Short answer: a kilowatt (kW) measures power: how fast energy is used or produced at a moment. A kilowatt-hour (kWh) measures energy: how much is used over time. Your utility bills you for kWh. A 1,500-watt (1.5 kW) heater running for 2 hours uses 3 kWh.
The water analogy
Think of a garden hose filling a bucket. kW is the flow rate (gallons per minute). kWh is how much water ends up in the bucket (gallons). A strong flow for a short time can fill the same bucket as a weak flow for a long time.
kWh = kW × hours
Everyday examples
| Device | Power (kW) | Time | Energy (kWh) | Cost at 18.2¢/kWh |
|---|---|---|---|---|
| LED bulb (10 W) | 0.01 | 5 hours | 0.05 | about 1¢ |
| Space heater on high | 1.5 | 4 hours | 6 | about $1.09 |
| Level 2 EV charger | 7.2 | 3 hours | 21.6 | about $3.93 |
| Level 1 EV charging | 1.9 | 10 hours | 19 | about $3.46 |
Notice the last two rows: Level 2 is almost four times as powerful, but both sessions put a similar amount of energy into the car. Power decides how fast; kWh decides the bill. Prices use the EIA’s 2026 forecast average of 18.2 cents per kWh (EIA).
Where the two numbers show up
- Your bill: kWh used, priced per kWh. Some plans add a demand charge based on your highest kW in the month; see fixed and demand charges, explained.
- Solar panels: rated in watts or kW (a 7 kW system), but they produce kWh over a year (for example, about 9,000-10,000 kWh). See how many solar panels you need.
- Home batteries: have both. A Tesla Powerwall 3 stores 13.5 kWh and can deliver 11.5 kW at once (EnergySage). Capacity (kWh) decides how long it lasts; power (kW) decides how many appliances run at the same time.
- EVs: battery size is in kWh (a 75 kWh pack); charging speed in kW (a 150 kW fast charger).
- Power plants and data centers: capacity in megawatts (MW = 1,000 kW) or gigawatts; yearly use in megawatt-hours or terawatt-hours.
Quick conversions
- 1 kW = 1,000 watts
- 1 kWh = 1,000 watt-hours (Wh); portable power stations are often rated in Wh
- 1 MWh = 1,000 kWh
- Amps to watts (roughly): watts = volts × amps. A 12-amp device on 120 volts draws about 1,440 W.
Common mix-ups
- “kW per hour” is not a thing. Say kW (rate) or kWh (amount).
- A bigger battery doesn’t charge faster on its own; the charger and the car’s charging limit set the kW.
- A higher-wattage appliance isn’t always costlier. A 1,000 W microwave used 5 minutes a day uses less energy than a 100 W device left on all day.
FAQ
How many kWh does a home use? The U.S. average was 865 kWh a month in 2024 (EIA), about 28 kWh a day.
How do I measure a device? A plug-in energy meter (about $20-30) shows watts and kWh over time.