Short answer: watts are power, kWh is energy over time
Watts tell you the rate of power use at a moment in time. A 100 W device is drawing power at that rate while it is operating. kWh adds time to the picture. A 100 W device running for 10 hours uses 1 kWh.
Why watts alone cannot tell you electricity cost
Cost depends on both power and runtime. A high-wattage heater used briefly can cost less than a lower-wattage device that runs all day. That is why a useful electricity estimate needs watts, hours, and your rate per kWh. For the full cost formula, read how to calculate electricity cost from watts.
Use this guide to understand the difference between power and energy. If you want to turn watts, hours, and a rate per kWh into an estimated cost, use the watts-to-cost guide.
Which number should you use next?
Choose the next step based on the number you have now. Watts, kWh, and cost answer different questions. This is an input-choice guide, not a second walkthrough of the full cost formula.
Scroll horizontally to see all columns.
| What you have | What it tells you | What you still need | Best next step |
|---|---|---|---|
| Watts from a label | Rated or selected power input; actual draw can vary by mode or cycle. | Runtime, or measured kWh when the load cycles or varies. | Use watts and hours for a stated scenario; use measured kWh when the draw varies. |
| kWh for a session or period | Energy used over time. | A rate per kWh before you can estimate cost. | Use kWh with your rate to estimate usage cost. |
| Watts, hours, and a rate | Enough inputs for a usage-cost estimate. | Bill-level items only if you are comparing with a full bill. | Use a cost calculator or follow the watts-to-cost formula. |
Simple formula
kWh = watts x hours / 1000
If you know the wattage and runtime but not the energy use, use the Watts to kWh Calculator first.
Watts
Power right now
A 100 W device draws power at that rate while it is running.
Hours
Runtime
The same device running longer uses more total energy.
kWh
Energy over time
100 W for 10 hours equals 1 kWh.
One neutral unit example
A 100 W device running for 10 hours uses 1 kWh. The example is deliberately about units, not an appliance recommendation: watts describe the power level, while kWh records that power over a defined amount of time.
Once the question becomes a heater session, overnight fan use, or a workday setup, the useful next step is to choose the scenario and inputs for that situation rather than reuse a generic unit example.
When to use a cost calculator instead
If you already know your electricity rate and want a money estimate, use the Appliance Electricity Cost Calculator for daily, monthly, and yearly cost. For one-hour comparisons, the Electricity Cost Per Hour Calculator shows one-hour, 8-hour, and 24-hour estimates. If you already have a kWh value, use the kWh to Cost Calculator.
Common mistakes
- Comparing watts without considering how long each appliance runs
- Using peak wattage as if the appliance always runs at that level
- Forgetting quantity when several identical devices run together
FAQ
Is 1,000 watts the same as 1 kWh?
Not by itself. 1,000 watts is 1 kW of power. It becomes 1 kWh of energy only when that load runs for one hour.
Do electricity bills use watts or kWh?
Bills commonly charge for kWh because billing is based on energy used over time, not just instantaneous power.
Why do batteries often list watt-hours?
Watt-hours describe stored energy. You can compare watt-hours with an appliance's estimated watt-hour use, but real runtime may be lower because of efficiency losses.