The formula
Energy (kWh) = watts ÷ 1,000 × hours per day × days. Cost = energy × tariff.
A worked example
A 1,500 W appliance used for 2 hours daily over 30 days uses 90 kWh. At 0.25 per kWh, it costs 22.50.
What this estimate includes
Rated power is not always average power. Refrigerators, heat pumps and air conditioners cycle on and off. Use measured average draw or an energy meter for a better estimate.
Common questions
Should I use the power printed on the appliance?
For a steady load it can be a useful starting point. For cycling appliances, use measured consumption or average power rather than assuming the rated draw continues all day.
Does this include standby use?
Only if standby use is included in your average power and operating hours.
Further reading
US Energy Information Administration: measuring electricity. Background reference; these tools are independent and are not endorsed by the source organization.