What Is a Kilowatt-Hour? Energy Explained Simply
A kilowatt-hour (kWh) is the amount of energy consumed by a 1,000-watt device running for one hour. It is the standard unit on your electricity bill. A 100-watt light bulb left on for 10 hours uses 1 kWh. Your EV battery holds 40–100 kWh.
Power vs Energy: The kW/kWh Distinction
Power (measured in watts or kilowatts) is the rate at which energy is used at any instant. Energy (measured in kWh) is the total amount used over time. The relationship: Energy (kWh) = Power (kW) × Time (hours).
A 2,000-watt (2 kW) electric heater running for 3 hours uses 6 kWh of energy. A 50-watt laptop charger running for 8 hours uses 0.4 kWh. The wattage tells you the rate; the kWh tells you the total.
On an electricity bill, you are charged per kWh. In the US, the average residential rate is about $0.13–0.17 per kWh (varies widely by state). In Europe, rates are typically €0.20–0.40 per kWh. Knowing your appliances' wattage lets you estimate daily energy costs.
Everyday Appliance Energy Use
Most home appliances have a wattage label (look for the "W" rating). Multiply the wattage by daily hours of use to estimate daily kWh consumption.
A typical household in the US uses about 30 kWh per day (900 kWh per month). Most of this comes from heating/cooling (HVAC), water heating, and large appliances like refrigerators and washing machines.
kWh and Electric Vehicles
EV batteries are rated in kWh — this is their energy storage capacity, analogous to the fuel tank size on a gasoline car. A Tesla Model 3 Standard Range has a 57.5 kWh battery; a Long Range version has 82 kWh. A Nissan Leaf has 40 kWh (standard) or 62 kWh (extended range).
Efficiency is measured in miles per kWh (or km per kWh). A typical EV gets 3–4 miles per kWh. So a 60 kWh battery gives roughly 180–240 miles of range. Charging speed is measured in kW: a standard home charger delivers 7–11 kW; a fast public charger can deliver 50–350 kW.
To estimate charging cost: battery size (kWh) × electricity rate ($/kWh) = cost to fully charge. A 60 kWh battery at $0.15/kWh costs $9.00 to charge from empty. At a commercial charger at $0.30/kWh, the same charge costs $18.00.
kWh vs Joules: The Scientific Relationship
The joule (J) is the SI unit of energy. 1 kWh = 3,600,000 joules (3.6 megajoules). The kWh is simply a more convenient scale for household and industrial energy use — a joule is a very small amount of energy (about the energy needed to lift an apple 10 cm), while a kWh is a practical everyday quantity.
In scientific contexts, you may also see megajoules (MJ) and gigajoules (GJ). 1 kWh = 3.6 MJ. 1 GJ = 277.8 kWh. Energy prices for natural gas are sometimes quoted in GJ in Canada and Australia.
Conclusion
A kilowatt-hour is power times time: a 1 kW device running for 1 hour. It is the unit on your electricity bill, the battery capacity unit of EVs, and the standard measure of household energy consumption. Knowing your appliances' wattage and your electricity rate lets you calculate the cost of anything you plug in.
Frequently Asked Questions
What is 1 kWh?
1 kilowatt-hour is the energy consumed by a 1,000-watt device running for 1 hour. A 100-watt bulb needs 10 hours to use 1 kWh.
How many kWh does a house use per day?
The US average is about 30 kWh per day (900 kWh/month). Homes in hot climates with heavy air conditioning use more; well-insulated homes use less.
How much does 1 kWh cost?
In the US, about $0.13–0.17. In Europe, typically €0.20–0.40. Prices vary by country, region, and time of day.
What is the difference between kW and kWh?
kW is the rate of energy use (power). kWh is the total energy used. A 2 kW heater used for 3 hours consumes 6 kWh.