How this calculator works
A power rating in watts tells you how fast an appliance uses electricity. Multiplying watts by hours gives watt-hours, and dividing by 1,000 gives kilowatt-hours, the unit on a Canadian electricity bill. Repeating the sum for each appliance and adding the results gives the household total. The daily figure is multiplied by the number of days in the month and by 365 for the year. Quantity multiplies the result for appliances you own more than one of, such as a second refrigerator.
Formula
kWh per day = watts x hours per day / 1,000 x quantity. cost = kWh x price per kWh. monthly kWh = daily kWh x days; annual kWh = daily kWh x 365.
Worked example
A 150 W refrigerator running all day uses 150 x 24 / 1,000 = 3.6 kWh. A 500 W washing machine run for one hour adds 0.5 kWh, a 1,200 W microwave run for 15 minutes adds 0.3 kWh and a 100 W television run for four hours adds 0.4 kWh. The total is 4.8 kWh a day, or about 144 kWh a month and $21.60 a month at 15 cents per kWh.
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Frequently asked questions
Should I use the label wattage or a measured figure?
The label is usually a maximum. A meter reading gives the real average draw, which is lower for anything that cycles on and off, such as a refrigerator or an oven.
Why does the running time matter so much?
Energy is power multiplied by time, so an appliance that draws a little power for many hours can beat a powerful appliance used briefly. A phone charger at 5 W for 24 hours uses 0.12 kWh, while a 2,000 W kettle for five minutes uses 0.17 kWh.
Does standby power count?
Only if you include it in the running time. Set-top boxes, game consoles and chargers can draw a few watts all day; a plug-in meter will show the standby figure so you can add it.
Are the costs the same in every province?
The arithmetic is the same, but the price per kilowatt-hour differs by province and by tariff. Enter the rate from your own bill to get a local figure.