You can estimate the electricity cost of almost any appliance with three inputs: its power draw in watts, how long it runs, and your electricity price in cents per kilowatt-hour.
Step 1: find the appliance's wattage
Check the product label, manual, manufacturer specifications, or an energy monitor. If the device lists amps and volts instead of watts, power can sometimes be estimated from electrical specifications, but nameplate values may represent maximum rather than typical use.
Step 2: convert watts to kilowatts
Divide watts by 1,000. A 1,500-watt heater is 1.5 kW. A 500-watt gaming PC is 0.5 kW.
Step 3: multiply by hours of use
This gives energy consumption in kilowatt-hours:
kWh = watts ÷ 1,000 × hours
A 1,500-watt heater used for four hours consumes 6 kWh if it draws that power continuously.
Step 4: multiply by the electricity price
Convert cents per kWh into dollars by dividing by 100, then multiply by kWh.
Monthly estimates need realistic usage assumptions
If you repeat that four-hour use every day for 30 days, the simple estimate is 180 kWh and $28.80 at 16¢/kWh. But “four hours per day” should mean four hours of actual power draw—not simply four hours that the appliance is switched on if it cycles.
Why thermostats and compressors complicate the math
Heating and cooling devices often turn on and off as needed. A 1,500-watt heater controlled by a thermostat may draw 1,500 watts while heating and nearly nothing when satisfied. Air conditioners and refrigerators cycle compressors. Inverter equipment can continuously change power.
For cycling equipment, an energy monitor or measured average power is better than assuming nameplate wattage for every minute.
Standby power can matter over long periods
A few watts of standby power look trivial, but devices that remain plugged in all year accumulate many hours. Ten watts continuously is 0.24 kWh per day, or about 87.6 kWh per year.
Use the right electricity price
Your utility bill may include fixed fees that should not be attributed to one extra appliance. For incremental appliance cost, use the usage-based electricity price that best represents another kWh of consumption. If you do not know it, a state average is useful for a rough estimate.
When this simple formula is enough
The watts × hours × rate method is excellent for comparing scenarios, understanding order of magnitude, and estimating straightforward loads. It becomes less exact when equipment cycles heavily, rates change by time of day, or the tariff has tiers, demand charges, or other special rules.
Source
For the underlying electrical units, see the U.S. Energy Information Administration's Measuring electricity guide.