulbs of 100 watts each–running for an hour. If you are trying to break down your bill to see how you are spending your power dollars, you can figure your watt hour usage by tracking your lighting and appliance usage. You can save money by using bulbs, such as compact fluorescent bulbs, and appliances that use fewer watts



Things You’ll Need: Calculator

1 Assess your electric usage by looking at your lights or appliances and estimating the monthly usage of each. The wattage of light bulbs is marked on the bulbs. The wattage of space heaters, dishwashers and other appliances is usually marked on the back or in the user’s manual.

2 Multiply the wattage by the average hours the bulb or appliance is used each month to get each one’s watt hours used per month. Add these together to get a watt hour total for your household.

3 Divide the watt hours by 1,000 to get the kilowatt hours (kwh) used–and check it against your utility bill.


Definition of Watt Hours

Wattage and watt-hours are often mistaken for each other. Watts are the measure of power that something needs to run. Watt-hours factor in time. Knowing how to calculate this helps manage household finances.

The watt-hour refers to the usage of one watt of energy over a one-hour time span, according to Princeton University’s WordNet.

Fun Fact
No measurement system accepts the watt-hour as a standard unit. But electronic devices commonly display their energy usage in watt-hours.