Watts and kilowatt-hours sound similar, but they measure different things. Understanding the difference makes almost every electricity-cost calculation easier.
Watts measure power
A watt measures the rate at which a device uses or produces energy at a particular moment. A 1,500-watt space heater has a much higher power draw than a 10-watt LED bulb. One kilowatt is 1,000 watts.
Kilowatt-hours measure energy
A kilowatt-hour measures how much energy was used over time. One kilowatt-hour is the energy used by a 1-kilowatt device running for one hour.
The formula
Convert watts to kilowatts by dividing by 1,000, then multiply by hours of use:
kWh = watts ÷ 1,000 × hours
To turn that energy into money, multiply by your electricity price. If electricity costs 16¢/kWh, one kWh costs $0.16.
Why runtime matters as much as wattage
A high-power appliance that runs briefly can use less electricity than a modest device that runs all day. A 1,200-watt microwave running for ten minutes uses only about 0.2 kWh. A 100-watt device running continuously for 24 hours uses 2.4 kWh.
This is why simply ranking appliances by wattage can be misleading. You need both power and time.
Rated watts are not always average watts
Many appliances cycle or modulate. An oven's heating elements switch on and off. An air conditioner cycles its compressor. A mini-split can vary its power continuously. A refrigerator may have a high compressor draw but spend much of the day at lower power or off.
For those devices, the nameplate wattage can be useful as a starting point but may overstate long-run average power. Energy monitors, smart plugs, or manufacturer energy-use specifications can provide a better estimate.
Why utilities bill in kWh
Utilities need a measure that captures both how much power a customer draws and how long that power is used. Kilowatt-hours do exactly that. Your meter accumulates energy use over the billing period, and the utility applies its rate structure to that usage.
kW and kWh are not interchangeable
This distinction matters in EV charging too. A Level 2 charger might deliver power at 7.2 kW, while the battery may need 45 kWh of energy. Dividing energy by power gives an approximate charging time before accounting for losses or charging-rate changes.
Source
The U.S. Energy Information Administration explains that watts measure power while watthours and kilowatthours measure energy over time in Measuring electricity.