Energy in kilowatt-hours (kWh) = watts ÷ 1,000 × hours. Multiply by your price per kWh to get the cost. The monthly figure uses the days per month above, and the yearly figure is twelve months of that.
Example: a 1,500 W space heater running 2 hours a day uses 3 kWh a day. At 0.15 per kWh that is 0.45 a day, 13.50 a month (30 days) and 162.00 a year.
It is printed on your electricity bill, usually as a rate per kWh. If the bill shows separate supply and delivery charges, add them together.
Look at the label on the back or bottom of the device, or in its manual. If it only lists volts and amps, multiply them: watts = volts × amps. Devices that cycle on and off, such as fridges, use less than their label wattage on average.
See also: Fuel Cost Calculator · Unit Price Calculator · Savings Goal Calculator