Watts, kilowatts and kilowatt-hours
Watts measure the rate of energy use; kilowatt-hours measure energy accumulated over time. They answer different questions and cannot be substituted on a bill.
Convert power into energy
1,000 W equals 1 kW. A 2 kW heater running for 1.5 hours consumes 3 kWh. At an illustrative USD 0.25/kWh, the energy charge is USD 0.75. A low-power device running all day can use more energy than a powerful device used briefly.
Count operating time
Hours must be total operating hours per day. If a device runs for ten minutes six times a day, use one hour, not ten hours. Enter standby as a separate load using only the hours spent in that state; use 24 hours only for a load that really remains in that state all day.
Price avoidable use
The calculator estimates the device energy charge only. Fixed account fees do not disappear when a device is switched off. Compare marginal costs when deciding whether a routine change is worthwhile.
Keep units attached to numbers
A 20 W device running for 24 hours uses 0.48 kWh. A 2,000 W kettle used for six minutes uses 0.20 kWh, because six minutes is 0.1 hours. These constant-power examples explain why wattage alone cannot rank monthly costs. Amperes and volt-amperes are not interchangeable with measured real watts.
Appliance Energy Cost Calculator
Worked example: Price a router and idle network accessory
Related reading: Measuring standby power without overstating the savings
Reference: Virginia Tech Extension: estimating appliance energy use.