Electricity basics
Watts vs kWh: What Your Appliance Label Means
Understand watts, kilowatts, and kilowatt-hours. Learn which number measures power, which measures energy, and which belongs in a cost calculation.
Watts describe power at a moment. Kilowatt-hours describe energy over time. One kilowatt equals 1,000 watts. Cost calculations use kWh multiplied by price per kWh.
On this page
Three similar terms can make an appliance label hard to read: W, kW, and kWh. They do different jobs. A useful cost estimate starts by keeping them separate.
The EIA defines electricity units. A watt measures power. A kilowatt is 1,000 watts. A kilowatt-hour measures the energy used when one kilowatt runs for one hour.
Read the unit before the number
| Unit | What it tells you | What it does not tell you |
|---|---|---|
| W | Power at a moment or a stated rating | Total daily energy without time |
| kW | Power in thousands of watts | A complete electricity cost |
| kWh | Energy for a period or cycle | The price of that energy |
A large power number does not always mean a large monthly energy total. Duration matters. A device that runs briefly can use less energy than a smaller device left on much longer.
Compare two clear examples
Suppose one hypothetical device draws 1,000 W for 15 minutes. Convert the time to 0.25 hours. Its energy use is 1 × 0.25 = 0.25 kWh.
A second device draws 50 W for ten hours. Its energy use is 0.05 × 10 = 0.5 kWh. The second has much lower power but uses twice the energy in these examples.
At the same electricity price, the second costs twice as much for those stated periods. For a buying decision, also check what each device does and whether it meets the same need.
Why a fridge label can look inconsistent
A fridge may display an electrical rating in watts or amps and an annual energy figure in kWh. Multiplying the rating by 24 hours can produce a much larger number than the annual label.
That is not enough to prove the label is wrong. The rated input and the annual test measure different things. Compressors, fans, and defrost heaters do not necessarily draw their rated power continuously.
The FTC's EnergyGuide explanation makes another distinction: a label estimate follows standard conditions. Your kitchen, door use, and temperature settings may differ.
Watts are different from volt-amps
Some electricity monitors show both W and VA. Do not swap them when estimating running cost. The P3 monitor manual labels watts and apparent power separately.
Voltage multiplied by current gives volt-amps. For some loads, that differs from real watts. If you have a safe, suitable monitor, its cumulative kWh reading avoids guessing from those electrical ratings.
Avoid four common calculation errors
- Convert watts to kilowatts before multiplying by hours.
- Convert minutes to hours rather than entering minutes as hours.
- Do not multiply a measured kWh figure by watts again.
- Do not treat cooling capacity in BTU per hour as electrical input power.
For example, 30 minutes means 0.5 hours. A steady 200 W device uses 200 ÷ 1,000 × 0.5 = 0.1 kWh during that half-hour.
Put the units into a cost estimate
Once you have energy in kWh, multiply by your price per kWh. At an example $0.20/kWh, 0.1 kWh costs $0.02.
Use the electricity calculator to convert your figures. Choose measured energy when available. Keep the unit and measurement period with every number you record.