Understand the distinction
A calculation can look tidy while mixing watts with kilowatts or minutes with hours. Consistent units explain what the result means. For a deliberately constant-power example, converting 500 watts to 0.5 kilowatts and multiplying by two hours gives one kilowatt-hour.
Put it in your home notes
Label this as a simplified example, not a measured household result. Check the source of each input and retain your conversion steps. If the appliance changes power during operation, note that the constant-power assumption no longer describes the full behaviour.