Some devices show a range of watts, such as “200–300W. " It may be more accurate to choose the middle of this range, or 250W in this example.
Example: A large window fan rated at 250 watts runs for an average of 5 hours a day. The fan’s daily watt-hours equal (250 watts) x (5 hours / day) = 1250 watt-hours per day. For air conditioning and heating units, make separate calculations for each season. Refrigerators only draw power about ⅓ of the time, or about 8 hours a day if you never unplug them. [3] X Trustworthy Source U. S. Department of Energy Official site for the U. S. Department of Energy, which provides resources related to energy safety, conservation, and efficiency Go to source
Example: You’ve calculated that your fan uses 1250 watt hours of energy per day. (1250 watt hours / day) ÷ (1000 watts / 1 kilowatt) = 1. 25 kilowatt hours per day.
Example: Over the course of a 30-day month, your fan would use (1. 25 kWh / day) x (30 days / month) = 37. 5 kWh per month. Example: If you fan runs every day for a year, it would use (1. 25 kWh / day) x (365 days / year) = 456. 25 kWh per year.
Example: If energy costs 17 cents/kWh, running the fan will cost (0. 17 dollars / kWh) x (456. 25 kWh / year) = $77. 56 per year (rounded to the nearest cent). Remember that estimates based on the listed wattage is a maximum. In reality you would be charged less than this. If you’re looking at a different area than the one you live in, search online to find the cost of electricity. For US locations, start with the EIA website.
Laptop and phone chargers may list two amp values. Use the one labeled input.
In the US, some large appliances such as washing machines may be plugged into special 240V circuits. Check the appliance label’s voltage rating to find out. (The label only tells you the recommended voltage, but you can assume a professionally installed appliance matches this recommendation. )
Example: A microwave label lists 6. 5 amps and is plugged into a 120V outlet. It consumes 6. 5 amps x 120 volts = 780 watts.
Example: If the microwave runs for half an hour each day, multiply 780 watts x 0. 5 hours / day = 390 watt hours per day.
Example: 390 watt hours / day ÷ 1000 watts / kilowatt = 0. 39 kilowatt hours per day.
Example: 0. 39 kilowatt hours / day x 31 days = 12. 09 kilowatt hours.
If you’re familiar with electrician’s tools, you may be able to use a multimeter instead. This requires access to the appliance’s wiring while it is plugged in. Needless to say, don’t disassemble anything unless you know what you’re doing.
If your power meter only measures watts, you can use the method above to calculate kilowatt hours from that measurement. Refer to the power meter instructions if you’re not sure how to change its settings.
For example, let’s say the meter has been running for 5 days, and you want to find the 30-day estimate. 30 divided by 5 is 6, so multiply the kWh displayed by 6.
