A cheaper appliance can cost more to run, but a wattage label alone does not tell you the annual bill. You also need to know how long the appliance operates and whether it draws that power continuously.
Build the calculation from your use
For a steady load, divide watts by 1,000 to get kilowatts, multiply by hours to estimate kilowatt-hours, then multiply by the relevant price per kWh. The US Department of Energy’s electricity overview provides background on power and electrical energy.
As an invented example, a 40-watt device running for five hours uses 0.2 kWh. At an illustrative rate of 0.30 per kWh, that costs 0.06 in the tariff’s currency. Neither the rate nor the usage is a quote for your home.
Know when the simple estimate is weak
A refrigerator, heater or variable-speed appliance may cycle or change power. Multiplying its maximum label rating by every hour it is plugged in can misrepresent consumption. A product’s tested energy figure or a suitable, safely used energy monitor may provide a better starting point.
Include standby use where relevant, but do not mix energy charges with fixed standing charges as though every appliance creates another daily fee. Time-of-use tariffs also require the price for the hours you intend to run it.
Compare two options over a plausible period using the same assumptions. A small running-cost difference may take years to offset a large purchase-price difference, while repairability and suitability still matter. Avoid presenting a calculated payback as certain when use or tariffs may change.
Keep the calculation beside its assumptions: watts or tested kWh, hours, tariff and date. That makes it easy to update when your routine changes, instead of treating one impressive annual-saving figure as a permanent fact.