Which number should you use next?
Start with the letters printed beside the value. Then note where the value came from and whether a time period is attached. A nameplate wattage, a meter reading, an annual energy label, and a bill total are not interchangeable inputs even when they all describe electricity use.
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| Number and likely source | What it describes | Time boundary | Next decision |
|---|---|---|---|
| W or kW on a nameplate | Power for a rated condition or selected operating mode. | No duration is included. | Find a representative runtime, or prefer measured kWh for a variable load. |
| Wh or kWh on a plug-in meter | Energy accumulated during a measurement window. | The meter start and stop times define the period. | Use energy added during that window, not a lifetime total. Convert Wh to kWh before applying a per-kWh rate. |
| Annual kWh on an energy label | A standardized yearly estimate for the labeled product. | A year under the label's test assumptions. | Use it as annual energy, not as instantaneous power or a measured household result. |
| kWh on an electricity bill | Energy for the whole account during the billing period. | The bill's start and end dates. | Do not assign it to one appliance unless you have a separate measurement. |
Why power and energy answer different questions
In an illustrative comparison, a steady 2 kW load running for 15 minutes uses 2 x (15 / 60) = 0.5 kWh. A steady 50 W load running for 12 hours uses 50 / 1000 x 12 = 0.6 kWh. The lower-power load uses more energy here because it runs longer.
1 kW = 1,000 W; 1 kWh = 1,000 Wh. Convert within the same kind of unit: a 750 Wh energy reading is 0.75 kWh, not 750 kWh. Do not multiply an energy reading by hours again; its period is already included. The U.S. Energy Information Administration's unit explanation gives the underlying power and energy definitions.
Run three checks before choosing a calculator
- Read the unit. W and kW are power; Wh and kWh are energy.
- Name the source. Record whether the value came from a label, meter, product specification, or bill.
- Attach the period. Write down the active runtime, measurement window, label year, or billing dates.
If the source or mode is unclear, resolve it with the appliance wattage guide before treating a printed number as actual draw.
Choose the path that matches the diagnosed input
You have watts and a known runtime
Use the Watts to kWh Calculator for a quick energy conversion. Use the watts-to-cost worksheet when you also need to document the wattage, runtime, rate, assumptions, and time period.
You already have kWh
Skip the watts conversion. Send the reading and its period to the kWh to Cost Calculator.
You want a recurring appliance estimate
Use the Appliance Electricity Cost Calculator for a daily schedule and annual use days. Use the Electricity Cost Per Hour Calculator only when an hourly comparison is the decision you need.